[v2,1/6] mm: userfaultfd: add new UFFDIO_POISON ioctl

Commit Message

Axel Rasmussen June 29, 2023, 8:50 p.m. UTC

The basic idea here is to "simulate" memory poisoning for VMs. A VM
running on some host might encounter a memory error, after which some
page(s) are poisoned (i.e., future accesses SIGBUS). They expect that
once poisoned, pages can never become "un-poisoned". So, when we live
migrate the VM, we need to preserve the poisoned status of these pages.

When live migrating, we try to get the guest running on its new host as
quickly as possible. So, we start it running before all memory has been
copied, and before we're certain which pages should be poisoned or not.

So the basic way to use this new feature is:

- On the new host, the guest's memory is registered with userfaultfd, in
  either MISSING or MINOR mode (doesn't really matter for this purpose).
- On any first access, we get a userfaultfd event. At this point we can
  communicate with the old host to find out if the page was poisoned.
- If so, we can respond with a UFFDIO_POISON - this places a swap marker
  so any future accesses will SIGBUS. Because the pte is now "present",
  future accesses won't generate more userfaultfd events, they'll just
  SIGBUS directly.

UFFDIO_POISON does not handle unmapping previously-present PTEs. This
isn't needed, because during live migration we want to intercept
all accesses with userfaultfd (not just writes, so WP mode isn't useful
for this). So whether minor or missing mode is being used (or both), the
PTE won't be present in any case, so handling that case isn't needed.

Why return VM_FAULT_HWPOISON instead of VM_FAULT_SIGBUS when one of
these markers is encountered? For "normal" userspace programs there
isn't a big difference, both yield a SIGBUS. The difference for KVM is
key though: VM_FAULT_HWPOISON will result in an MCE being injected into
the guest (which is the behavior we want). With VM_FAULT_SIGBUS, the
hypervisor would need to catch the SIGBUS and deal with the MCE
injection itself.

Signed-off-by: Axel Rasmussen <[e-mail]>
---
 fs/userfaultfd.c                 | 63 ++++++++++++++++++++++++++++++++
 include/linux/swapops.h          |  3 +-
 include/linux/userfaultfd_k.h    |  4 ++
 include/uapi/linux/userfaultfd.h | 25 +++++++++++--
 mm/memory.c                      |  4 ++
 mm/userfaultfd.c                 | 62 ++++++++++++++++++++++++++++++-
 6 files changed, 156 insertions(+), 5 deletions(-)

Comments

Peter Xu July 4, 2023, 8:38 p.m. UTC | #1

On Thu, Jun 29, 2023 at 01:50:35PM -0700, Axel Rasmussen wrote:
> The basic idea here is to "simulate" memory poisoning for VMs. A VM
> running on some host might encounter a memory error, after which some
> page(s) are poisoned (i.e., future accesses SIGBUS). They expect that
> once poisoned, pages can never become "un-poisoned". So, when we live
> migrate the VM, we need to preserve the poisoned status of these pages.
> 
> When live migrating, we try to get the guest running on its new host as
> quickly as possible. So, we start it running before all memory has been
> copied, and before we're certain which pages should be poisoned or not.
> 
> So the basic way to use this new feature is:
> 
> - On the new host, the guest's memory is registered with userfaultfd, in
>   either MISSING or MINOR mode (doesn't really matter for this purpose).
> - On any first access, we get a userfaultfd event. At this point we can
>   communicate with the old host to find out if the page was poisoned.
> - If so, we can respond with a UFFDIO_POISON - this places a swap marker
>   so any future accesses will SIGBUS. Because the pte is now "present",
>   future accesses won't generate more userfaultfd events, they'll just
>   SIGBUS directly.
> 
> UFFDIO_POISON does not handle unmapping previously-present PTEs. This
> isn't needed, because during live migration we want to intercept
> all accesses with userfaultfd (not just writes, so WP mode isn't useful
> for this). So whether minor or missing mode is being used (or both), the
> PTE won't be present in any case, so handling that case isn't needed.
> 
> Why return VM_FAULT_HWPOISON instead of VM_FAULT_SIGBUS when one of
> these markers is encountered? For "normal" userspace programs there
> isn't a big difference, both yield a SIGBUS. The difference for KVM is
> key though: VM_FAULT_HWPOISON will result in an MCE being injected into
> the guest (which is the behavior we want). With VM_FAULT_SIGBUS, the
> hypervisor would need to catch the SIGBUS and deal with the MCE
> injection itself.
> 
> Signed-off-by: Axel Rasmussen <[e-mail]>

Maybe have a cover letter for the next version?

> ---
>  fs/userfaultfd.c                 | 63 ++++++++++++++++++++++++++++++++
>  include/linux/swapops.h          |  3 +-
>  include/linux/userfaultfd_k.h    |  4 ++
>  include/uapi/linux/userfaultfd.h | 25 +++++++++++--
>  mm/memory.c                      |  4 ++
>  mm/userfaultfd.c                 | 62 ++++++++++++++++++++++++++++++-
>  6 files changed, 156 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index 7cecd49e078b..c26a883399c9 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -1965,6 +1965,66 @@ static int userfaultfd_continue(struct userfaultfd_ctx *ctx, unsigned long arg)
>  	return ret;
>  }
>  
> +static inline int userfaultfd_poison(struct userfaultfd_ctx *ctx, unsigned long arg)
> +{
> +	__s64 ret;
> +	struct uffdio_poison uffdio_poison;
> +	struct uffdio_poison __user *user_uffdio_poison;
> +	struct userfaultfd_wake_range range;
> +
> +	user_uffdio_poison = (struct uffdio_poison __user *)arg;
> +
> +	ret = -EAGAIN;
> +	if (atomic_read(&ctx->mmap_changing))
> +		goto out;
> +
> +	ret = -EFAULT;
> +	if (copy_from_user(&uffdio_poison, user_uffdio_poison,
> +			   /* don't copy the output fields */
> +			   sizeof(uffdio_poison) - (sizeof(__s64))))
> +		goto out;
> +
> +	ret = validate_range(ctx->mm, uffdio_poison.range.start,
> +			     uffdio_poison.range.len);
> +	if (ret)
> +		goto out;
> +
> +	ret = -EINVAL;
> +	/* double check for wraparound just in case. */
> +	if (uffdio_poison.range.start + uffdio_poison.range.len <=
> +	    uffdio_poison.range.start) {

Brackets not needed here.

> +		goto out;
> +	}
> +	if (uffdio_poison.mode & ~UFFDIO_POISON_MODE_DONTWAKE)
> +		goto out;
> +
> +	if (mmget_not_zero(ctx->mm)) {
> +		ret = mfill_atomic_poison(ctx->mm, uffdio_poison.range.start,
> +					  uffdio_poison.range.len,
> +					  &ctx->mmap_changing, 0);
> +		mmput(ctx->mm);
> +	} else {
> +		return -ESRCH;
> +	}
> +
> +	if (unlikely(put_user(ret, &user_uffdio_poison->updated)))
> +		return -EFAULT;
> +	if (ret < 0)
> +		goto out;
> +
> +	/* len == 0 would wake all */
> +	BUG_ON(!ret);
> +	range.len = ret;
> +	if (!(uffdio_poison.mode & UFFDIO_POISON_MODE_DONTWAKE)) {
> +		range.start = uffdio_poison.range.start;
> +		wake_userfault(ctx, &range);
> +	}
> +	ret = range.len == uffdio_poison.range.len ? 0 : -EAGAIN;
> +
> +out:
> +	return ret;
> +}
> +
>  static inline unsigned int uffd_ctx_features(__u64 user_features)
>  {
>  	/*
> @@ -2066,6 +2126,9 @@ static long userfaultfd_ioctl(struct file *file, unsigned cmd,
>  	case UFFDIO_CONTINUE:
>  		ret = userfaultfd_continue(ctx, arg);
>  		break;
> +	case UFFDIO_POISON:
> +		ret = userfaultfd_poison(ctx, arg);
> +		break;
>  	}
>  	return ret;
>  }
> diff --git a/include/linux/swapops.h b/include/linux/swapops.h
> index 4c932cb45e0b..8259fee32421 100644
> --- a/include/linux/swapops.h
> +++ b/include/linux/swapops.h
> @@ -394,7 +394,8 @@ typedef unsigned long pte_marker;
>  
>  #define  PTE_MARKER_UFFD_WP			BIT(0)
>  #define  PTE_MARKER_SWAPIN_ERROR		BIT(1)
> -#define  PTE_MARKER_MASK			(BIT(2) - 1)
> +#define  PTE_MARKER_UFFD_POISON			BIT(2)

One more tab.

Though I remembered the last time we discussed IIRC we plan to rename
SWAPIN_ERROR and reuse it, could you explain why a new bit is still needed?

I think I commented this but I'll do it again: IIUC any existing host
swapin errors for guest pages should be reported as MCE too, afaict,
happened in kvm context.

> +#define  PTE_MARKER_MASK			(BIT(3) - 1)
>  
>  static inline swp_entry_t make_pte_marker_entry(pte_marker marker)
>  {
> diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
> index ac7b0c96d351..ac8c6854097c 100644
> --- a/include/linux/userfaultfd_k.h
> +++ b/include/linux/userfaultfd_k.h
> @@ -46,6 +46,7 @@ enum mfill_atomic_mode {
>  	MFILL_ATOMIC_COPY,
>  	MFILL_ATOMIC_ZEROPAGE,
>  	MFILL_ATOMIC_CONTINUE,
> +	MFILL_ATOMIC_POISON,
>  	NR_MFILL_ATOMIC_MODES,
>  };
>  
> @@ -83,6 +84,9 @@ extern ssize_t mfill_atomic_zeropage(struct mm_struct *dst_mm,
>  extern ssize_t mfill_atomic_continue(struct mm_struct *dst_mm, unsigned long dst_start,
>  				     unsigned long len, atomic_t *mmap_changing,
>  				     uffd_flags_t flags);
> +extern ssize_t mfill_atomic_poison(struct mm_struct *dst_mm, unsigned long start,
> +				   unsigned long len, atomic_t *mmap_changing,
> +				   uffd_flags_t flags);
>  extern int mwriteprotect_range(struct mm_struct *dst_mm,
>  			       unsigned long start, unsigned long len,
>  			       bool enable_wp, atomic_t *mmap_changing);
> diff --git a/include/uapi/linux/userfaultfd.h b/include/uapi/linux/userfaultfd.h
> index 66dd4cd277bd..62151706c5a3 100644
> --- a/include/uapi/linux/userfaultfd.h
> +++ b/include/uapi/linux/userfaultfd.h
> @@ -39,7 +39,8 @@
>  			   UFFD_FEATURE_MINOR_SHMEM |		\
>  			   UFFD_FEATURE_EXACT_ADDRESS |		\
>  			   UFFD_FEATURE_WP_HUGETLBFS_SHMEM |	\
> -			   UFFD_FEATURE_WP_UNPOPULATED)
> +			   UFFD_FEATURE_WP_UNPOPULATED |	\
> +			   UFFD_FEATURE_POISON)
>  #define UFFD_API_IOCTLS				\
>  	((__u64)1 << _UFFDIO_REGISTER |		\
>  	 (__u64)1 << _UFFDIO_UNREGISTER |	\
> @@ -49,12 +50,14 @@
>  	 (__u64)1 << _UFFDIO_COPY |		\
>  	 (__u64)1 << _UFFDIO_ZEROPAGE |		\
>  	 (__u64)1 << _UFFDIO_WRITEPROTECT |	\
> -	 (__u64)1 << _UFFDIO_CONTINUE)
> +	 (__u64)1 << _UFFDIO_CONTINUE |		\
> +	 (__u64)1 << _UFFDIO_POISON)
>  #define UFFD_API_RANGE_IOCTLS_BASIC		\
>  	((__u64)1 << _UFFDIO_WAKE |		\
>  	 (__u64)1 << _UFFDIO_COPY |		\
> +	 (__u64)1 << _UFFDIO_WRITEPROTECT |	\
>  	 (__u64)1 << _UFFDIO_CONTINUE |		\
> -	 (__u64)1 << _UFFDIO_WRITEPROTECT)
> +	 (__u64)1 << _UFFDIO_POISON)

May not be a large deal, but it's still better to declare the feature &
ioctls after all things implemented.  Maybe make these few lines
(UFFD_API*, and the new feature bit) as the last patch to enable the
feature?

>  
>  /*
>   * Valid ioctl command number range with this API is from 0x00 to
> @@ -71,6 +74,7 @@
>  #define _UFFDIO_ZEROPAGE		(0x04)
>  #define _UFFDIO_WRITEPROTECT		(0x06)
>  #define _UFFDIO_CONTINUE		(0x07)
> +#define _UFFDIO_POISON			(0x08)
>  #define _UFFDIO_API			(0x3F)
>  
>  /* userfaultfd ioctl ids */
> @@ -91,6 +95,8 @@
>  				      struct uffdio_writeprotect)
>  #define UFFDIO_CONTINUE		_IOWR(UFFDIO, _UFFDIO_CONTINUE,	\
>  				      struct uffdio_continue)
> +#define UFFDIO_POISON		_IOWR(UFFDIO, _UFFDIO_POISON, \
> +				      struct uffdio_poison)
>  
>  /* read() structure */
>  struct uffd_msg {
> @@ -225,6 +231,7 @@ struct uffdio_api {
>  #define UFFD_FEATURE_EXACT_ADDRESS		(1<<11)
>  #define UFFD_FEATURE_WP_HUGETLBFS_SHMEM		(1<<12)
>  #define UFFD_FEATURE_WP_UNPOPULATED		(1<<13)
> +#define UFFD_FEATURE_POISON			(1<<14)
>  	__u64 features;
>  
>  	__u64 ioctls;
> @@ -321,6 +328,18 @@ struct uffdio_continue {
>  	__s64 mapped;
>  };
>  
> +struct uffdio_poison {
> +	struct uffdio_range range;
> +#define UFFDIO_POISON_MODE_DONTWAKE		((__u64)1<<0)
> +	__u64 mode;
> +
> +	/*
> +	 * Fields below here are written by the ioctl and must be at the end:
> +	 * the copy_from_user will not read past here.
> +	 */
> +	__s64 updated;
> +};
> +
>  /*
>   * Flags for the userfaultfd(2) system call itself.
>   */
> diff --git a/mm/memory.c b/mm/memory.c
> index d8a9a770b1f1..7fbda39e060d 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3692,6 +3692,10 @@ static vm_fault_t handle_pte_marker(struct vm_fault *vmf)
>  	if (WARN_ON_ONCE(!marker))
>  		return VM_FAULT_SIGBUS;
>  
> +	/* Poison emulation explicitly requested for this PTE. */
> +	if (marker & PTE_MARKER_UFFD_POISON)
> +		return VM_FAULT_HWPOISON;
> +
>  	/* Higher priority than uffd-wp when data corrupted */
>  	if (marker & PTE_MARKER_SWAPIN_ERROR)
>  		return VM_FAULT_SIGBUS;
> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index a2bf37ee276d..87b62ca1e09e 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -286,6 +286,51 @@ static int mfill_atomic_pte_continue(pmd_t *dst_pmd,
>  	goto out;
>  }
>  
> +/* Handles UFFDIO_POISON for all non-hugetlb VMAs. */
> +static int mfill_atomic_pte_poison(pmd_t *dst_pmd,
> +				   struct vm_area_struct *dst_vma,
> +				   unsigned long dst_addr,
> +				   uffd_flags_t flags)
> +{
> +	int ret;
> +	struct mm_struct *dst_mm = dst_vma->vm_mm;
> +	pte_t _dst_pte, *dst_pte;
> +	spinlock_t *ptl;
> +
> +	_dst_pte = make_pte_marker(PTE_MARKER_UFFD_POISON);
> +	dst_pte = pte_offset_map_lock(dst_mm, dst_pmd, dst_addr, &ptl);
> +
> +	if (vma_is_shmem(dst_vma)) {
> +		struct inode *inode;
> +		pgoff_t offset, max_off;
> +
> +		/* serialize against truncate with the page table lock */
> +		inode = dst_vma->vm_file->f_inode;
> +		offset = linear_page_index(dst_vma, dst_addr);
> +		max_off = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
> +		ret = -EFAULT;
> +		if (unlikely(offset >= max_off))
> +			goto out_unlock;
> +	}

Maybe good chance to have a mfill_file_over_size() helper?  Other potential
users are mfill_atomic_pte_zeropage() and mfill_atomic_install_pte().

> +
> +	ret = -EEXIST;
> +	/*
> +	 * For now, we don't handle unmapping pages, so only support filling in
> +	 * none PTEs, or replacing PTE markers.
> +	 */
> +	if (!pte_none_mostly(*dst_pte))
> +		goto out_unlock;
> +
> +	set_pte_at(dst_mm, dst_addr, dst_pte, _dst_pte);
> +
> +	/* No need to invalidate - it was non-present before */
> +	update_mmu_cache(dst_vma, dst_addr, dst_pte);
> +	ret = 0;
> +out_unlock:
> +	pte_unmap_unlock(dst_pte, ptl);
> +	return ret;
> +}
> +
>  static pmd_t *mm_alloc_pmd(struct mm_struct *mm, unsigned long address)
>  {
>  	pgd_t *pgd;
> @@ -336,8 +381,12 @@ static __always_inline ssize_t mfill_atomic_hugetlb(
>  	 * supported by hugetlb.  A PMD_SIZE huge pages may exist as used
>  	 * by THP.  Since we can not reliably insert a zero page, this
>  	 * feature is not supported.
> +	 *
> +	 * PTE marker handling for hugetlb is a bit special, so for now
> +	 * UFFDIO_POISON is not supported.
>  	 */
> -	if (uffd_flags_mode_is(flags, MFILL_ATOMIC_ZEROPAGE)) {
> +	if (uffd_flags_mode_is(flags, MFILL_ATOMIC_ZEROPAGE) ||
> +	    uffd_flags_mode_is(flags, MFILL_ATOMIC_POISON)) {
>  		mmap_read_unlock(dst_mm);
>  		return -EINVAL;
>  	}
> @@ -481,6 +530,9 @@ static __always_inline ssize_t mfill_atomic_pte(pmd_t *dst_pmd,
>  	if (uffd_flags_mode_is(flags, MFILL_ATOMIC_CONTINUE)) {
>  		return mfill_atomic_pte_continue(dst_pmd, dst_vma,
>  						 dst_addr, flags);
> +	} else if (uffd_flags_mode_is(flags, MFILL_ATOMIC_POISON)) {
> +		return mfill_atomic_pte_poison(dst_pmd, dst_vma,
> +					       dst_addr, flags);
>  	}
>  
>  	/*
> @@ -702,6 +754,14 @@ ssize_t mfill_atomic_continue(struct mm_struct *dst_mm, unsigned long start,
>  			    uffd_flags_set_mode(flags, MFILL_ATOMIC_CONTINUE));
>  }
>  
> +ssize_t mfill_atomic_poison(struct mm_struct *dst_mm, unsigned long start,
> +			    unsigned long len, atomic_t *mmap_changing,
> +			    uffd_flags_t flags)
> +{
> +	return mfill_atomic(dst_mm, start, 0, len, mmap_changing,
> +			    uffd_flags_set_mode(flags, MFILL_ATOMIC_POISON));
> +}
> +
>  long uffd_wp_range(struct vm_area_struct *dst_vma,
>  		   unsigned long start, unsigned long len, bool enable_wp)
>  {
> -- 
> 2.41.0.255.g8b1d071c50-goog
>

James Houghton July 5, 2023, 3:58 p.m. UTC | #2

On Thu, Jun 29, 2023 at 1:50 PM Axel Rasmussen <[e-mail]> wrote:
>
> The basic idea here is to "simulate" memory poisoning for VMs. A VM
> running on some host might encounter a memory error, after which some
> page(s) are poisoned (i.e., future accesses SIGBUS). They expect that
> once poisoned, pages can never become "un-poisoned". So, when we live
> migrate the VM, we need to preserve the poisoned status of these pages.
>
> When live migrating, we try to get the guest running on its new host as
> quickly as possible. So, we start it running before all memory has been
> copied, and before we're certain which pages should be poisoned or not.
>
> So the basic way to use this new feature is:
>
> - On the new host, the guest's memory is registered with userfaultfd, in
>   either MISSING or MINOR mode (doesn't really matter for this purpose).
> - On any first access, we get a userfaultfd event. At this point we can
>   communicate with the old host to find out if the page was poisoned.
> - If so, we can respond with a UFFDIO_POISON - this places a swap marker
>   so any future accesses will SIGBUS. Because the pte is now "present",
>   future accesses won't generate more userfaultfd events, they'll just
>   SIGBUS directly.
>
> UFFDIO_POISON does not handle unmapping previously-present PTEs. This
> isn't needed, because during live migration we want to intercept
> all accesses with userfaultfd (not just writes, so WP mode isn't useful
> for this). So whether minor or missing mode is being used (or both), the
> PTE won't be present in any case, so handling that case isn't needed.
>
> Why return VM_FAULT_HWPOISON instead of VM_FAULT_SIGBUS when one of
> these markers is encountered? For "normal" userspace programs there
> isn't a big difference, both yield a SIGBUS. The difference for KVM is
> key though: VM_FAULT_HWPOISON will result in an MCE being injected into
> the guest (which is the behavior we want). With VM_FAULT_SIGBUS, the
> hypervisor would need to catch the SIGBUS and deal with the MCE
> injection itself.

For a normal userspace program, the si_code will be different
(BUS_MCEERR_AR vs. BUS_ADRERR), maybe that's worth mentioning?

Also KVM doesn't inject an MCE automatically. When KVM_RUN gets
-EHWPOISON back from GUP (for x86 and arm64 anyway), KVM will queue up
a BUS_MCERR_AR signal for userspace and return 0 to userspace. Then
userspace may choose to inject an MCE. If we gave back VM_FAULT_SIGBUS
instead of VM_FAULT_HWPOISON, KVM_RUN would instead return -EFAULT to
userspace without queueing a signal. Today, without the signal, it
becomes impossible (I think) to determine what the appropriate MCE to
inject is.

>
> Signed-off-by: Axel Rasmussen <[e-mail]>
> ---
>  fs/userfaultfd.c                 | 63 ++++++++++++++++++++++++++++++++
>  include/linux/swapops.h          |  3 +-
>  include/linux/userfaultfd_k.h    |  4 ++
>  include/uapi/linux/userfaultfd.h | 25 +++++++++++--
>  mm/memory.c                      |  4 ++
>  mm/userfaultfd.c                 | 62 ++++++++++++++++++++++++++++++-
>  6 files changed, 156 insertions(+), 5 deletions(-)
>
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index 7cecd49e078b..c26a883399c9 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -1965,6 +1965,66 @@ static int userfaultfd_continue(struct userfaultfd_ctx *ctx, unsigned long arg)
>         return ret;
>  }
>
> +static inline int userfaultfd_poison(struct userfaultfd_ctx *ctx, unsigned long arg)
> +{
> +       __s64 ret;
> +       struct uffdio_poison uffdio_poison;
> +       struct uffdio_poison __user *user_uffdio_poison;
> +       struct userfaultfd_wake_range range;
> +
> +       user_uffdio_poison = (struct uffdio_poison __user *)arg;
> +
> +       ret = -EAGAIN;
> +       if (atomic_read(&ctx->mmap_changing))
> +               goto out;
> +
> +       ret = -EFAULT;
> +       if (copy_from_user(&uffdio_poison, user_uffdio_poison,
> +                          /* don't copy the output fields */
> +                          sizeof(uffdio_poison) - (sizeof(__s64))))
> +               goto out;
> +
> +       ret = validate_range(ctx->mm, uffdio_poison.range.start,
> +                            uffdio_poison.range.len);
> +       if (ret)
> +               goto out;
> +
> +       ret = -EINVAL;
> +       /* double check for wraparound just in case. */
> +       if (uffdio_poison.range.start + uffdio_poison.range.len <=
> +           uffdio_poison.range.start) {
> +               goto out;
> +       }
> +       if (uffdio_poison.mode & ~UFFDIO_POISON_MODE_DONTWAKE)
> +               goto out;
> +
> +       if (mmget_not_zero(ctx->mm)) {
> +               ret = mfill_atomic_poison(ctx->mm, uffdio_poison.range.start,
> +                                         uffdio_poison.range.len,
> +                                         &ctx->mmap_changing, 0);
> +               mmput(ctx->mm);
> +       } else {
> +               return -ESRCH;
> +       }
> +
> +       if (unlikely(put_user(ret, &user_uffdio_poison->updated)))
> +               return -EFAULT;
> +       if (ret < 0)
> +               goto out;
> +
> +       /* len == 0 would wake all */
> +       BUG_ON(!ret);
> +       range.len = ret;
> +       if (!(uffdio_poison.mode & UFFDIO_POISON_MODE_DONTWAKE)) {
> +               range.start = uffdio_poison.range.start;
> +               wake_userfault(ctx, &range);
> +       }
> +       ret = range.len == uffdio_poison.range.len ? 0 : -EAGAIN;
> +
> +out:
> +       return ret;
> +}
> +
>  static inline unsigned int uffd_ctx_features(__u64 user_features)
>  {
>         /*
> @@ -2066,6 +2126,9 @@ static long userfaultfd_ioctl(struct file *file, unsigned cmd,
>         case UFFDIO_CONTINUE:
>                 ret = userfaultfd_continue(ctx, arg);
>                 break;
> +       case UFFDIO_POISON:
> +               ret = userfaultfd_poison(ctx, arg);
> +               break;
>         }
>         return ret;
>  }
> diff --git a/include/linux/swapops.h b/include/linux/swapops.h
> index 4c932cb45e0b..8259fee32421 100644
> --- a/include/linux/swapops.h
> +++ b/include/linux/swapops.h
> @@ -394,7 +394,8 @@ typedef unsigned long pte_marker;
>
>  #define  PTE_MARKER_UFFD_WP                    BIT(0)
>  #define  PTE_MARKER_SWAPIN_ERROR               BIT(1)
> -#define  PTE_MARKER_MASK                       (BIT(2) - 1)
> +#define  PTE_MARKER_UFFD_POISON                        BIT(2)
> +#define  PTE_MARKER_MASK                       (BIT(3) - 1)
>
>  static inline swp_entry_t make_pte_marker_entry(pte_marker marker)
>  {
> diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
> index ac7b0c96d351..ac8c6854097c 100644
> --- a/include/linux/userfaultfd_k.h
> +++ b/include/linux/userfaultfd_k.h
> @@ -46,6 +46,7 @@ enum mfill_atomic_mode {
>         MFILL_ATOMIC_COPY,
>         MFILL_ATOMIC_ZEROPAGE,
>         MFILL_ATOMIC_CONTINUE,
> +       MFILL_ATOMIC_POISON,
>         NR_MFILL_ATOMIC_MODES,
>  };
>
> @@ -83,6 +84,9 @@ extern ssize_t mfill_atomic_zeropage(struct mm_struct *dst_mm,
>  extern ssize_t mfill_atomic_continue(struct mm_struct *dst_mm, unsigned long dst_start,
>                                      unsigned long len, atomic_t *mmap_changing,
>                                      uffd_flags_t flags);
> +extern ssize_t mfill_atomic_poison(struct mm_struct *dst_mm, unsigned long start,
> +                                  unsigned long len, atomic_t *mmap_changing,
> +                                  uffd_flags_t flags);
>  extern int mwriteprotect_range(struct mm_struct *dst_mm,
>                                unsigned long start, unsigned long len,
>                                bool enable_wp, atomic_t *mmap_changing);
> diff --git a/include/uapi/linux/userfaultfd.h b/include/uapi/linux/userfaultfd.h
> index 66dd4cd277bd..62151706c5a3 100644
> --- a/include/uapi/linux/userfaultfd.h
> +++ b/include/uapi/linux/userfaultfd.h
> @@ -39,7 +39,8 @@
>                            UFFD_FEATURE_MINOR_SHMEM |           \
>                            UFFD_FEATURE_EXACT_ADDRESS |         \
>                            UFFD_FEATURE_WP_HUGETLBFS_SHMEM |    \
> -                          UFFD_FEATURE_WP_UNPOPULATED)
> +                          UFFD_FEATURE_WP_UNPOPULATED |        \
> +                          UFFD_FEATURE_POISON)
>  #define UFFD_API_IOCTLS                                \
>         ((__u64)1 << _UFFDIO_REGISTER |         \
>          (__u64)1 << _UFFDIO_UNREGISTER |       \
> @@ -49,12 +50,14 @@
>          (__u64)1 << _UFFDIO_COPY |             \
>          (__u64)1 << _UFFDIO_ZEROPAGE |         \
>          (__u64)1 << _UFFDIO_WRITEPROTECT |     \
> -        (__u64)1 << _UFFDIO_CONTINUE)
> +        (__u64)1 << _UFFDIO_CONTINUE |         \
> +        (__u64)1 << _UFFDIO_POISON)
>  #define UFFD_API_RANGE_IOCTLS_BASIC            \
>         ((__u64)1 << _UFFDIO_WAKE |             \
>          (__u64)1 << _UFFDIO_COPY |             \
> +        (__u64)1 << _UFFDIO_WRITEPROTECT |     \
>          (__u64)1 << _UFFDIO_CONTINUE |         \
> -        (__u64)1 << _UFFDIO_WRITEPROTECT)
> +        (__u64)1 << _UFFDIO_POISON)
>
>  /*
>   * Valid ioctl command number range with this API is from 0x00 to
> @@ -71,6 +74,7 @@
>  #define _UFFDIO_ZEROPAGE               (0x04)
>  #define _UFFDIO_WRITEPROTECT           (0x06)
>  #define _UFFDIO_CONTINUE               (0x07)
> +#define _UFFDIO_POISON                 (0x08)
>  #define _UFFDIO_API                    (0x3F)
>
>  /* userfaultfd ioctl ids */
> @@ -91,6 +95,8 @@
>                                       struct uffdio_writeprotect)
>  #define UFFDIO_CONTINUE                _IOWR(UFFDIO, _UFFDIO_CONTINUE, \
>                                       struct uffdio_continue)
> +#define UFFDIO_POISON          _IOWR(UFFDIO, _UFFDIO_POISON, \
> +                                     struct uffdio_poison)
>
>  /* read() structure */
>  struct uffd_msg {
> @@ -225,6 +231,7 @@ struct uffdio_api {
>  #define UFFD_FEATURE_EXACT_ADDRESS             (1<<11)
>  #define UFFD_FEATURE_WP_HUGETLBFS_SHMEM                (1<<12)
>  #define UFFD_FEATURE_WP_UNPOPULATED            (1<<13)
> +#define UFFD_FEATURE_POISON                    (1<<14)
>         __u64 features;
>
>         __u64 ioctls;
> @@ -321,6 +328,18 @@ struct uffdio_continue {
>         __s64 mapped;
>  };
>
> +struct uffdio_poison {
> +       struct uffdio_range range;
> +#define UFFDIO_POISON_MODE_DONTWAKE            ((__u64)1<<0)
> +       __u64 mode;
> +
> +       /*
> +        * Fields below here are written by the ioctl and must be at the end:
> +        * the copy_from_user will not read past here.
> +        */
> +       __s64 updated;
> +};
> +
>  /*
>   * Flags for the userfaultfd(2) system call itself.
>   */
> diff --git a/mm/memory.c b/mm/memory.c
> index d8a9a770b1f1..7fbda39e060d 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3692,6 +3692,10 @@ static vm_fault_t handle_pte_marker(struct vm_fault *vmf)
>         if (WARN_ON_ONCE(!marker))
>                 return VM_FAULT_SIGBUS;
>
> +       /* Poison emulation explicitly requested for this PTE. */
> +       if (marker & PTE_MARKER_UFFD_POISON)
> +               return VM_FAULT_HWPOISON;
> +
>         /* Higher priority than uffd-wp when data corrupted */
>         if (marker & PTE_MARKER_SWAPIN_ERROR)
>                 return VM_FAULT_SIGBUS;
> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index a2bf37ee276d..87b62ca1e09e 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -286,6 +286,51 @@ static int mfill_atomic_pte_continue(pmd_t *dst_pmd,
>         goto out;
>  }
>
> +/* Handles UFFDIO_POISON for all non-hugetlb VMAs. */
> +static int mfill_atomic_pte_poison(pmd_t *dst_pmd,
> +                                  struct vm_area_struct *dst_vma,
> +                                  unsigned long dst_addr,
> +                                  uffd_flags_t flags)
> +{
> +       int ret;
> +       struct mm_struct *dst_mm = dst_vma->vm_mm;
> +       pte_t _dst_pte, *dst_pte;
> +       spinlock_t *ptl;
> +
> +       _dst_pte = make_pte_marker(PTE_MARKER_UFFD_POISON);
> +       dst_pte = pte_offset_map_lock(dst_mm, dst_pmd, dst_addr, &ptl);
> +
> +       if (vma_is_shmem(dst_vma)) {
> +               struct inode *inode;
> +               pgoff_t offset, max_off;
> +
> +               /* serialize against truncate with the page table lock */
> +               inode = dst_vma->vm_file->f_inode;
> +               offset = linear_page_index(dst_vma, dst_addr);
> +               max_off = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
> +               ret = -EFAULT;
> +               if (unlikely(offset >= max_off))
> +                       goto out_unlock;
> +       }
> +
> +       ret = -EEXIST;
> +       /*
> +        * For now, we don't handle unmapping pages, so only support filling in
> +        * none PTEs, or replacing PTE markers.
> +        */
> +       if (!pte_none_mostly(*dst_pte))
> +               goto out_unlock;
> +
> +       set_pte_at(dst_mm, dst_addr, dst_pte, _dst_pte);
> +
> +       /* No need to invalidate - it was non-present before */
> +       update_mmu_cache(dst_vma, dst_addr, dst_pte);
> +       ret = 0;
> +out_unlock:
> +       pte_unmap_unlock(dst_pte, ptl);
> +       return ret;
> +}
> +
>  static pmd_t *mm_alloc_pmd(struct mm_struct *mm, unsigned long address)
>  {
>         pgd_t *pgd;
> @@ -336,8 +381,12 @@ static __always_inline ssize_t mfill_atomic_hugetlb(
>          * supported by hugetlb.  A PMD_SIZE huge pages may exist as used
>          * by THP.  Since we can not reliably insert a zero page, this
>          * feature is not supported.
> +        *
> +        * PTE marker handling for hugetlb is a bit special, so for now
> +        * UFFDIO_POISON is not supported.
>          */
> -       if (uffd_flags_mode_is(flags, MFILL_ATOMIC_ZEROPAGE)) {
> +       if (uffd_flags_mode_is(flags, MFILL_ATOMIC_ZEROPAGE) ||
> +           uffd_flags_mode_is(flags, MFILL_ATOMIC_POISON)) {
>                 mmap_read_unlock(dst_mm);
>                 return -EINVAL;
>         }
> @@ -481,6 +530,9 @@ static __always_inline ssize_t mfill_atomic_pte(pmd_t *dst_pmd,
>         if (uffd_flags_mode_is(flags, MFILL_ATOMIC_CONTINUE)) {
>                 return mfill_atomic_pte_continue(dst_pmd, dst_vma,
>                                                  dst_addr, flags);
> +       } else if (uffd_flags_mode_is(flags, MFILL_ATOMIC_POISON)) {
> +               return mfill_atomic_pte_poison(dst_pmd, dst_vma,
> +                                              dst_addr, flags);
>         }
>
>         /*
> @@ -702,6 +754,14 @@ ssize_t mfill_atomic_continue(struct mm_struct *dst_mm, unsigned long start,
>                             uffd_flags_set_mode(flags, MFILL_ATOMIC_CONTINUE));
>  }
>
> +ssize_t mfill_atomic_poison(struct mm_struct *dst_mm, unsigned long start,
> +                           unsigned long len, atomic_t *mmap_changing,
> +                           uffd_flags_t flags)
> +{
> +       return mfill_atomic(dst_mm, start, 0, len, mmap_changing,
> +                           uffd_flags_set_mode(flags, MFILL_ATOMIC_POISON));
> +}
> +
>  long uffd_wp_range(struct vm_area_struct *dst_vma,
>                    unsigned long start, unsigned long len, bool enable_wp)
>  {
> --
> 2.41.0.255.g8b1d071c50-goog
>

James Houghton July 5, 2023, 4:09 p.m. UTC | #3

On Tue, Jul 4, 2023 at 1:38 PM Peter Xu <[e-mail]> wrote:
>
> On Thu, Jun 29, 2023 at 01:50:35PM -0700, Axel Rasmussen wrote:
> > The basic idea here is to "simulate" memory poisoning for VMs. A VM
> > running on some host might encounter a memory error, after which some
> > page(s) are poisoned (i.e., future accesses SIGBUS). They expect that
> > once poisoned, pages can never become "un-poisoned". So, when we live
> > migrate the VM, we need to preserve the poisoned status of these pages.
> >
> > When live migrating, we try to get the guest running on its new host as
> > quickly as possible. So, we start it running before all memory has been
> > copied, and before we're certain which pages should be poisoned or not.
> >
> > So the basic way to use this new feature is:
> >
> > - On the new host, the guest's memory is registered with userfaultfd, in
> >   either MISSING or MINOR mode (doesn't really matter for this purpose).
> > - On any first access, we get a userfaultfd event. At this point we can
> >   communicate with the old host to find out if the page was poisoned.
> > - If so, we can respond with a UFFDIO_POISON - this places a swap marker
> >   so any future accesses will SIGBUS. Because the pte is now "present",
> >   future accesses won't generate more userfaultfd events, they'll just
> >   SIGBUS directly.
> >
> > UFFDIO_POISON does not handle unmapping previously-present PTEs. This
> > isn't needed, because during live migration we want to intercept
> > all accesses with userfaultfd (not just writes, so WP mode isn't useful
> > for this). So whether minor or missing mode is being used (or both), the
> > PTE won't be present in any case, so handling that case isn't needed.
> >
> > Why return VM_FAULT_HWPOISON instead of VM_FAULT_SIGBUS when one of
> > these markers is encountered? For "normal" userspace programs there
> > isn't a big difference, both yield a SIGBUS. The difference for KVM is
> > key though: VM_FAULT_HWPOISON will result in an MCE being injected into
> > the guest (which is the behavior we want). With VM_FAULT_SIGBUS, the
> > hypervisor would need to catch the SIGBUS and deal with the MCE
> > injection itself.
> >
> > Signed-off-by: Axel Rasmussen <[e-mail]>
>
> Maybe have a cover letter for the next version?
>
> > ---
> >  fs/userfaultfd.c                 | 63 ++++++++++++++++++++++++++++++++
> >  include/linux/swapops.h          |  3 +-
> >  include/linux/userfaultfd_k.h    |  4 ++
> >  include/uapi/linux/userfaultfd.h | 25 +++++++++++--
> >  mm/memory.c                      |  4 ++
> >  mm/userfaultfd.c                 | 62 ++++++++++++++++++++++++++++++-
> >  6 files changed, 156 insertions(+), 5 deletions(-)
> >
> > diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> > index 7cecd49e078b..c26a883399c9 100644
> > --- a/fs/userfaultfd.c
> > +++ b/fs/userfaultfd.c
> > @@ -1965,6 +1965,66 @@ static int userfaultfd_continue(struct userfaultfd_ctx *ctx, unsigned long arg)
> >       return ret;
> >  }
> >
> > +static inline int userfaultfd_poison(struct userfaultfd_ctx *ctx, unsigned long arg)
> > +{
> > +     __s64 ret;
> > +     struct uffdio_poison uffdio_poison;
> > +     struct uffdio_poison __user *user_uffdio_poison;
> > +     struct userfaultfd_wake_range range;
> > +
> > +     user_uffdio_poison = (struct uffdio_poison __user *)arg;
> > +
> > +     ret = -EAGAIN;
> > +     if (atomic_read(&ctx->mmap_changing))
> > +             goto out;
> > +
> > +     ret = -EFAULT;
> > +     if (copy_from_user(&uffdio_poison, user_uffdio_poison,
> > +                        /* don't copy the output fields */
> > +                        sizeof(uffdio_poison) - (sizeof(__s64))))
> > +             goto out;
> > +
> > +     ret = validate_range(ctx->mm, uffdio_poison.range.start,
> > +                          uffdio_poison.range.len);
> > +     if (ret)
> > +             goto out;
> > +
> > +     ret = -EINVAL;
> > +     /* double check for wraparound just in case. */
> > +     if (uffdio_poison.range.start + uffdio_poison.range.len <=
> > +         uffdio_poison.range.start) {
>
> Brackets not needed here.
>
> > +             goto out;
> > +     }
> > +     if (uffdio_poison.mode & ~UFFDIO_POISON_MODE_DONTWAKE)
> > +             goto out;
> > +
> > +     if (mmget_not_zero(ctx->mm)) {
> > +             ret = mfill_atomic_poison(ctx->mm, uffdio_poison.range.start,
> > +                                       uffdio_poison.range.len,
> > +                                       &ctx->mmap_changing, 0);
> > +             mmput(ctx->mm);
> > +     } else {
> > +             return -ESRCH;
> > +     }
> > +
> > +     if (unlikely(put_user(ret, &user_uffdio_poison->updated)))
> > +             return -EFAULT;
> > +     if (ret < 0)
> > +             goto out;
> > +
> > +     /* len == 0 would wake all */
> > +     BUG_ON(!ret);
> > +     range.len = ret;
> > +     if (!(uffdio_poison.mode & UFFDIO_POISON_MODE_DONTWAKE)) {
> > +             range.start = uffdio_poison.range.start;
> > +             wake_userfault(ctx, &range);
> > +     }
> > +     ret = range.len == uffdio_poison.range.len ? 0 : -EAGAIN;
> > +
> > +out:
> > +     return ret;
> > +}
> > +
> >  static inline unsigned int uffd_ctx_features(__u64 user_features)
> >  {
> >       /*
> > @@ -2066,6 +2126,9 @@ static long userfaultfd_ioctl(struct file *file, unsigned cmd,
> >       case UFFDIO_CONTINUE:
> >               ret = userfaultfd_continue(ctx, arg);
> >               break;
> > +     case UFFDIO_POISON:
> > +             ret = userfaultfd_poison(ctx, arg);
> > +             break;
> >       }
> >       return ret;
> >  }
> > diff --git a/include/linux/swapops.h b/include/linux/swapops.h
> > index 4c932cb45e0b..8259fee32421 100644
> > --- a/include/linux/swapops.h
> > +++ b/include/linux/swapops.h
> > @@ -394,7 +394,8 @@ typedef unsigned long pte_marker;
> >
> >  #define  PTE_MARKER_UFFD_WP                  BIT(0)
> >  #define  PTE_MARKER_SWAPIN_ERROR             BIT(1)
> > -#define  PTE_MARKER_MASK                     (BIT(2) - 1)
> > +#define  PTE_MARKER_UFFD_POISON                      BIT(2)
>
> One more tab.
>
> Though I remembered the last time we discussed IIRC we plan to rename
> SWAPIN_ERROR and reuse it, could you explain why a new bit is still needed?
>
> I think I commented this but I'll do it again: IIUC any existing host
> swapin errors for guest pages should be reported as MCE too, afaict,
> happened in kvm context.

I think swapin errors are treated differently than poison. Swapin
errors get VM_FAULT_SIGBUS, and poison gets VM_FAULT_HWPOISON, so
UFFDIO_POISON should also get VM_FAULT_HWPOISON (so that's what Axel
has implemented). And I think that needs a separate PTE marker.

>
> > +#define  PTE_MARKER_MASK                     (BIT(3) - 1)
> >
> >  static inline swp_entry_t make_pte_marker_entry(pte_marker marker)
> >  {
> > diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
> > index ac7b0c96d351..ac8c6854097c 100644
> > --- a/include/linux/userfaultfd_k.h
> > +++ b/include/linux/userfaultfd_k.h
> > @@ -46,6 +46,7 @@ enum mfill_atomic_mode {
> >       MFILL_ATOMIC_COPY,
> >       MFILL_ATOMIC_ZEROPAGE,
> >       MFILL_ATOMIC_CONTINUE,
> > +     MFILL_ATOMIC_POISON,
> >       NR_MFILL_ATOMIC_MODES,
> >  };
> >
> > @@ -83,6 +84,9 @@ extern ssize_t mfill_atomic_zeropage(struct mm_struct *dst_mm,
> >  extern ssize_t mfill_atomic_continue(struct mm_struct *dst_mm, unsigned long dst_start,
> >                                    unsigned long len, atomic_t *mmap_changing,
> >                                    uffd_flags_t flags);
> > +extern ssize_t mfill_atomic_poison(struct mm_struct *dst_mm, unsigned long start,
> > +                                unsigned long len, atomic_t *mmap_changing,
> > +                                uffd_flags_t flags);
> >  extern int mwriteprotect_range(struct mm_struct *dst_mm,
> >                              unsigned long start, unsigned long len,
> >                              bool enable_wp, atomic_t *mmap_changing);
> > diff --git a/include/uapi/linux/userfaultfd.h b/include/uapi/linux/userfaultfd.h
> > index 66dd4cd277bd..62151706c5a3 100644
> > --- a/include/uapi/linux/userfaultfd.h
> > +++ b/include/uapi/linux/userfaultfd.h
> > @@ -39,7 +39,8 @@
> >                          UFFD_FEATURE_MINOR_SHMEM |           \
> >                          UFFD_FEATURE_EXACT_ADDRESS |         \
> >                          UFFD_FEATURE_WP_HUGETLBFS_SHMEM |    \
> > -                        UFFD_FEATURE_WP_UNPOPULATED)
> > +                        UFFD_FEATURE_WP_UNPOPULATED |        \
> > +                        UFFD_FEATURE_POISON)
> >  #define UFFD_API_IOCTLS                              \
> >       ((__u64)1 << _UFFDIO_REGISTER |         \
> >        (__u64)1 << _UFFDIO_UNREGISTER |       \
> > @@ -49,12 +50,14 @@
> >        (__u64)1 << _UFFDIO_COPY |             \
> >        (__u64)1 << _UFFDIO_ZEROPAGE |         \
> >        (__u64)1 << _UFFDIO_WRITEPROTECT |     \
> > -      (__u64)1 << _UFFDIO_CONTINUE)
> > +      (__u64)1 << _UFFDIO_CONTINUE |         \
> > +      (__u64)1 << _UFFDIO_POISON)
> >  #define UFFD_API_RANGE_IOCTLS_BASIC          \
> >       ((__u64)1 << _UFFDIO_WAKE |             \
> >        (__u64)1 << _UFFDIO_COPY |             \
> > +      (__u64)1 << _UFFDIO_WRITEPROTECT |     \
> >        (__u64)1 << _UFFDIO_CONTINUE |         \
> > -      (__u64)1 << _UFFDIO_WRITEPROTECT)
> > +      (__u64)1 << _UFFDIO_POISON)
>
> May not be a large deal, but it's still better to declare the feature &
> ioctls after all things implemented.  Maybe make these few lines
> (UFFD_API*, and the new feature bit) as the last patch to enable the
> feature?

I agree. Another option would be to have a separate feature for
UFFDIO_POISON for hugetlb, but I don't think we should do that. :)

>
> >
> >  /*
> >   * Valid ioctl command number range with this API is from 0x00 to
> > @@ -71,6 +74,7 @@
> >  #define _UFFDIO_ZEROPAGE             (0x04)
> >  #define _UFFDIO_WRITEPROTECT         (0x06)
> >  #define _UFFDIO_CONTINUE             (0x07)
> > +#define _UFFDIO_POISON                       (0x08)
> >  #define _UFFDIO_API                  (0x3F)
> >
> >  /* userfaultfd ioctl ids */
> > @@ -91,6 +95,8 @@
> >                                     struct uffdio_writeprotect)
> >  #define UFFDIO_CONTINUE              _IOWR(UFFDIO, _UFFDIO_CONTINUE, \
> >                                     struct uffdio_continue)
> > +#define UFFDIO_POISON                _IOWR(UFFDIO, _UFFDIO_POISON, \
> > +                                   struct uffdio_poison)
> >
> >  /* read() structure */
> >  struct uffd_msg {
> > @@ -225,6 +231,7 @@ struct uffdio_api {
> >  #define UFFD_FEATURE_EXACT_ADDRESS           (1<<11)
> >  #define UFFD_FEATURE_WP_HUGETLBFS_SHMEM              (1<<12)
> >  #define UFFD_FEATURE_WP_UNPOPULATED          (1<<13)
> > +#define UFFD_FEATURE_POISON                  (1<<14)
> >       __u64 features;
> >
> >       __u64 ioctls;
> > @@ -321,6 +328,18 @@ struct uffdio_continue {
> >       __s64 mapped;
> >  };
> >
> > +struct uffdio_poison {
> > +     struct uffdio_range range;
> > +#define UFFDIO_POISON_MODE_DONTWAKE          ((__u64)1<<0)
> > +     __u64 mode;
> > +
> > +     /*
> > +      * Fields below here are written by the ioctl and must be at the end:
> > +      * the copy_from_user will not read past here.
> > +      */
> > +     __s64 updated;
> > +};
> > +
> >  /*
> >   * Flags for the userfaultfd(2) system call itself.
> >   */
> > diff --git a/mm/memory.c b/mm/memory.c
> > index d8a9a770b1f1..7fbda39e060d 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -3692,6 +3692,10 @@ static vm_fault_t handle_pte_marker(struct vm_fault *vmf)
> >       if (WARN_ON_ONCE(!marker))
> >               return VM_FAULT_SIGBUS;
> >
> > +     /* Poison emulation explicitly requested for this PTE. */
> > +     if (marker & PTE_MARKER_UFFD_POISON)
> > +             return VM_FAULT_HWPOISON;
> > +
> >       /* Higher priority than uffd-wp when data corrupted */
> >       if (marker & PTE_MARKER_SWAPIN_ERROR)
> >               return VM_FAULT_SIGBUS;
> > diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> > index a2bf37ee276d..87b62ca1e09e 100644
> > --- a/mm/userfaultfd.c
> > +++ b/mm/userfaultfd.c
> > @@ -286,6 +286,51 @@ static int mfill_atomic_pte_continue(pmd_t *dst_pmd,
> >       goto out;
> >  }
> >
> > +/* Handles UFFDIO_POISON for all non-hugetlb VMAs. */
> > +static int mfill_atomic_pte_poison(pmd_t *dst_pmd,
> > +                                struct vm_area_struct *dst_vma,
> > +                                unsigned long dst_addr,
> > +                                uffd_flags_t flags)
> > +{
> > +     int ret;
> > +     struct mm_struct *dst_mm = dst_vma->vm_mm;
> > +     pte_t _dst_pte, *dst_pte;
> > +     spinlock_t *ptl;
> > +
> > +     _dst_pte = make_pte_marker(PTE_MARKER_UFFD_POISON);
> > +     dst_pte = pte_offset_map_lock(dst_mm, dst_pmd, dst_addr, &ptl);
> > +
> > +     if (vma_is_shmem(dst_vma)) {
> > +             struct inode *inode;
> > +             pgoff_t offset, max_off;
> > +
> > +             /* serialize against truncate with the page table lock */
> > +             inode = dst_vma->vm_file->f_inode;
> > +             offset = linear_page_index(dst_vma, dst_addr);
> > +             max_off = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
> > +             ret = -EFAULT;
> > +             if (unlikely(offset >= max_off))
> > +                     goto out_unlock;
> > +     }
>
> Maybe good chance to have a mfill_file_over_size() helper?  Other potential
> users are mfill_atomic_pte_zeropage() and mfill_atomic_install_pte().
>
> > +
> > +     ret = -EEXIST;
> > +     /*
> > +      * For now, we don't handle unmapping pages, so only support filling in
> > +      * none PTEs, or replacing PTE markers.
> > +      */
> > +     if (!pte_none_mostly(*dst_pte))
> > +             goto out_unlock;
> > +
> > +     set_pte_at(dst_mm, dst_addr, dst_pte, _dst_pte);
> > +
> > +     /* No need to invalidate - it was non-present before */
> > +     update_mmu_cache(dst_vma, dst_addr, dst_pte);
> > +     ret = 0;
> > +out_unlock:
> > +     pte_unmap_unlock(dst_pte, ptl);
> > +     return ret;
> > +}
> > +
> >  static pmd_t *mm_alloc_pmd(struct mm_struct *mm, unsigned long address)
> >  {
> >       pgd_t *pgd;
> > @@ -336,8 +381,12 @@ static __always_inline ssize_t mfill_atomic_hugetlb(
> >        * supported by hugetlb.  A PMD_SIZE huge pages may exist as used
> >        * by THP.  Since we can not reliably insert a zero page, this
> >        * feature is not supported.
> > +      *
> > +      * PTE marker handling for hugetlb is a bit special, so for now
> > +      * UFFDIO_POISON is not supported.
> >        */
> > -     if (uffd_flags_mode_is(flags, MFILL_ATOMIC_ZEROPAGE)) {
> > +     if (uffd_flags_mode_is(flags, MFILL_ATOMIC_ZEROPAGE) ||
> > +         uffd_flags_mode_is(flags, MFILL_ATOMIC_POISON)) {
> >               mmap_read_unlock(dst_mm);
> >               return -EINVAL;
> >       }
> > @@ -481,6 +530,9 @@ static __always_inline ssize_t mfill_atomic_pte(pmd_t *dst_pmd,
> >       if (uffd_flags_mode_is(flags, MFILL_ATOMIC_CONTINUE)) {
> >               return mfill_atomic_pte_continue(dst_pmd, dst_vma,
> >                                                dst_addr, flags);
> > +     } else if (uffd_flags_mode_is(flags, MFILL_ATOMIC_POISON)) {
> > +             return mfill_atomic_pte_poison(dst_pmd, dst_vma,
> > +                                            dst_addr, flags);
> >       }
> >
> >       /*
> > @@ -702,6 +754,14 @@ ssize_t mfill_atomic_continue(struct mm_struct *dst_mm, unsigned long start,
> >                           uffd_flags_set_mode(flags, MFILL_ATOMIC_CONTINUE));
> >  }
> >
> > +ssize_t mfill_atomic_poison(struct mm_struct *dst_mm, unsigned long start,
> > +                         unsigned long len, atomic_t *mmap_changing,
> > +                         uffd_flags_t flags)
> > +{
> > +     return mfill_atomic(dst_mm, start, 0, len, mmap_changing,
> > +                         uffd_flags_set_mode(flags, MFILL_ATOMIC_POISON));
> > +}
> > +
> >  long uffd_wp_range(struct vm_area_struct *dst_vma,
> >                  unsigned long start, unsigned long len, bool enable_wp)
> >  {
> > --
> > 2.41.0.255.g8b1d071c50-goog
> >
>
> --
> Peter Xu
>

Peter Xu July 5, 2023, 4:15 p.m. UTC | #4

On Wed, Jul 05, 2023 at 09:09:19AM -0700, James Houghton wrote:
> > > diff --git a/include/linux/swapops.h b/include/linux/swapops.h
> > > index 4c932cb45e0b..8259fee32421 100644
> > > --- a/include/linux/swapops.h
> > > +++ b/include/linux/swapops.h
> > > @@ -394,7 +394,8 @@ typedef unsigned long pte_marker;
> > >
> > >  #define  PTE_MARKER_UFFD_WP                  BIT(0)
> > >  #define  PTE_MARKER_SWAPIN_ERROR             BIT(1)
> > > -#define  PTE_MARKER_MASK                     (BIT(2) - 1)
> > > +#define  PTE_MARKER_UFFD_POISON                      BIT(2)
> >
> > One more tab.
> >
> > Though I remembered the last time we discussed IIRC we plan to rename
> > SWAPIN_ERROR and reuse it, could you explain why a new bit is still needed?
> >
> > I think I commented this but I'll do it again: IIUC any existing host
> > swapin errors for guest pages should be reported as MCE too, afaict,
> > happened in kvm context.
> 
> I think swapin errors are treated differently than poison. Swapin
> errors get VM_FAULT_SIGBUS, and poison gets VM_FAULT_HWPOISON, so
> UFFDIO_POISON should also get VM_FAULT_HWPOISON (so that's what Axel
> has implemented). And I think that needs a separate PTE marker.

My question was, should we also make SWAPIN_ERROR return VM_FAULT_HWPOISON
always?

Just to recap from what I already commented above - if a guest page got
error in swapin due to block sector failures, it should be treated as
VM_FAULT_HWPOISON too, IMHO.  IOW, I think current SWAPIN_ERROR is wrong
when in kvm context and we should fix it first.

> 
> >
> > > +#define  PTE_MARKER_MASK                     (BIT(3) - 1)
> > >
> > >  static inline swp_entry_t make_pte_marker_entry(pte_marker marker)
> > >  {
> > > diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
> > > index ac7b0c96d351..ac8c6854097c 100644
> > > --- a/include/linux/userfaultfd_k.h
> > > +++ b/include/linux/userfaultfd_k.h
> > > @@ -46,6 +46,7 @@ enum mfill_atomic_mode {
> > >       MFILL_ATOMIC_COPY,
> > >       MFILL_ATOMIC_ZEROPAGE,
> > >       MFILL_ATOMIC_CONTINUE,
> > > +     MFILL_ATOMIC_POISON,
> > >       NR_MFILL_ATOMIC_MODES,
> > >  };
> > >
> > > @@ -83,6 +84,9 @@ extern ssize_t mfill_atomic_zeropage(struct mm_struct *dst_mm,
> > >  extern ssize_t mfill_atomic_continue(struct mm_struct *dst_mm, unsigned long dst_start,
> > >                                    unsigned long len, atomic_t *mmap_changing,
> > >                                    uffd_flags_t flags);
> > > +extern ssize_t mfill_atomic_poison(struct mm_struct *dst_mm, unsigned long start,
> > > +                                unsigned long len, atomic_t *mmap_changing,
> > > +                                uffd_flags_t flags);
> > >  extern int mwriteprotect_range(struct mm_struct *dst_mm,
> > >                              unsigned long start, unsigned long len,
> > >                              bool enable_wp, atomic_t *mmap_changing);
> > > diff --git a/include/uapi/linux/userfaultfd.h b/include/uapi/linux/userfaultfd.h
> > > index 66dd4cd277bd..62151706c5a3 100644
> > > --- a/include/uapi/linux/userfaultfd.h
> > > +++ b/include/uapi/linux/userfaultfd.h
> > > @@ -39,7 +39,8 @@
> > >                          UFFD_FEATURE_MINOR_SHMEM |           \
> > >                          UFFD_FEATURE_EXACT_ADDRESS |         \
> > >                          UFFD_FEATURE_WP_HUGETLBFS_SHMEM |    \
> > > -                        UFFD_FEATURE_WP_UNPOPULATED)
> > > +                        UFFD_FEATURE_WP_UNPOPULATED |        \
> > > +                        UFFD_FEATURE_POISON)
> > >  #define UFFD_API_IOCTLS                              \
> > >       ((__u64)1 << _UFFDIO_REGISTER |         \
> > >        (__u64)1 << _UFFDIO_UNREGISTER |       \
> > > @@ -49,12 +50,14 @@
> > >        (__u64)1 << _UFFDIO_COPY |             \
> > >        (__u64)1 << _UFFDIO_ZEROPAGE |         \
> > >        (__u64)1 << _UFFDIO_WRITEPROTECT |     \
> > > -      (__u64)1 << _UFFDIO_CONTINUE)
> > > +      (__u64)1 << _UFFDIO_CONTINUE |         \
> > > +      (__u64)1 << _UFFDIO_POISON)
> > >  #define UFFD_API_RANGE_IOCTLS_BASIC          \
> > >       ((__u64)1 << _UFFDIO_WAKE |             \
> > >        (__u64)1 << _UFFDIO_COPY |             \
> > > +      (__u64)1 << _UFFDIO_WRITEPROTECT |     \
> > >        (__u64)1 << _UFFDIO_CONTINUE |         \
> > > -      (__u64)1 << _UFFDIO_WRITEPROTECT)
> > > +      (__u64)1 << _UFFDIO_POISON)
> >
> > May not be a large deal, but it's still better to declare the feature &
> > ioctls after all things implemented.  Maybe make these few lines
> > (UFFD_API*, and the new feature bit) as the last patch to enable the
> > feature?
> 
> I agree. Another option would be to have a separate feature for
> UFFDIO_POISON for hugetlb, but I don't think we should do that. :)

Yeah let's make the features "memory-type-free" if possible. :)

Thanks,

James Houghton July 5, 2023, 4:27 p.m. UTC | #5

On Wed, Jul 5, 2023 at 9:15 AM Peter Xu <[e-mail]> wrote:
>
> On Wed, Jul 05, 2023 at 09:09:19AM -0700, James Houghton wrote:
> > > > diff --git a/include/linux/swapops.h b/include/linux/swapops.h
> > > > index 4c932cb45e0b..8259fee32421 100644
> > > > --- a/include/linux/swapops.h
> > > > +++ b/include/linux/swapops.h
> > > > @@ -394,7 +394,8 @@ typedef unsigned long pte_marker;
> > > >
> > > >  #define  PTE_MARKER_UFFD_WP                  BIT(0)
> > > >  #define  PTE_MARKER_SWAPIN_ERROR             BIT(1)
> > > > -#define  PTE_MARKER_MASK                     (BIT(2) - 1)
> > > > +#define  PTE_MARKER_UFFD_POISON                      BIT(2)
> > >
> > > One more tab.
> > >
> > > Though I remembered the last time we discussed IIRC we plan to rename
> > > SWAPIN_ERROR and reuse it, could you explain why a new bit is still needed?
> > >
> > > I think I commented this but I'll do it again: IIUC any existing host
> > > swapin errors for guest pages should be reported as MCE too, afaict,
> > > happened in kvm context.
> >
> > I think swapin errors are treated differently than poison. Swapin
> > errors get VM_FAULT_SIGBUS, and poison gets VM_FAULT_HWPOISON, so
> > UFFDIO_POISON should also get VM_FAULT_HWPOISON (so that's what Axel
> > has implemented). And I think that needs a separate PTE marker.
>
> My question was, should we also make SWAPIN_ERROR return VM_FAULT_HWPOISON
> always?
>
> Just to recap from what I already commented above - if a guest page got
> error in swapin due to block sector failures, it should be treated as
> VM_FAULT_HWPOISON too, IMHO.  IOW, I think current SWAPIN_ERROR is wrong
> when in kvm context and we should fix it first.

Oh! Yes, I agree, though I'm not familiar enough with the users of
SWAPIN_ERROR to know if we can actually make this change.

Peter Xu July 5, 2023, 4:38 p.m. UTC | #6

On Wed, Jul 05, 2023 at 09:27:15AM -0700, James Houghton wrote:
> On Wed, Jul 5, 2023 at 9:15 AM Peter Xu <[e-mail]> wrote:
> >
> > On Wed, Jul 05, 2023 at 09:09:19AM -0700, James Houghton wrote:
> > > > > diff --git a/include/linux/swapops.h b/include/linux/swapops.h
> > > > > index 4c932cb45e0b..8259fee32421 100644
> > > > > --- a/include/linux/swapops.h
> > > > > +++ b/include/linux/swapops.h
> > > > > @@ -394,7 +394,8 @@ typedef unsigned long pte_marker;
> > > > >
> > > > >  #define  PTE_MARKER_UFFD_WP                  BIT(0)
> > > > >  #define  PTE_MARKER_SWAPIN_ERROR             BIT(1)
> > > > > -#define  PTE_MARKER_MASK                     (BIT(2) - 1)
> > > > > +#define  PTE_MARKER_UFFD_POISON                      BIT(2)
> > > >
> > > > One more tab.
> > > >
> > > > Though I remembered the last time we discussed IIRC we plan to rename
> > > > SWAPIN_ERROR and reuse it, could you explain why a new bit is still needed?
> > > >
> > > > I think I commented this but I'll do it again: IIUC any existing host
> > > > swapin errors for guest pages should be reported as MCE too, afaict,
> > > > happened in kvm context.
> > >
> > > I think swapin errors are treated differently than poison. Swapin
> > > errors get VM_FAULT_SIGBUS, and poison gets VM_FAULT_HWPOISON, so
> > > UFFDIO_POISON should also get VM_FAULT_HWPOISON (so that's what Axel
> > > has implemented). And I think that needs a separate PTE marker.
> >
> > My question was, should we also make SWAPIN_ERROR return VM_FAULT_HWPOISON
> > always?
> >
> > Just to recap from what I already commented above - if a guest page got
> > error in swapin due to block sector failures, it should be treated as
> > VM_FAULT_HWPOISON too, IMHO.  IOW, I think current SWAPIN_ERROR is wrong
> > when in kvm context and we should fix it first.
> 
> Oh! Yes, I agree, though I'm not familiar enough with the users of
> SWAPIN_ERROR to know if we can actually make this change.

Miaohe initially proposed this swapin error facility, let's see whether he
can comment; he's already in the cc list.

AFAICT that's the right thing to do, and it shouldn't affect any existing
user of swapin error if there is.

Or say, VM_FAULT_HWPOISON should be the same as VM_FAULT_SIGBUS when not in
kvm context, so shouldn't change a thing in !kvm, while changing that
should fix kvm from crashing a guest where we shouldn't need to.

Axel Rasmussen July 5, 2023, 5:56 p.m. UTC | #7

On Wed, Jul 5, 2023 at 9:38 AM Peter Xu <[e-mail]> wrote:
>
> On Wed, Jul 05, 2023 at 09:27:15AM -0700, James Houghton wrote:
> > On Wed, Jul 5, 2023 at 9:15 AM Peter Xu <[e-mail]> wrote:
> > >
> > > On Wed, Jul 05, 2023 at 09:09:19AM -0700, James Houghton wrote:
> > > > > > diff --git a/include/linux/swapops.h b/include/linux/swapops.h
> > > > > > index 4c932cb45e0b..8259fee32421 100644
> > > > > > --- a/include/linux/swapops.h
> > > > > > +++ b/include/linux/swapops.h
> > > > > > @@ -394,7 +394,8 @@ typedef unsigned long pte_marker;
> > > > > >
> > > > > >  #define  PTE_MARKER_UFFD_WP                  BIT(0)
> > > > > >  #define  PTE_MARKER_SWAPIN_ERROR             BIT(1)
> > > > > > -#define  PTE_MARKER_MASK                     (BIT(2) - 1)
> > > > > > +#define  PTE_MARKER_UFFD_POISON                      BIT(2)
> > > > >
> > > > > One more tab.
> > > > >
> > > > > Though I remembered the last time we discussed IIRC we plan to rename
> > > > > SWAPIN_ERROR and reuse it, could you explain why a new bit is still needed?
> > > > >
> > > > > I think I commented this but I'll do it again: IIUC any existing host
> > > > > swapin errors for guest pages should be reported as MCE too, afaict,
> > > > > happened in kvm context.
> > > >
> > > > I think swapin errors are treated differently than poison. Swapin
> > > > errors get VM_FAULT_SIGBUS, and poison gets VM_FAULT_HWPOISON, so
> > > > UFFDIO_POISON should also get VM_FAULT_HWPOISON (so that's what Axel
> > > > has implemented). And I think that needs a separate PTE marker.
> > >
> > > My question was, should we also make SWAPIN_ERROR return VM_FAULT_HWPOISON
> > > always?
> > >
> > > Just to recap from what I already commented above - if a guest page got
> > > error in swapin due to block sector failures, it should be treated as
> > > VM_FAULT_HWPOISON too, IMHO.  IOW, I think current SWAPIN_ERROR is wrong
> > > when in kvm context and we should fix it first.
> >
> > Oh! Yes, I agree, though I'm not familiar enough with the users of
> > SWAPIN_ERROR to know if we can actually make this change.

Sorry I missed this, I'll take another pass looking at existing
SWAPIN_ERROR uses, and see if this can be done.

Thanks for the thorough review Peter, I'll address this comment and
the others in a v3. :)

>
> Miaohe initially proposed this swapin error facility, let's see whether he
> can comment; he's already in the cc list.
>
> AFAICT that's the right thing to do, and it shouldn't affect any existing
> user of swapin error if there is.
>
> Or say, VM_FAULT_HWPOISON should be the same as VM_FAULT_SIGBUS when not in
> kvm context, so shouldn't change a thing in !kvm, while changing that
> should fix kvm from crashing a guest where we shouldn't need to.
>
> --
> Peter Xu
>

Jiaqi Yan July 5, 2023, 6:17 p.m. UTC | #8

On Thu, Jun 29, 2023 at 1:50 PM Axel Rasmussen <[e-mail]> wrote:
>
> The basic idea here is to "simulate" memory poisoning for VMs. A VM
> running on some host might encounter a memory error, after which some
> page(s) are poisoned (i.e., future accesses SIGBUS). They expect that
> once poisoned, pages can never become "un-poisoned". So, when we live
> migrate the VM, we need to preserve the poisoned status of these pages.
>
> When live migrating, we try to get the guest running on its new host as
> quickly as possible. So, we start it running before all memory has been
> copied, and before we're certain which pages should be poisoned or not.
>
> So the basic way to use this new feature is:
>
> - On the new host, the guest's memory is registered with userfaultfd, in
>   either MISSING or MINOR mode (doesn't really matter for this purpose).
> - On any first access, we get a userfaultfd event. At this point we can
>   communicate with the old host to find out if the page was poisoned.
> - If so, we can respond with a UFFDIO_POISON - this places a swap marker
>   so any future accesses will SIGBUS. Because the pte is now "present",
>   future accesses won't generate more userfaultfd events, they'll just
>   SIGBUS directly.
>
> UFFDIO_POISON does not handle unmapping previously-present PTEs. This

A minor suggestion, would UFFDIO_HWPOISON be better? so that readers
won't be confused with CONFIG_PAGE_POISONING (a feature to fill the
pages with poison patterns after free).

> isn't needed, because during live migration we want to intercept
> all accesses with userfaultfd (not just writes, so WP mode isn't useful
> for this). So whether minor or missing mode is being used (or both), the
> PTE won't be present in any case, so handling that case isn't needed.
>
> Why return VM_FAULT_HWPOISON instead of VM_FAULT_SIGBUS when one of
> these markers is encountered? For "normal" userspace programs there
> isn't a big difference, both yield a SIGBUS. The difference for KVM is
> key though: VM_FAULT_HWPOISON will result in an MCE being injected into
> the guest (which is the behavior we want). With VM_FAULT_SIGBUS, the
> hypervisor would need to catch the SIGBUS and deal with the MCE
> injection itself.
>
> Signed-off-by: Axel Rasmussen <[e-mail]>
> ---
>  fs/userfaultfd.c                 | 63 ++++++++++++++++++++++++++++++++
>  include/linux/swapops.h          |  3 +-
>  include/linux/userfaultfd_k.h    |  4 ++
>  include/uapi/linux/userfaultfd.h | 25 +++++++++++--
>  mm/memory.c                      |  4 ++
>  mm/userfaultfd.c                 | 62 ++++++++++++++++++++++++++++++-
>  6 files changed, 156 insertions(+), 5 deletions(-)
>
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index 7cecd49e078b..c26a883399c9 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -1965,6 +1965,66 @@ static int userfaultfd_continue(struct userfaultfd_ctx *ctx, unsigned long arg)
>         return ret;
>  }
>
> +static inline int userfaultfd_poison(struct userfaultfd_ctx *ctx, unsigned long arg)
> +{
> +       __s64 ret;
> +       struct uffdio_poison uffdio_poison;
> +       struct uffdio_poison __user *user_uffdio_poison;
> +       struct userfaultfd_wake_range range;
> +
> +       user_uffdio_poison = (struct uffdio_poison __user *)arg;
> +
> +       ret = -EAGAIN;
> +       if (atomic_read(&ctx->mmap_changing))
> +               goto out;
> +
> +       ret = -EFAULT;
> +       if (copy_from_user(&uffdio_poison, user_uffdio_poison,
> +                          /* don't copy the output fields */
> +                          sizeof(uffdio_poison) - (sizeof(__s64))))
> +               goto out;
> +
> +       ret = validate_range(ctx->mm, uffdio_poison.range.start,
> +                            uffdio_poison.range.len);
> +       if (ret)
> +               goto out;
> +
> +       ret = -EINVAL;
> +       /* double check for wraparound just in case. */
> +       if (uffdio_poison.range.start + uffdio_poison.range.len <=
> +           uffdio_poison.range.start) {
> +               goto out;
> +       }
> +       if (uffdio_poison.mode & ~UFFDIO_POISON_MODE_DONTWAKE)
> +               goto out;
> +
> +       if (mmget_not_zero(ctx->mm)) {
> +               ret = mfill_atomic_poison(ctx->mm, uffdio_poison.range.start,
> +                                         uffdio_poison.range.len,
> +                                         &ctx->mmap_changing, 0);
> +               mmput(ctx->mm);
> +       } else {
> +               return -ESRCH;
> +       }
> +
> +       if (unlikely(put_user(ret, &user_uffdio_poison->updated)))
> +               return -EFAULT;
> +       if (ret < 0)
> +               goto out;
> +
> +       /* len == 0 would wake all */
> +       BUG_ON(!ret);
> +       range.len = ret;
> +       if (!(uffdio_poison.mode & UFFDIO_POISON_MODE_DONTWAKE)) {
> +               range.start = uffdio_poison.range.start;
> +               wake_userfault(ctx, &range);
> +       }
> +       ret = range.len == uffdio_poison.range.len ? 0 : -EAGAIN;
> +
> +out:
> +       return ret;
> +}
> +
>  static inline unsigned int uffd_ctx_features(__u64 user_features)
>  {
>         /*
> @@ -2066,6 +2126,9 @@ static long userfaultfd_ioctl(struct file *file, unsigned cmd,
>         case UFFDIO_CONTINUE:
>                 ret = userfaultfd_continue(ctx, arg);
>                 break;
> +       case UFFDIO_POISON:
> +               ret = userfaultfd_poison(ctx, arg);
> +               break;
>         }
>         return ret;
>  }
> diff --git a/include/linux/swapops.h b/include/linux/swapops.h
> index 4c932cb45e0b..8259fee32421 100644
> --- a/include/linux/swapops.h
> +++ b/include/linux/swapops.h
> @@ -394,7 +394,8 @@ typedef unsigned long pte_marker;
>
>  #define  PTE_MARKER_UFFD_WP                    BIT(0)
>  #define  PTE_MARKER_SWAPIN_ERROR               BIT(1)
> -#define  PTE_MARKER_MASK                       (BIT(2) - 1)
> +#define  PTE_MARKER_UFFD_POISON                        BIT(2)
> +#define  PTE_MARKER_MASK                       (BIT(3) - 1)
>
>  static inline swp_entry_t make_pte_marker_entry(pte_marker marker)
>  {
> diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
> index ac7b0c96d351..ac8c6854097c 100644
> --- a/include/linux/userfaultfd_k.h
> +++ b/include/linux/userfaultfd_k.h
> @@ -46,6 +46,7 @@ enum mfill_atomic_mode {
>         MFILL_ATOMIC_COPY,
>         MFILL_ATOMIC_ZEROPAGE,
>         MFILL_ATOMIC_CONTINUE,
> +       MFILL_ATOMIC_POISON,
>         NR_MFILL_ATOMIC_MODES,
>  };
>
> @@ -83,6 +84,9 @@ extern ssize_t mfill_atomic_zeropage(struct mm_struct *dst_mm,
>  extern ssize_t mfill_atomic_continue(struct mm_struct *dst_mm, unsigned long dst_start,
>                                      unsigned long len, atomic_t *mmap_changing,
>                                      uffd_flags_t flags);
> +extern ssize_t mfill_atomic_poison(struct mm_struct *dst_mm, unsigned long start,
> +                                  unsigned long len, atomic_t *mmap_changing,
> +                                  uffd_flags_t flags);
>  extern int mwriteprotect_range(struct mm_struct *dst_mm,
>                                unsigned long start, unsigned long len,
>                                bool enable_wp, atomic_t *mmap_changing);
> diff --git a/include/uapi/linux/userfaultfd.h b/include/uapi/linux/userfaultfd.h
> index 66dd4cd277bd..62151706c5a3 100644
> --- a/include/uapi/linux/userfaultfd.h
> +++ b/include/uapi/linux/userfaultfd.h
> @@ -39,7 +39,8 @@
>                            UFFD_FEATURE_MINOR_SHMEM |           \
>                            UFFD_FEATURE_EXACT_ADDRESS |         \
>                            UFFD_FEATURE_WP_HUGETLBFS_SHMEM |    \
> -                          UFFD_FEATURE_WP_UNPOPULATED)
> +                          UFFD_FEATURE_WP_UNPOPULATED |        \
> +                          UFFD_FEATURE_POISON)
>  #define UFFD_API_IOCTLS                                \
>         ((__u64)1 << _UFFDIO_REGISTER |         \
>          (__u64)1 << _UFFDIO_UNREGISTER |       \
> @@ -49,12 +50,14 @@
>          (__u64)1 << _UFFDIO_COPY |             \
>          (__u64)1 << _UFFDIO_ZEROPAGE |         \
>          (__u64)1 << _UFFDIO_WRITEPROTECT |     \
> -        (__u64)1 << _UFFDIO_CONTINUE)
> +        (__u64)1 << _UFFDIO_CONTINUE |         \
> +        (__u64)1 << _UFFDIO_POISON)
>  #define UFFD_API_RANGE_IOCTLS_BASIC            \
>         ((__u64)1 << _UFFDIO_WAKE |             \
>          (__u64)1 << _UFFDIO_COPY |             \
> +        (__u64)1 << _UFFDIO_WRITEPROTECT |     \
>          (__u64)1 << _UFFDIO_CONTINUE |         \
> -        (__u64)1 << _UFFDIO_WRITEPROTECT)
> +        (__u64)1 << _UFFDIO_POISON)
>
>  /*
>   * Valid ioctl command number range with this API is from 0x00 to
> @@ -71,6 +74,7 @@
>  #define _UFFDIO_ZEROPAGE               (0x04)
>  #define _UFFDIO_WRITEPROTECT           (0x06)
>  #define _UFFDIO_CONTINUE               (0x07)
> +#define _UFFDIO_POISON                 (0x08)
>  #define _UFFDIO_API                    (0x3F)
>
>  /* userfaultfd ioctl ids */
> @@ -91,6 +95,8 @@
>                                       struct uffdio_writeprotect)
>  #define UFFDIO_CONTINUE                _IOWR(UFFDIO, _UFFDIO_CONTINUE, \
>                                       struct uffdio_continue)
> +#define UFFDIO_POISON          _IOWR(UFFDIO, _UFFDIO_POISON, \
> +                                     struct uffdio_poison)
>
>  /* read() structure */
>  struct uffd_msg {
> @@ -225,6 +231,7 @@ struct uffdio_api {
>  #define UFFD_FEATURE_EXACT_ADDRESS             (1<<11)
>  #define UFFD_FEATURE_WP_HUGETLBFS_SHMEM                (1<<12)
>  #define UFFD_FEATURE_WP_UNPOPULATED            (1<<13)
> +#define UFFD_FEATURE_POISON                    (1<<14)
>         __u64 features;
>
>         __u64 ioctls;
> @@ -321,6 +328,18 @@ struct uffdio_continue {
>         __s64 mapped;
>  };
>
> +struct uffdio_poison {
> +       struct uffdio_range range;
> +#define UFFDIO_POISON_MODE_DONTWAKE            ((__u64)1<<0)
> +       __u64 mode;
> +
> +       /*
> +        * Fields below here are written by the ioctl and must be at the end:
> +        * the copy_from_user will not read past here.
> +        */
> +       __s64 updated;
> +};
> +
>  /*
>   * Flags for the userfaultfd(2) system call itself.
>   */
> diff --git a/mm/memory.c b/mm/memory.c
> index d8a9a770b1f1..7fbda39e060d 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3692,6 +3692,10 @@ static vm_fault_t handle_pte_marker(struct vm_fault *vmf)
>         if (WARN_ON_ONCE(!marker))
>                 return VM_FAULT_SIGBUS;
>
> +       /* Poison emulation explicitly requested for this PTE. */
> +       if (marker & PTE_MARKER_UFFD_POISON)
> +               return VM_FAULT_HWPOISON;
> +
>         /* Higher priority than uffd-wp when data corrupted */
>         if (marker & PTE_MARKER_SWAPIN_ERROR)
>                 return VM_FAULT_SIGBUS;
> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index a2bf37ee276d..87b62ca1e09e 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -286,6 +286,51 @@ static int mfill_atomic_pte_continue(pmd_t *dst_pmd,
>         goto out;
>  }
>
> +/* Handles UFFDIO_POISON for all non-hugetlb VMAs. */
> +static int mfill_atomic_pte_poison(pmd_t *dst_pmd,
> +                                  struct vm_area_struct *dst_vma,
> +                                  unsigned long dst_addr,
> +                                  uffd_flags_t flags)
> +{
> +       int ret;
> +       struct mm_struct *dst_mm = dst_vma->vm_mm;
> +       pte_t _dst_pte, *dst_pte;
> +       spinlock_t *ptl;
> +
> +       _dst_pte = make_pte_marker(PTE_MARKER_UFFD_POISON);
> +       dst_pte = pte_offset_map_lock(dst_mm, dst_pmd, dst_addr, &ptl);
> +
> +       if (vma_is_shmem(dst_vma)) {
> +               struct inode *inode;
> +               pgoff_t offset, max_off;
> +
> +               /* serialize against truncate with the page table lock */
> +               inode = dst_vma->vm_file->f_inode;
> +               offset = linear_page_index(dst_vma, dst_addr);
> +               max_off = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
> +               ret = -EFAULT;
> +               if (unlikely(offset >= max_off))
> +                       goto out_unlock;
> +       }
> +
> +       ret = -EEXIST;
> +       /*
> +        * For now, we don't handle unmapping pages, so only support filling in
> +        * none PTEs, or replacing PTE markers.
> +        */
> +       if (!pte_none_mostly(*dst_pte))
> +               goto out_unlock;
> +
> +       set_pte_at(dst_mm, dst_addr, dst_pte, _dst_pte);
> +
> +       /* No need to invalidate - it was non-present before */
> +       update_mmu_cache(dst_vma, dst_addr, dst_pte);
> +       ret = 0;
> +out_unlock:
> +       pte_unmap_unlock(dst_pte, ptl);
> +       return ret;
> +}
> +
>  static pmd_t *mm_alloc_pmd(struct mm_struct *mm, unsigned long address)
>  {
>         pgd_t *pgd;
> @@ -336,8 +381,12 @@ static __always_inline ssize_t mfill_atomic_hugetlb(
>          * supported by hugetlb.  A PMD_SIZE huge pages may exist as used
>          * by THP.  Since we can not reliably insert a zero page, this
>          * feature is not supported.
> +        *
> +        * PTE marker handling for hugetlb is a bit special, so for now
> +        * UFFDIO_POISON is not supported.
>          */
> -       if (uffd_flags_mode_is(flags, MFILL_ATOMIC_ZEROPAGE)) {
> +       if (uffd_flags_mode_is(flags, MFILL_ATOMIC_ZEROPAGE) ||
> +           uffd_flags_mode_is(flags, MFILL_ATOMIC_POISON)) {
>                 mmap_read_unlock(dst_mm);
>                 return -EINVAL;
>         }
> @@ -481,6 +530,9 @@ static __always_inline ssize_t mfill_atomic_pte(pmd_t *dst_pmd,
>         if (uffd_flags_mode_is(flags, MFILL_ATOMIC_CONTINUE)) {
>                 return mfill_atomic_pte_continue(dst_pmd, dst_vma,
>                                                  dst_addr, flags);
> +       } else if (uffd_flags_mode_is(flags, MFILL_ATOMIC_POISON)) {
> +               return mfill_atomic_pte_poison(dst_pmd, dst_vma,
> +                                              dst_addr, flags);
>         }
>
>         /*
> @@ -702,6 +754,14 @@ ssize_t mfill_atomic_continue(struct mm_struct *dst_mm, unsigned long start,
>                             uffd_flags_set_mode(flags, MFILL_ATOMIC_CONTINUE));
>  }
>
> +ssize_t mfill_atomic_poison(struct mm_struct *dst_mm, unsigned long start,
> +                           unsigned long len, atomic_t *mmap_changing,
> +                           uffd_flags_t flags)
> +{
> +       return mfill_atomic(dst_mm, start, 0, len, mmap_changing,
> +                           uffd_flags_set_mode(flags, MFILL_ATOMIC_POISON));
> +}
> +
>  long uffd_wp_range(struct vm_area_struct *dst_vma,
>                    unsigned long start, unsigned long len, bool enable_wp)
>  {
> --
> 2.41.0.255.g8b1d071c50-goog
>

Axel Rasmussen July 5, 2023, 6:23 p.m. UTC | #9

On Wed, Jul 5, 2023 at 11:18 AM Jiaqi Yan <[e-mail]> wrote:
>
> On Thu, Jun 29, 2023 at 1:50 PM Axel Rasmussen <[e-mail]> wrote:
> >
> > The basic idea here is to "simulate" memory poisoning for VMs. A VM
> > running on some host might encounter a memory error, after which some
> > page(s) are poisoned (i.e., future accesses SIGBUS). They expect that
> > once poisoned, pages can never become "un-poisoned". So, when we live
> > migrate the VM, we need to preserve the poisoned status of these pages.
> >
> > When live migrating, we try to get the guest running on its new host as
> > quickly as possible. So, we start it running before all memory has been
> > copied, and before we're certain which pages should be poisoned or not.
> >
> > So the basic way to use this new feature is:
> >
> > - On the new host, the guest's memory is registered with userfaultfd, in
> >   either MISSING or MINOR mode (doesn't really matter for this purpose).
> > - On any first access, we get a userfaultfd event. At this point we can
> >   communicate with the old host to find out if the page was poisoned.
> > - If so, we can respond with a UFFDIO_POISON - this places a swap marker
> >   so any future accesses will SIGBUS. Because the pte is now "present",
> >   future accesses won't generate more userfaultfd events, they'll just
> >   SIGBUS directly.
> >
> > UFFDIO_POISON does not handle unmapping previously-present PTEs. This
>
> A minor suggestion, would UFFDIO_HWPOISON be better? so that readers
> won't be confused with CONFIG_PAGE_POISONING (a feature to fill the
> pages with poison patterns after free).

I was a bit wary of using "HWPOISON" because this is more like
"simulated" poisoning, not "real" hardware poisoning. :/

But, maybe folks feel that distinction is less important than the one
you've raised here Jiaqi. I don't have the strongest opinion.

>
> > isn't needed, because during live migration we want to intercept
> > all accesses with userfaultfd (not just writes, so WP mode isn't useful
> > for this). So whether minor or missing mode is being used (or both), the
> > PTE won't be present in any case, so handling that case isn't needed.
> >
> > Why return VM_FAULT_HWPOISON instead of VM_FAULT_SIGBUS when one of
> > these markers is encountered? For "normal" userspace programs there
> > isn't a big difference, both yield a SIGBUS. The difference for KVM is
> > key though: VM_FAULT_HWPOISON will result in an MCE being injected into
> > the guest (which is the behavior we want). With VM_FAULT_SIGBUS, the
> > hypervisor would need to catch the SIGBUS and deal with the MCE
> > injection itself.
> >
> > Signed-off-by: Axel Rasmussen <[e-mail]>
> > ---
> >  fs/userfaultfd.c                 | 63 ++++++++++++++++++++++++++++++++
> >  include/linux/swapops.h          |  3 +-
> >  include/linux/userfaultfd_k.h    |  4 ++
> >  include/uapi/linux/userfaultfd.h | 25 +++++++++++--
> >  mm/memory.c                      |  4 ++
> >  mm/userfaultfd.c                 | 62 ++++++++++++++++++++++++++++++-
> >  6 files changed, 156 insertions(+), 5 deletions(-)
> >
> > diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> > index 7cecd49e078b..c26a883399c9 100644
> > --- a/fs/userfaultfd.c
> > +++ b/fs/userfaultfd.c
> > @@ -1965,6 +1965,66 @@ static int userfaultfd_continue(struct userfaultfd_ctx *ctx, unsigned long arg)
> >         return ret;
> >  }
> >
> > +static inline int userfaultfd_poison(struct userfaultfd_ctx *ctx, unsigned long arg)
> > +{
> > +       __s64 ret;
> > +       struct uffdio_poison uffdio_poison;
> > +       struct uffdio_poison __user *user_uffdio_poison;
> > +       struct userfaultfd_wake_range range;
> > +
> > +       user_uffdio_poison = (struct uffdio_poison __user *)arg;
> > +
> > +       ret = -EAGAIN;
> > +       if (atomic_read(&ctx->mmap_changing))
> > +               goto out;
> > +
> > +       ret = -EFAULT;
> > +       if (copy_from_user(&uffdio_poison, user_uffdio_poison,
> > +                          /* don't copy the output fields */
> > +                          sizeof(uffdio_poison) - (sizeof(__s64))))
> > +               goto out;
> > +
> > +       ret = validate_range(ctx->mm, uffdio_poison.range.start,
> > +                            uffdio_poison.range.len);
> > +       if (ret)
> > +               goto out;
> > +
> > +       ret = -EINVAL;
> > +       /* double check for wraparound just in case. */
> > +       if (uffdio_poison.range.start + uffdio_poison.range.len <=
> > +           uffdio_poison.range.start) {
> > +               goto out;
> > +       }
> > +       if (uffdio_poison.mode & ~UFFDIO_POISON_MODE_DONTWAKE)
> > +               goto out;
> > +
> > +       if (mmget_not_zero(ctx->mm)) {
> > +               ret = mfill_atomic_poison(ctx->mm, uffdio_poison.range.start,
> > +                                         uffdio_poison.range.len,
> > +                                         &ctx->mmap_changing, 0);
> > +               mmput(ctx->mm);
> > +       } else {
> > +               return -ESRCH;
> > +       }
> > +
> > +       if (unlikely(put_user(ret, &user_uffdio_poison->updated)))
> > +               return -EFAULT;
> > +       if (ret < 0)
> > +               goto out;
> > +
> > +       /* len == 0 would wake all */
> > +       BUG_ON(!ret);
> > +       range.len = ret;
> > +       if (!(uffdio_poison.mode & UFFDIO_POISON_MODE_DONTWAKE)) {
> > +               range.start = uffdio_poison.range.start;
> > +               wake_userfault(ctx, &range);
> > +       }
> > +       ret = range.len == uffdio_poison.range.len ? 0 : -EAGAIN;
> > +
> > +out:
> > +       return ret;
> > +}
> > +
> >  static inline unsigned int uffd_ctx_features(__u64 user_features)
> >  {
> >         /*
> > @@ -2066,6 +2126,9 @@ static long userfaultfd_ioctl(struct file *file, unsigned cmd,
> >         case UFFDIO_CONTINUE:
> >                 ret = userfaultfd_continue(ctx, arg);
> >                 break;
> > +       case UFFDIO_POISON:
> > +               ret = userfaultfd_poison(ctx, arg);
> > +               break;
> >         }
> >         return ret;
> >  }
> > diff --git a/include/linux/swapops.h b/include/linux/swapops.h
> > index 4c932cb45e0b..8259fee32421 100644
> > --- a/include/linux/swapops.h
> > +++ b/include/linux/swapops.h
> > @@ -394,7 +394,8 @@ typedef unsigned long pte_marker;
> >
> >  #define  PTE_MARKER_UFFD_WP                    BIT(0)
> >  #define  PTE_MARKER_SWAPIN_ERROR               BIT(1)
> > -#define  PTE_MARKER_MASK                       (BIT(2) - 1)
> > +#define  PTE_MARKER_UFFD_POISON                        BIT(2)
> > +#define  PTE_MARKER_MASK                       (BIT(3) - 1)
> >
> >  static inline swp_entry_t make_pte_marker_entry(pte_marker marker)
> >  {
> > diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
> > index ac7b0c96d351..ac8c6854097c 100644
> > --- a/include/linux/userfaultfd_k.h
> > +++ b/include/linux/userfaultfd_k.h
> > @@ -46,6 +46,7 @@ enum mfill_atomic_mode {
> >         MFILL_ATOMIC_COPY,
> >         MFILL_ATOMIC_ZEROPAGE,
> >         MFILL_ATOMIC_CONTINUE,
> > +       MFILL_ATOMIC_POISON,
> >         NR_MFILL_ATOMIC_MODES,
> >  };
> >
> > @@ -83,6 +84,9 @@ extern ssize_t mfill_atomic_zeropage(struct mm_struct *dst_mm,
> >  extern ssize_t mfill_atomic_continue(struct mm_struct *dst_mm, unsigned long dst_start,
> >                                      unsigned long len, atomic_t *mmap_changing,
> >                                      uffd_flags_t flags);
> > +extern ssize_t mfill_atomic_poison(struct mm_struct *dst_mm, unsigned long start,
> > +                                  unsigned long len, atomic_t *mmap_changing,
> > +                                  uffd_flags_t flags);
> >  extern int mwriteprotect_range(struct mm_struct *dst_mm,
> >                                unsigned long start, unsigned long len,
> >                                bool enable_wp, atomic_t *mmap_changing);
> > diff --git a/include/uapi/linux/userfaultfd.h b/include/uapi/linux/userfaultfd.h
> > index 66dd4cd277bd..62151706c5a3 100644
> > --- a/include/uapi/linux/userfaultfd.h
> > +++ b/include/uapi/linux/userfaultfd.h
> > @@ -39,7 +39,8 @@
> >                            UFFD_FEATURE_MINOR_SHMEM |           \
> >                            UFFD_FEATURE_EXACT_ADDRESS |         \
> >                            UFFD_FEATURE_WP_HUGETLBFS_SHMEM |    \
> > -                          UFFD_FEATURE_WP_UNPOPULATED)
> > +                          UFFD_FEATURE_WP_UNPOPULATED |        \
> > +                          UFFD_FEATURE_POISON)
> >  #define UFFD_API_IOCTLS                                \
> >         ((__u64)1 << _UFFDIO_REGISTER |         \
> >          (__u64)1 << _UFFDIO_UNREGISTER |       \
> > @@ -49,12 +50,14 @@
> >          (__u64)1 << _UFFDIO_COPY |             \
> >          (__u64)1 << _UFFDIO_ZEROPAGE |         \
> >          (__u64)1 << _UFFDIO_WRITEPROTECT |     \
> > -        (__u64)1 << _UFFDIO_CONTINUE)
> > +        (__u64)1 << _UFFDIO_CONTINUE |         \
> > +        (__u64)1 << _UFFDIO_POISON)
> >  #define UFFD_API_RANGE_IOCTLS_BASIC            \
> >         ((__u64)1 << _UFFDIO_WAKE |             \
> >          (__u64)1 << _UFFDIO_COPY |             \
> > +        (__u64)1 << _UFFDIO_WRITEPROTECT |     \
> >          (__u64)1 << _UFFDIO_CONTINUE |         \
> > -        (__u64)1 << _UFFDIO_WRITEPROTECT)
> > +        (__u64)1 << _UFFDIO_POISON)
> >
> >  /*
> >   * Valid ioctl command number range with this API is from 0x00 to
> > @@ -71,6 +74,7 @@
> >  #define _UFFDIO_ZEROPAGE               (0x04)
> >  #define _UFFDIO_WRITEPROTECT           (0x06)
> >  #define _UFFDIO_CONTINUE               (0x07)
> > +#define _UFFDIO_POISON                 (0x08)
> >  #define _UFFDIO_API                    (0x3F)
> >
> >  /* userfaultfd ioctl ids */
> > @@ -91,6 +95,8 @@
> >                                       struct uffdio_writeprotect)
> >  #define UFFDIO_CONTINUE                _IOWR(UFFDIO, _UFFDIO_CONTINUE, \
> >                                       struct uffdio_continue)
> > +#define UFFDIO_POISON          _IOWR(UFFDIO, _UFFDIO_POISON, \
> > +                                     struct uffdio_poison)
> >
> >  /* read() structure */
> >  struct uffd_msg {
> > @@ -225,6 +231,7 @@ struct uffdio_api {
> >  #define UFFD_FEATURE_EXACT_ADDRESS             (1<<11)
> >  #define UFFD_FEATURE_WP_HUGETLBFS_SHMEM                (1<<12)
> >  #define UFFD_FEATURE_WP_UNPOPULATED            (1<<13)
> > +#define UFFD_FEATURE_POISON                    (1<<14)
> >         __u64 features;
> >
> >         __u64 ioctls;
> > @@ -321,6 +328,18 @@ struct uffdio_continue {
> >         __s64 mapped;
> >  };
> >
> > +struct uffdio_poison {
> > +       struct uffdio_range range;
> > +#define UFFDIO_POISON_MODE_DONTWAKE            ((__u64)1<<0)
> > +       __u64 mode;
> > +
> > +       /*
> > +        * Fields below here are written by the ioctl and must be at the end:
> > +        * the copy_from_user will not read past here.
> > +        */
> > +       __s64 updated;
> > +};
> > +
> >  /*
> >   * Flags for the userfaultfd(2) system call itself.
> >   */
> > diff --git a/mm/memory.c b/mm/memory.c
> > index d8a9a770b1f1..7fbda39e060d 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -3692,6 +3692,10 @@ static vm_fault_t handle_pte_marker(struct vm_fault *vmf)
> >         if (WARN_ON_ONCE(!marker))
> >                 return VM_FAULT_SIGBUS;
> >
> > +       /* Poison emulation explicitly requested for this PTE. */
> > +       if (marker & PTE_MARKER_UFFD_POISON)
> > +               return VM_FAULT_HWPOISON;
> > +
> >         /* Higher priority than uffd-wp when data corrupted */
> >         if (marker & PTE_MARKER_SWAPIN_ERROR)
> >                 return VM_FAULT_SIGBUS;
> > diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> > index a2bf37ee276d..87b62ca1e09e 100644
> > --- a/mm/userfaultfd.c
> > +++ b/mm/userfaultfd.c
> > @@ -286,6 +286,51 @@ static int mfill_atomic_pte_continue(pmd_t *dst_pmd,
> >         goto out;
> >  }
> >
> > +/* Handles UFFDIO_POISON for all non-hugetlb VMAs. */
> > +static int mfill_atomic_pte_poison(pmd_t *dst_pmd,
> > +                                  struct vm_area_struct *dst_vma,
> > +                                  unsigned long dst_addr,
> > +                                  uffd_flags_t flags)
> > +{
> > +       int ret;
> > +       struct mm_struct *dst_mm = dst_vma->vm_mm;
> > +       pte_t _dst_pte, *dst_pte;
> > +       spinlock_t *ptl;
> > +
> > +       _dst_pte = make_pte_marker(PTE_MARKER_UFFD_POISON);
> > +       dst_pte = pte_offset_map_lock(dst_mm, dst_pmd, dst_addr, &ptl);
> > +
> > +       if (vma_is_shmem(dst_vma)) {
> > +               struct inode *inode;
> > +               pgoff_t offset, max_off;
> > +
> > +               /* serialize against truncate with the page table lock */
> > +               inode = dst_vma->vm_file->f_inode;
> > +               offset = linear_page_index(dst_vma, dst_addr);
> > +               max_off = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
> > +               ret = -EFAULT;
> > +               if (unlikely(offset >= max_off))
> > +                       goto out_unlock;
> > +       }
> > +
> > +       ret = -EEXIST;
> > +       /*
> > +        * For now, we don't handle unmapping pages, so only support filling in
> > +        * none PTEs, or replacing PTE markers.
> > +        */
> > +       if (!pte_none_mostly(*dst_pte))
> > +               goto out_unlock;
> > +
> > +       set_pte_at(dst_mm, dst_addr, dst_pte, _dst_pte);
> > +
> > +       /* No need to invalidate - it was non-present before */
> > +       update_mmu_cache(dst_vma, dst_addr, dst_pte);
> > +       ret = 0;
> > +out_unlock:
> > +       pte_unmap_unlock(dst_pte, ptl);
> > +       return ret;
> > +}
> > +
> >  static pmd_t *mm_alloc_pmd(struct mm_struct *mm, unsigned long address)
> >  {
> >         pgd_t *pgd;
> > @@ -336,8 +381,12 @@ static __always_inline ssize_t mfill_atomic_hugetlb(
> >          * supported by hugetlb.  A PMD_SIZE huge pages may exist as used
> >          * by THP.  Since we can not reliably insert a zero page, this
> >          * feature is not supported.
> > +        *
> > +        * PTE marker handling for hugetlb is a bit special, so for now
> > +        * UFFDIO_POISON is not supported.
> >          */
> > -       if (uffd_flags_mode_is(flags, MFILL_ATOMIC_ZEROPAGE)) {
> > +       if (uffd_flags_mode_is(flags, MFILL_ATOMIC_ZEROPAGE) ||
> > +           uffd_flags_mode_is(flags, MFILL_ATOMIC_POISON)) {
> >                 mmap_read_unlock(dst_mm);
> >                 return -EINVAL;
> >         }
> > @@ -481,6 +530,9 @@ static __always_inline ssize_t mfill_atomic_pte(pmd_t *dst_pmd,
> >         if (uffd_flags_mode_is(flags, MFILL_ATOMIC_CONTINUE)) {
> >                 return mfill_atomic_pte_continue(dst_pmd, dst_vma,
> >                                                  dst_addr, flags);
> > +       } else if (uffd_flags_mode_is(flags, MFILL_ATOMIC_POISON)) {
> > +               return mfill_atomic_pte_poison(dst_pmd, dst_vma,
> > +                                              dst_addr, flags);
> >         }
> >
> >         /*
> > @@ -702,6 +754,14 @@ ssize_t mfill_atomic_continue(struct mm_struct *dst_mm, unsigned long start,
> >                             uffd_flags_set_mode(flags, MFILL_ATOMIC_CONTINUE));
> >  }
> >
> > +ssize_t mfill_atomic_poison(struct mm_struct *dst_mm, unsigned long start,
> > +                           unsigned long len, atomic_t *mmap_changing,
> > +                           uffd_flags_t flags)
> > +{
> > +       return mfill_atomic(dst_mm, start, 0, len, mmap_changing,
> > +                           uffd_flags_set_mode(flags, MFILL_ATOMIC_POISON));
> > +}
> > +
> >  long uffd_wp_range(struct vm_area_struct *dst_vma,
> >                    unsigned long start, unsigned long len, bool enable_wp)
> >  {
> > --
> > 2.41.0.255.g8b1d071c50-goog
> >

Patch

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 7cecd49e078b..c26a883399c9 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -1965,6 +1965,66 @@  static int userfaultfd_continue(struct userfaultfd_ctx *ctx, unsigned long arg)
 	return ret;
 }
 
+static inline int userfaultfd_poison(struct userfaultfd_ctx *ctx, unsigned long arg)
+{
+	__s64 ret;
+	struct uffdio_poison uffdio_poison;
+	struct uffdio_poison __user *user_uffdio_poison;
+	struct userfaultfd_wake_range range;
+
+	user_uffdio_poison = (struct uffdio_poison __user *)arg;
+
+	ret = -EAGAIN;
+	if (atomic_read(&ctx->mmap_changing))
+		goto out;
+
+	ret = -EFAULT;
+	if (copy_from_user(&uffdio_poison, user_uffdio_poison,
+			   /* don't copy the output fields */
+			   sizeof(uffdio_poison) - (sizeof(__s64))))
+		goto out;
+
+	ret = validate_range(ctx->mm, uffdio_poison.range.start,
+			     uffdio_poison.range.len);
+	if (ret)
+		goto out;
+
+	ret = -EINVAL;
+	/* double check for wraparound just in case. */
+	if (uffdio_poison.range.start + uffdio_poison.range.len <=
+	    uffdio_poison.range.start) {
+		goto out;
+	}
+	if (uffdio_poison.mode & ~UFFDIO_POISON_MODE_DONTWAKE)
+		goto out;
+
+	if (mmget_not_zero(ctx->mm)) {
+		ret = mfill_atomic_poison(ctx->mm, uffdio_poison.range.start,
+					  uffdio_poison.range.len,
+					  &ctx->mmap_changing, 0);
+		mmput(ctx->mm);
+	} else {
+		return -ESRCH;
+	}
+
+	if (unlikely(put_user(ret, &user_uffdio_poison->updated)))
+		return -EFAULT;
+	if (ret < 0)
+		goto out;
+
+	/* len == 0 would wake all */
+	BUG_ON(!ret);
+	range.len = ret;
+	if (!(uffdio_poison.mode & UFFDIO_POISON_MODE_DONTWAKE)) {
+		range.start = uffdio_poison.range.start;
+		wake_userfault(ctx, &range);
+	}
+	ret = range.len == uffdio_poison.range.len ? 0 : -EAGAIN;
+
+out:
+	return ret;
+}
+
 static inline unsigned int uffd_ctx_features(__u64 user_features)
 {
 	/*
@@ -2066,6 +2126,9 @@  static long userfaultfd_ioctl(struct file *file, unsigned cmd,
 	case UFFDIO_CONTINUE:
 		ret = userfaultfd_continue(ctx, arg);
 		break;
+	case UFFDIO_POISON:
+		ret = userfaultfd_poison(ctx, arg);
+		break;
 	}
 	return ret;
 }
diff --git a/include/linux/swapops.h b/include/linux/swapops.h
index 4c932cb45e0b..8259fee32421 100644
--- a/include/linux/swapops.h
+++ b/include/linux/swapops.h
@@ -394,7 +394,8 @@  typedef unsigned long pte_marker;
 
 #define  PTE_MARKER_UFFD_WP			BIT(0)
 #define  PTE_MARKER_SWAPIN_ERROR		BIT(1)
-#define  PTE_MARKER_MASK			(BIT(2) - 1)
+#define  PTE_MARKER_UFFD_POISON			BIT(2)
+#define  PTE_MARKER_MASK			(BIT(3) - 1)
 
 static inline swp_entry_t make_pte_marker_entry(pte_marker marker)
 {
diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
index ac7b0c96d351..ac8c6854097c 100644
--- a/include/linux/userfaultfd_k.h
+++ b/include/linux/userfaultfd_k.h
@@ -46,6 +46,7 @@  enum mfill_atomic_mode {
 	MFILL_ATOMIC_COPY,
 	MFILL_ATOMIC_ZEROPAGE,
 	MFILL_ATOMIC_CONTINUE,
+	MFILL_ATOMIC_POISON,
 	NR_MFILL_ATOMIC_MODES,
 };
 
@@ -83,6 +84,9 @@  extern ssize_t mfill_atomic_zeropage(struct mm_struct *dst_mm,
 extern ssize_t mfill_atomic_continue(struct mm_struct *dst_mm, unsigned long dst_start,
 				     unsigned long len, atomic_t *mmap_changing,
 				     uffd_flags_t flags);
+extern ssize_t mfill_atomic_poison(struct mm_struct *dst_mm, unsigned long start,
+				   unsigned long len, atomic_t *mmap_changing,
+				   uffd_flags_t flags);
 extern int mwriteprotect_range(struct mm_struct *dst_mm,
 			       unsigned long start, unsigned long len,
 			       bool enable_wp, atomic_t *mmap_changing);
diff --git a/include/uapi/linux/userfaultfd.h b/include/uapi/linux/userfaultfd.h
index 66dd4cd277bd..62151706c5a3 100644
--- a/include/uapi/linux/userfaultfd.h
+++ b/include/uapi/linux/userfaultfd.h
@@ -39,7 +39,8 @@ 
 			   UFFD_FEATURE_MINOR_SHMEM |		\
 			   UFFD_FEATURE_EXACT_ADDRESS |		\
 			   UFFD_FEATURE_WP_HUGETLBFS_SHMEM |	\
-			   UFFD_FEATURE_WP_UNPOPULATED)
+			   UFFD_FEATURE_WP_UNPOPULATED |	\
+			   UFFD_FEATURE_POISON)
 #define UFFD_API_IOCTLS				\
 	((__u64)1 << _UFFDIO_REGISTER |		\
 	 (__u64)1 << _UFFDIO_UNREGISTER |	\
@@ -49,12 +50,14 @@ 
 	 (__u64)1 << _UFFDIO_COPY |		\
 	 (__u64)1 << _UFFDIO_ZEROPAGE |		\
 	 (__u64)1 << _UFFDIO_WRITEPROTECT |	\
-	 (__u64)1 << _UFFDIO_CONTINUE)
+	 (__u64)1 << _UFFDIO_CONTINUE |		\
+	 (__u64)1 << _UFFDIO_POISON)
 #define UFFD_API_RANGE_IOCTLS_BASIC		\
 	((__u64)1 << _UFFDIO_WAKE |		\
 	 (__u64)1 << _UFFDIO_COPY |		\
+	 (__u64)1 << _UFFDIO_WRITEPROTECT |	\
 	 (__u64)1 << _UFFDIO_CONTINUE |		\
-	 (__u64)1 << _UFFDIO_WRITEPROTECT)
+	 (__u64)1 << _UFFDIO_POISON)
 
 /*
  * Valid ioctl command number range with this API is from 0x00 to
@@ -71,6 +74,7 @@ 
 #define _UFFDIO_ZEROPAGE		(0x04)
 #define _UFFDIO_WRITEPROTECT		(0x06)
 #define _UFFDIO_CONTINUE		(0x07)
+#define _UFFDIO_POISON			(0x08)
 #define _UFFDIO_API			(0x3F)
 
 /* userfaultfd ioctl ids */
@@ -91,6 +95,8 @@ 
 				      struct uffdio_writeprotect)
 #define UFFDIO_CONTINUE		_IOWR(UFFDIO, _UFFDIO_CONTINUE,	\
 				      struct uffdio_continue)
+#define UFFDIO_POISON		_IOWR(UFFDIO, _UFFDIO_POISON, \
+				      struct uffdio_poison)
 
 /* read() structure */
 struct uffd_msg {
@@ -225,6 +231,7 @@  struct uffdio_api {
 #define UFFD_FEATURE_EXACT_ADDRESS		(1<<11)
 #define UFFD_FEATURE_WP_HUGETLBFS_SHMEM		(1<<12)
 #define UFFD_FEATURE_WP_UNPOPULATED		(1<<13)
+#define UFFD_FEATURE_POISON			(1<<14)
 	__u64 features;
 
 	__u64 ioctls;
@@ -321,6 +328,18 @@  struct uffdio_continue {
 	__s64 mapped;
 };
 
+struct uffdio_poison {
+	struct uffdio_range range;
+#define UFFDIO_POISON_MODE_DONTWAKE		((__u64)1<<0)
+	__u64 mode;
+
+	/*
+	 * Fields below here are written by the ioctl and must be at the end:
+	 * the copy_from_user will not read past here.
+	 */
+	__s64 updated;
+};
+
 /*
  * Flags for the userfaultfd(2) system call itself.
  */
diff --git a/mm/memory.c b/mm/memory.c
index d8a9a770b1f1..7fbda39e060d 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3692,6 +3692,10 @@  static vm_fault_t handle_pte_marker(struct vm_fault *vmf)
 	if (WARN_ON_ONCE(!marker))
 		return VM_FAULT_SIGBUS;
 
+	/* Poison emulation explicitly requested for this PTE. */
+	if (marker & PTE_MARKER_UFFD_POISON)
+		return VM_FAULT_HWPOISON;
+
 	/* Higher priority than uffd-wp when data corrupted */
 	if (marker & PTE_MARKER_SWAPIN_ERROR)
 		return VM_FAULT_SIGBUS;
diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index a2bf37ee276d..87b62ca1e09e 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -286,6 +286,51 @@  static int mfill_atomic_pte_continue(pmd_t *dst_pmd,
 	goto out;
 }
 
+/* Handles UFFDIO_POISON for all non-hugetlb VMAs. */
+static int mfill_atomic_pte_poison(pmd_t *dst_pmd,
+				   struct vm_area_struct *dst_vma,
+				   unsigned long dst_addr,
+				   uffd_flags_t flags)
+{
+	int ret;
+	struct mm_struct *dst_mm = dst_vma->vm_mm;
+	pte_t _dst_pte, *dst_pte;
+	spinlock_t *ptl;
+
+	_dst_pte = make_pte_marker(PTE_MARKER_UFFD_POISON);
+	dst_pte = pte_offset_map_lock(dst_mm, dst_pmd, dst_addr, &ptl);
+
+	if (vma_is_shmem(dst_vma)) {
+		struct inode *inode;
+		pgoff_t offset, max_off;
+
+		/* serialize against truncate with the page table lock */
+		inode = dst_vma->vm_file->f_inode;
+		offset = linear_page_index(dst_vma, dst_addr);
+		max_off = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
+		ret = -EFAULT;
+		if (unlikely(offset >= max_off))
+			goto out_unlock;
+	}
+
+	ret = -EEXIST;
+	/*
+	 * For now, we don't handle unmapping pages, so only support filling in
+	 * none PTEs, or replacing PTE markers.
+	 */
+	if (!pte_none_mostly(*dst_pte))
+		goto out_unlock;
+
+	set_pte_at(dst_mm, dst_addr, dst_pte, _dst_pte);
+
+	/* No need to invalidate - it was non-present before */
+	update_mmu_cache(dst_vma, dst_addr, dst_pte);
+	ret = 0;
+out_unlock:
+	pte_unmap_unlock(dst_pte, ptl);
+	return ret;
+}
+
 static pmd_t *mm_alloc_pmd(struct mm_struct *mm, unsigned long address)
 {
 	pgd_t *pgd;
@@ -336,8 +381,12 @@  static __always_inline ssize_t mfill_atomic_hugetlb(
 	 * supported by hugetlb.  A PMD_SIZE huge pages may exist as used
 	 * by THP.  Since we can not reliably insert a zero page, this
 	 * feature is not supported.
+	 *
+	 * PTE marker handling for hugetlb is a bit special, so for now
+	 * UFFDIO_POISON is not supported.
 	 */
-	if (uffd_flags_mode_is(flags, MFILL_ATOMIC_ZEROPAGE)) {
+	if (uffd_flags_mode_is(flags, MFILL_ATOMIC_ZEROPAGE) ||
+	    uffd_flags_mode_is(flags, MFILL_ATOMIC_POISON)) {
 		mmap_read_unlock(dst_mm);
 		return -EINVAL;
 	}
@@ -481,6 +530,9 @@  static __always_inline ssize_t mfill_atomic_pte(pmd_t *dst_pmd,
 	if (uffd_flags_mode_is(flags, MFILL_ATOMIC_CONTINUE)) {
 		return mfill_atomic_pte_continue(dst_pmd, dst_vma,
 						 dst_addr, flags);
+	} else if (uffd_flags_mode_is(flags, MFILL_ATOMIC_POISON)) {
+		return mfill_atomic_pte_poison(dst_pmd, dst_vma,
+					       dst_addr, flags);
 	}
 
 	/*
@@ -702,6 +754,14 @@  ssize_t mfill_atomic_continue(struct mm_struct *dst_mm, unsigned long start,
 			    uffd_flags_set_mode(flags, MFILL_ATOMIC_CONTINUE));
 }
 
+ssize_t mfill_atomic_poison(struct mm_struct *dst_mm, unsigned long start,
+			    unsigned long len, atomic_t *mmap_changing,
+			    uffd_flags_t flags)
+{
+	return mfill_atomic(dst_mm, start, 0, len, mmap_changing,
+			    uffd_flags_set_mode(flags, MFILL_ATOMIC_POISON));
+}
+
 long uffd_wp_range(struct vm_area_struct *dst_vma,
 		   unsigned long start, unsigned long len, bool enable_wp)
 {