| Message ID | 20210317110644.25343-3-david@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | mm/madvise: introduce MADV_POPULATE_(READ|WRITE) to prefault/prealloc memory | expand |
On Wed, Mar 17, 2021 at 12:07 PM David Hildenbrand <david@redhat.com> wrote: > I. Background: Sparse Memory Mappings > > When we manage sparse memory mappings dynamically in user space - also > sometimes involving MAP_NORESERVE - we want to dynamically populate/ > discard memory inside such a sparse memory region. Example users are > hypervisors (especially implementing memory ballooning or similar > technologies like virtio-mem) and memory allocators. In addition, we want > to fail in a nice way (instead of generating SIGBUS) if populating does not > succeed because we are out of backend memory (which can happen easily with > file-based mappings, especially tmpfs and hugetlbfs). > > While MADV_DONTNEED, MADV_REMOVE and FALLOC_FL_PUNCH_HOLE allow for > reliably discarding memory, there is no generic approach to populate > page tables and preallocate memory. > > Although mmap() supports MAP_POPULATE, it is not applicable to the concept > of sparse memory mappings, where we want to do populate/discard > dynamically and avoid expensive/problematic remappings. In addition, > we never actually report errors during the final populate phase - it is > best-effort only. > > fallocate() can be used to preallocate file-based memory and fail in a safe > way. However, it cannot really be used for any private mappings on > anonymous files via memfd due to COW semantics. In addition, fallocate() > does not actually populate page tables, so we still always get > pagefaults on first access - which is sometimes undesired (i.e., real-time > workloads) and requires real prefaulting of page tables, not just a > preallocation of backend storage. There might be interesting use cases > for sparse memory regions along with mlockall(MCL_ONFAULT) which > fallocate() cannot satisfy as it does not prefault page tables. > > II. On preallcoation/prefaulting from user space > > Because we don't have a proper interface, what applications > (like QEMU and databases) end up doing is touching (i.e., reading+writing > one byte to not overwrite existing data) all individual pages. > > However, that approach > 1) Can result in wear on storage backing, because we end up writing > and thereby dirtying each page --- i.e., disks or pmem. > 2) Can result in mmap_sem contention when prefaulting via multiple > threads. > 3) Requires expensive signal handling, especially to catch SIGBUS in case > of hugetlbfs/shmem/file-backed memory. For example, this is > problematic in hypervisors like QEMU where SIGBUS handlers might already > be used by other subsystems concurrently to e.g, handle hardware errors. > "Simply" doing preallocation concurrently from other thread is not that > easy. > > III. On MADV_WILLNEED > > Extending MADV_WILLNEED is not an option because > 1. It would change the semantics: "Expect access in the near future." and > "might be a good idea to read some pages" vs. "Definitely populate/ > preallocate all memory and definitely fail on errors.". > 2. Existing users (like virtio-balloon in QEMU when deflating the balloon) > don't want populate/prealloc semantics. They treat this rather as a hint > to give a little performance boost without too much overhead - and don't > expect that a lot of memory might get consumed or a lot of time > might be spent. > > IV. MADV_POPULATE_READ and MADV_POPULATE_WRITE > > Let's introduce MADV_POPULATE_READ and MADV_POPULATE_WRITE with the > following semantics: > 1. MADV_POPULATE_READ can be used to preallocate backend memory and > prefault page tables just like manually reading each individual page. > This will not break any COW mappings -- e.g., it will populate the > shared zeropage when applicable. Please clarify what is meant by "backend memory". As far as I can tell from looking at the code, MADV_POPULATE_READ on file mappings will allocate zeroed memory in the page cache, and map it as readonly pages into userspace, but any attempt to actually write to that memory will trigger the filesystem's ->page_mkwrite handler; and e.g. ext4 will only try to allocate disk blocks at that point, which may fail. So as far as I can tell, for files on filesystems like ext4, the current implementation of MADV_POPULATE_READ does not replace fallocate(). Am I missing something? If the desired semantics are that disk blocks should be preallocated, I think you may have to look up the ->vm_file and then internally call vfs_fallocate() to address this, kinda like in madvise_remove()? > 2. If MADV_POPULATE_READ succeeds, all page tables have been populated > (prefaulted) readable once. > 3. MADV_POPULATE_WRITE can be used to preallocate backend memory and > prefault page tables just like manually writing (or > reading+writing) each individual page. This will break any COW > mappings -- e.g., the shared zeropage is never populated. > 4. If MADV_POPULATE_WRITE succeeds, all page tables have been populated > (prefaulted) writable once. > 5. MADV_POPULATE_READ and MADV_POPULATE_WRITE cannot be applied to special > mappings marked with VM_PFNMAP and VM_IO. Also, proper access > permissions (e.g., PROT_READ, PROT_WRITE) are required. If any such > mapping is encountered, madvise() fails with -EINVAL. > 6. If MADV_POPULATE_READ or MADV_POPULATE_WRITE fails, some page tables > might have been populated. In that case, madvise() fails with > -ENOMEM. AFAICS that's not true (or misphrased). If MADV_POPULATE_* successfully populates a bunch of pages, then fails because of an error (e.g. EHWPOISON), it will return EHWPOISON, not ENOMEM, right? > 7. MADV_POPULATE_READ and MADV_POPULATE_WRITE will return -EHWPOISON > when encountering a HW poisoned page in the range. > 8. Similar to MAP_POPULATE, MADV_POPULATE_READ and MADV_POPULATE_WRITE > cannot protect from the OOM (Out Of Memory) handler killing the > process. > > While the use case for MADV_POPULATE_WRITE is fairly obvious (i.e., > preallocate memory and prefault page tables for VMs), there are valid use > cases for MADV_POPULATE_READ: > 1. Efficiently populate page tables with zero pages (i.e., shared > zeropage). This is necessary when using userfaultfd() WP (Write-Protect > to properly catch all modifications within a mapping: for > write-protection to be effective for a virtual address, there has to be > a page already mapped -- even if it's the shared zeropage. This sounds like a hack to work around issues that would be better addressed by improving userfaultfd? > 2. Pre-read a whole mapping from backend storage without marking it > dirty, such that eviction won't have to write it back. If no backend > memory has been allocated yet, allocate the backend memory. Helpful > when preallocating/prefaulting a file stored on disk without having > to writeback each and every page on eviction. This sounds reasonable to me. > Although sparse memory mappings are the primary use case, this will > also be useful for ordinary preallocations where MAP_POPULATE is not > desired especially in QEMU, where users can trigger preallocation of > guest RAM after the mapping was created. > > Looking at the history, MADV_POPULATE was already proposed in 2013 [1], > however, the main motivation back than was performance improvements > (which should also still be the case, but it is a secondary concern). > > V. Single-threaded performance comparison > > There is a performance benefit when using POPULATE_READ / POPULATE_WRITE > already when only using a single thread to do prefaulting/preallocation. As > we have less pagefaults for huge pages, the performance benefit is > negligible with small mappings. [...] > diff --git a/mm/gup.c b/mm/gup.c [...] > +long faultin_vma_page_range(struct vm_area_struct *vma, unsigned long start, > + unsigned long end, bool write, int *locked) > +{ > + struct mm_struct *mm = vma->vm_mm; > + unsigned long nr_pages = (end - start) / PAGE_SIZE; > + int gup_flags; > + > + VM_BUG_ON(!PAGE_ALIGNED(start)); > + VM_BUG_ON(!PAGE_ALIGNED(end)); > + VM_BUG_ON_VMA(start < vma->vm_start, vma); > + VM_BUG_ON_VMA(end > vma->vm_end, vma); > + mmap_assert_locked(mm); > + > + /* > + * FOLL_HWPOISON: Return -EHWPOISON instead of -EFAULT when we hit > + * a poisoned page. > + * FOLL_POPULATE: Always populate memory with VM_LOCKONFAULT. > + * !FOLL_FORCE: Require proper access permissions. > + */ > + gup_flags = FOLL_TOUCH | FOLL_POPULATE | FOLL_MLOCK | FOLL_HWPOISON; > + if (write) > + gup_flags |= FOLL_WRITE; > + > + /* > + * See check_vma_flags(): Will return -EFAULT on incompatible mappings > + * or with insufficient permissions. > + */ > + return __get_user_pages(mm, start, nr_pages, gup_flags, > + NULL, NULL, locked); You mentioned in the commit message that you don't want to actually dirty all the file pages and force writeback; but doesn't POPULATE_WRITE still do exactly that? In follow_page_pte(), if FOLL_TOUCH and FOLL_WRITE are set, we mark the page as dirty: if (flags & FOLL_TOUCH) { if ((flags & FOLL_WRITE) && !pte_dirty(pte) && !PageDirty(page)) set_page_dirty(page); /* * pte_mkyoung() would be more correct here, but atomic care * is needed to avoid losing the dirty bit: it is easier to use * mark_page_accessed(). */ mark_page_accessed(page); } > +} > + > /* > * __mm_populate - populate and/or mlock pages within a range of address space. > * > diff --git a/mm/internal.h b/mm/internal.h > index 3f22c4ceb7b5..ee398696380f 100644 > --- a/mm/internal.h > +++ b/mm/internal.h > @@ -335,6 +335,9 @@ void __vma_unlink_list(struct mm_struct *mm, struct vm_area_struct *vma); > #ifdef CONFIG_MMU > extern long populate_vma_page_range(struct vm_area_struct *vma, > unsigned long start, unsigned long end, int *locked); > +extern long faultin_vma_page_range(struct vm_area_struct *vma, > + unsigned long start, unsigned long end, > + bool write, int *locked); > extern void munlock_vma_pages_range(struct vm_area_struct *vma, > unsigned long start, unsigned long end); > static inline void munlock_vma_pages_all(struct vm_area_struct *vma) > diff --git a/mm/madvise.c b/mm/madvise.c > index 01fef79ac761..857460873f7a 100644 > --- a/mm/madvise.c > +++ b/mm/madvise.c > @@ -53,6 +53,8 @@ static int madvise_need_mmap_write(int behavior) > case MADV_COLD: > case MADV_PAGEOUT: > case MADV_FREE: > + case MADV_POPULATE_READ: > + case MADV_POPULATE_WRITE: > return 0; > default: > /* be safe, default to 1. list exceptions explicitly */ > @@ -822,6 +824,64 @@ static long madvise_dontneed_free(struct vm_area_struct *vma, > return -EINVAL; > } > > +static long madvise_populate(struct vm_area_struct *vma, > + struct vm_area_struct **prev, > + unsigned long start, unsigned long end, > + int behavior) > +{ > + const bool write = behavior == MADV_POPULATE_WRITE; > + struct mm_struct *mm = vma->vm_mm; > + unsigned long tmp_end; > + int locked = 1; > + long pages; > + > + *prev = vma; > + > + while (start < end) { > + /* > + * We might have temporarily dropped the lock. For example, > + * our VMA might have been split. > + */ > + if (!vma || start >= vma->vm_end) { > + vma = find_vma(mm, start); > + if (!vma || start < vma->vm_start) > + return -ENOMEM; > + } > + > + tmp_end = min_t(unsigned long, end, vma->vm_end); > + /* Populate (prefault) page tables readable/writable. */ > + pages = faultin_vma_page_range(vma, start, tmp_end, write, > + &locked); > + if (!locked) { > + mmap_read_lock(mm); > + locked = 1; > + *prev = NULL; > + vma = NULL; > + } > + if (pages < 0) { > + switch (pages) { > + case -EINTR: > + return -EINTR; > + case -EFAULT: /* Incompatible mappings / permissions. */ > + return -EINVAL; > + case -EHWPOISON: > + return -EHWPOISON; > + case -EBUSY: What is -EBUSY doing here? __get_user_pages() fixes up -EBUSY from faultin_page() to 0, right? > + case -EAGAIN: Where can -EAGAIN come from? > + continue; > + default: > + pr_warn_once("%s: unhandled return value: %ld\n", > + __func__, pages); > + fallthrough; > + case -ENOMEM: > + return -ENOMEM; > + } > + } > + start += pages * PAGE_SIZE; > + } > + return 0; > +}
[...] >> >> Let's introduce MADV_POPULATE_READ and MADV_POPULATE_WRITE with the >> following semantics: >> 1. MADV_POPULATE_READ can be used to preallocate backend memory and >> prefault page tables just like manually reading each individual page. >> This will not break any COW mappings -- e.g., it will populate the >> shared zeropage when applicable. > > Please clarify what is meant by "backend memory". As far as I can tell > from looking at the code, MADV_POPULATE_READ on file mappings will > allocate zeroed memory in the page cache, and map it as readonly pages > into userspace, but any attempt to actually write to that memory will > trigger the filesystem's ->page_mkwrite handler; and e.g. ext4 will > only try to allocate disk blocks at that point, which may fail. So as > far as I can tell, for files on filesystems like ext4, the current > implementation of MADV_POPULATE_READ does not replace fallocate(). Am > I missing something? Thanks for pointing that out, I guess I was blinded by tmpfs/hugetlbfs behavior. There might be cases (!tmpfs, !hugetlbfs) where we indeed need fallocate()+MADV_POPULATE_READ on file mappings. The logic is essentially what mlock()/MAP_POPULATE does via populate_vma_page_range() on shared mappings, so I assumed it would always properly allocate backend memory. /* * We want to touch writable mappings with a write fault in order * to break COW, except for shared mappings because these don't COW * and we would not want to dirty them for nothing. */ My tests with MADV_POPULATE_READ: 1. MAP_SHARED on tmpfs: memory in the file is allocated 2. MAP_PRIVATE on tmpfs: memory in the file is allocated 3. MAP_SHARED on hugetlbfs: memory in the file is allocated 4. MAP_PRIVATE on hugetlbfs: memory in the file is *not* allocated 5. MAP_SHARED on ext4: memory in the file is *not* allocated 6. MAP_PRIVATE on ext4: memory in the file is *not* allocated 1..4 are also the reason why it works with memfd as expected. For 4 and 6 it's not bad: writing to the private mapping will not result in backend storage/blocks having to get allocated. So the backend storage is actually RAM (although we don't allocate backend storage here but use the shared zero page, but that's a different story). For 5. we indeed need fallocate() before MADV_POPULATE_READ in case we could have holes. Thanks for pointing that out. > > If the desired semantics are that disk blocks should be preallocated, > I think you may have to look up the ->vm_file and then internally call > vfs_fallocate() to address this, kinda like in madvise_remove()? Does not sound too complicated, but devil might be in the details. At least for MAP_SHARED this might be the right thing to do. As discussed above, for MAP_PRIVATE we usually don't want to do that (and SHMEM is just weird). I honestly do wonder if breaking with MAP_POPULATE semantics is beneficial. For my use cases, doing fallocate() plus MADV_POPULATE_READ on shared, file-backed mappings would certainly be sufficient. But having a simple, consistent behavior would be much nicer. I'll give it a thought! >> 2. If MADV_POPULATE_READ succeeds, all page tables have been populated >> (prefaulted) readable once. >> 3. MADV_POPULATE_WRITE can be used to preallocate backend memory and >> prefault page tables just like manually writing (or >> reading+writing) each individual page. This will break any COW >> mappings -- e.g., the shared zeropage is never populated. >> 4. If MADV_POPULATE_WRITE succeeds, all page tables have been populated >> (prefaulted) writable once. >> 5. MADV_POPULATE_READ and MADV_POPULATE_WRITE cannot be applied to special >> mappings marked with VM_PFNMAP and VM_IO. Also, proper access >> permissions (e.g., PROT_READ, PROT_WRITE) are required. If any such >> mapping is encountered, madvise() fails with -EINVAL. >> 6. If MADV_POPULATE_READ or MADV_POPULATE_WRITE fails, some page tables >> might have been populated. In that case, madvise() fails with >> -ENOMEM. > > AFAICS that's not true (or misphrased). If MADV_POPULATE_* > successfully populates a bunch of pages, then fails because of an > error (e.g. EHWPOISON), it will return EHWPOISON, not ENOMEM, right? Indeed, leftover from previous version. It's clearer in the man page I prepared, will fix it up. > >> 7. MADV_POPULATE_READ and MADV_POPULATE_WRITE will return -EHWPOISON >> when encountering a HW poisoned page in the range. >> 8. Similar to MAP_POPULATE, MADV_POPULATE_READ and MADV_POPULATE_WRITE >> cannot protect from the OOM (Out Of Memory) handler killing the >> process. >> >> While the use case for MADV_POPULATE_WRITE is fairly obvious (i.e., >> preallocate memory and prefault page tables for VMs), there are valid use >> cases for MADV_POPULATE_READ: >> 1. Efficiently populate page tables with zero pages (i.e., shared >> zeropage). This is necessary when using userfaultfd() WP (Write-Protect >> to properly catch all modifications within a mapping: for >> write-protection to be effective for a virtual address, there has to be >> a page already mapped -- even if it's the shared zeropage. > > This sounds like a hack to work around issues that would be better > addressed by improving userfaultfd? There are plans to do that, indeed. > >> 2. Pre-read a whole mapping from backend storage without marking it >> dirty, such that eviction won't have to write it back. If no backend >> memory has been allocated yet, allocate the backend memory. Helpful >> when preallocating/prefaulting a file stored on disk without having >> to writeback each and every page on eviction. > > This sounds reasonable to me. Yes, the case with holes / backend memory has to be clarified. > >> Although sparse memory mappings are the primary use case, this will >> also be useful for ordinary preallocations where MAP_POPULATE is not >> desired especially in QEMU, where users can trigger preallocation of >> guest RAM after the mapping was created. >> >> Looking at the history, MADV_POPULATE was already proposed in 2013 [1], >> however, the main motivation back than was performance improvements >> (which should also still be the case, but it is a secondary concern). >> >> V. Single-threaded performance comparison >> >> There is a performance benefit when using POPULATE_READ / POPULATE_WRITE >> already when only using a single thread to do prefaulting/preallocation. As >> we have less pagefaults for huge pages, the performance benefit is >> negligible with small mappings. > [...] >> diff --git a/mm/gup.c b/mm/gup.c > [...] >> +long faultin_vma_page_range(struct vm_area_struct *vma, unsigned long start, >> + unsigned long end, bool write, int *locked) >> +{ >> + struct mm_struct *mm = vma->vm_mm; >> + unsigned long nr_pages = (end - start) / PAGE_SIZE; >> + int gup_flags; >> + >> + VM_BUG_ON(!PAGE_ALIGNED(start)); >> + VM_BUG_ON(!PAGE_ALIGNED(end)); >> + VM_BUG_ON_VMA(start < vma->vm_start, vma); >> + VM_BUG_ON_VMA(end > vma->vm_end, vma); >> + mmap_assert_locked(mm); >> + >> + /* >> + * FOLL_HWPOISON: Return -EHWPOISON instead of -EFAULT when we hit >> + * a poisoned page. >> + * FOLL_POPULATE: Always populate memory with VM_LOCKONFAULT. >> + * !FOLL_FORCE: Require proper access permissions. >> + */ >> + gup_flags = FOLL_TOUCH | FOLL_POPULATE | FOLL_MLOCK | FOLL_HWPOISON; >> + if (write) >> + gup_flags |= FOLL_WRITE; >> + >> + /* >> + * See check_vma_flags(): Will return -EFAULT on incompatible mappings >> + * or with insufficient permissions. >> + */ >> + return __get_user_pages(mm, start, nr_pages, gup_flags, >> + NULL, NULL, locked); > > You mentioned in the commit message that you don't want to actually > dirty all the file pages and force writeback; but doesn't > POPULATE_WRITE still do exactly that? In follow_page_pte(), if > FOLL_TOUCH and FOLL_WRITE are set, we mark the page as dirty: Well, I mention that POPULATE_READ explicitly doesn't do that. I primarily set it because populate_vma_page_range() also sets it. Is it safe to *not* set it? IOW, fault something writable into a page table (where the CPU could dirty it without additional page faults) without marking it accessed? For me, this made logically sense. Thus I also understood why populate_vma_page_range() set it. > > if (flags & FOLL_TOUCH) { > if ((flags & FOLL_WRITE) && > !pte_dirty(pte) && !PageDirty(page)) > set_page_dirty(page); > /* > * pte_mkyoung() would be more correct here, but atomic care > * is needed to avoid losing the dirty bit: it is easier to use > * mark_page_accessed(). > */ > mark_page_accessed(page); > } > > >> +} >> + >> /* >> * __mm_populate - populate and/or mlock pages within a range of address space. >> * >> diff --git a/mm/internal.h b/mm/internal.h >> index 3f22c4ceb7b5..ee398696380f 100644 >> --- a/mm/internal.h >> +++ b/mm/internal.h >> @@ -335,6 +335,9 @@ void __vma_unlink_list(struct mm_struct *mm, struct vm_area_struct *vma); >> #ifdef CONFIG_MMU >> extern long populate_vma_page_range(struct vm_area_struct *vma, >> unsigned long start, unsigned long end, int *locked); >> +extern long faultin_vma_page_range(struct vm_area_struct *vma, >> + unsigned long start, unsigned long end, >> + bool write, int *locked); >> extern void munlock_vma_pages_range(struct vm_area_struct *vma, >> unsigned long start, unsigned long end); >> static inline void munlock_vma_pages_all(struct vm_area_struct *vma) >> diff --git a/mm/madvise.c b/mm/madvise.c >> index 01fef79ac761..857460873f7a 100644 >> --- a/mm/madvise.c >> +++ b/mm/madvise.c >> @@ -53,6 +53,8 @@ static int madvise_need_mmap_write(int behavior) >> case MADV_COLD: >> case MADV_PAGEOUT: >> case MADV_FREE: >> + case MADV_POPULATE_READ: >> + case MADV_POPULATE_WRITE: >> return 0; >> default: >> /* be safe, default to 1. list exceptions explicitly */ >> @@ -822,6 +824,64 @@ static long madvise_dontneed_free(struct vm_area_struct *vma, >> return -EINVAL; >> } >> >> +static long madvise_populate(struct vm_area_struct *vma, >> + struct vm_area_struct **prev, >> + unsigned long start, unsigned long end, >> + int behavior) >> +{ >> + const bool write = behavior == MADV_POPULATE_WRITE; >> + struct mm_struct *mm = vma->vm_mm; >> + unsigned long tmp_end; >> + int locked = 1; >> + long pages; >> + >> + *prev = vma; >> + >> + while (start < end) { >> + /* >> + * We might have temporarily dropped the lock. For example, >> + * our VMA might have been split. >> + */ >> + if (!vma || start >= vma->vm_end) { >> + vma = find_vma(mm, start); >> + if (!vma || start < vma->vm_start) >> + return -ENOMEM; >> + } >> + >> + tmp_end = min_t(unsigned long, end, vma->vm_end); >> + /* Populate (prefault) page tables readable/writable. */ >> + pages = faultin_vma_page_range(vma, start, tmp_end, write, >> + &locked); >> + if (!locked) { >> + mmap_read_lock(mm); >> + locked = 1; >> + *prev = NULL; >> + vma = NULL; >> + } >> + if (pages < 0) { >> + switch (pages) { >> + case -EINTR: >> + return -EINTR; >> + case -EFAULT: /* Incompatible mappings / permissions. */ >> + return -EINVAL; >> + case -EHWPOISON: >> + return -EHWPOISON; >> + case -EBUSY: > > What is -EBUSY doing here? __get_user_pages() fixes up -EBUSY from > faultin_page() to 0, right? > >> + case -EAGAIN: > > Where can -EAGAIN come from? On both points: the lack of documentation on return values made me add these. The faultin_page() path is indeed fine. If the other paths don't yield any such return values, we can drop both. Thanks for the review!
On Tue, Mar 30, 2021 at 5:01 PM David Hildenbrand <david@redhat.com> wrote: > >> +long faultin_vma_page_range(struct vm_area_struct *vma, unsigned long start, > >> + unsigned long end, bool write, int *locked) > >> +{ > >> + struct mm_struct *mm = vma->vm_mm; > >> + unsigned long nr_pages = (end - start) / PAGE_SIZE; > >> + int gup_flags; > >> + > >> + VM_BUG_ON(!PAGE_ALIGNED(start)); > >> + VM_BUG_ON(!PAGE_ALIGNED(end)); > >> + VM_BUG_ON_VMA(start < vma->vm_start, vma); > >> + VM_BUG_ON_VMA(end > vma->vm_end, vma); > >> + mmap_assert_locked(mm); > >> + > >> + /* > >> + * FOLL_HWPOISON: Return -EHWPOISON instead of -EFAULT when we hit > >> + * a poisoned page. > >> + * FOLL_POPULATE: Always populate memory with VM_LOCKONFAULT. > >> + * !FOLL_FORCE: Require proper access permissions. > >> + */ > >> + gup_flags = FOLL_TOUCH | FOLL_POPULATE | FOLL_MLOCK | FOLL_HWPOISON; > >> + if (write) > >> + gup_flags |= FOLL_WRITE; > >> + > >> + /* > >> + * See check_vma_flags(): Will return -EFAULT on incompatible mappings > >> + * or with insufficient permissions. > >> + */ > >> + return __get_user_pages(mm, start, nr_pages, gup_flags, > >> + NULL, NULL, locked); > > > > You mentioned in the commit message that you don't want to actually > > dirty all the file pages and force writeback; but doesn't > > POPULATE_WRITE still do exactly that? In follow_page_pte(), if > > FOLL_TOUCH and FOLL_WRITE are set, we mark the page as dirty: > > Well, I mention that POPULATE_READ explicitly doesn't do that. I > primarily set it because populate_vma_page_range() also sets it. > > Is it safe to *not* set it? IOW, fault something writable into a page > table (where the CPU could dirty it without additional page faults) > without marking it accessed? For me, this made logically sense. Thus I > also understood why populate_vma_page_range() set it. FOLL_TOUCH doesn't have anything to do with installing the PTE - it essentially means "the caller of get_user_pages wants to read/write the contents of the returned page, so please do the same things you would do if userspace was accessing the page". So in particular, if you look up a page via get_user_pages() with FOLL_WRITE|FOLL_TOUCH, that tells the MM subsystem "I will be writing into this page directly from the kernel, bypassing the userspace page tables, so please mark it as dirty now so that it will be properly written back later". Part of that is that it marks the page as recently used, which has an effect on LRU pageout behavior, I think - as far as I understand, that is why populate_vma_page_range() uses FOLL_TOUCH. If you look at __get_user_pages(), you can see that it is split up into two major parts: faultin_page() for creating PTEs, and follow_page_mask() for grabbing pages from PTEs. faultin_page() ignores FOLL_TOUCH completely; only follow_page_mask() uses it. In a way I guess maybe you do want the "mark as recently accessed" part that FOLL_TOUCH would give you without FOLL_WRITE? But I think you very much don't want the dirtying that FOLL_TOUCH|FOLL_WRITE leads to. Maybe the ideal approach would be to add a new FOLL flag to say "I only want to mark as recently used, I don't want to dirty". Or maybe it's enough to just leave out the FOLL_TOUCH entirely, I don't know.
On 30.03.21 18:21, Jann Horn wrote: > On Tue, Mar 30, 2021 at 5:01 PM David Hildenbrand <david@redhat.com> wrote: >>>> +long faultin_vma_page_range(struct vm_area_struct *vma, unsigned long start, >>>> + unsigned long end, bool write, int *locked) >>>> +{ >>>> + struct mm_struct *mm = vma->vm_mm; >>>> + unsigned long nr_pages = (end - start) / PAGE_SIZE; >>>> + int gup_flags; >>>> + >>>> + VM_BUG_ON(!PAGE_ALIGNED(start)); >>>> + VM_BUG_ON(!PAGE_ALIGNED(end)); >>>> + VM_BUG_ON_VMA(start < vma->vm_start, vma); >>>> + VM_BUG_ON_VMA(end > vma->vm_end, vma); >>>> + mmap_assert_locked(mm); >>>> + >>>> + /* >>>> + * FOLL_HWPOISON: Return -EHWPOISON instead of -EFAULT when we hit >>>> + * a poisoned page. >>>> + * FOLL_POPULATE: Always populate memory with VM_LOCKONFAULT. >>>> + * !FOLL_FORCE: Require proper access permissions. >>>> + */ >>>> + gup_flags = FOLL_TOUCH | FOLL_POPULATE | FOLL_MLOCK | FOLL_HWPOISON; >>>> + if (write) >>>> + gup_flags |= FOLL_WRITE; >>>> + >>>> + /* >>>> + * See check_vma_flags(): Will return -EFAULT on incompatible mappings >>>> + * or with insufficient permissions. >>>> + */ >>>> + return __get_user_pages(mm, start, nr_pages, gup_flags, >>>> + NULL, NULL, locked); >>> >>> You mentioned in the commit message that you don't want to actually >>> dirty all the file pages and force writeback; but doesn't >>> POPULATE_WRITE still do exactly that? In follow_page_pte(), if >>> FOLL_TOUCH and FOLL_WRITE are set, we mark the page as dirty: >> >> Well, I mention that POPULATE_READ explicitly doesn't do that. I >> primarily set it because populate_vma_page_range() also sets it. >> >> Is it safe to *not* set it? IOW, fault something writable into a page >> table (where the CPU could dirty it without additional page faults) >> without marking it accessed? For me, this made logically sense. Thus I >> also understood why populate_vma_page_range() set it. > > FOLL_TOUCH doesn't have anything to do with installing the PTE - it > essentially means "the caller of get_user_pages wants to read/write > the contents of the returned page, so please do the same things you > would do if userspace was accessing the page". So in particular, if > you look up a page via get_user_pages() with FOLL_WRITE|FOLL_TOUCH, > that tells the MM subsystem "I will be writing into this page directly > from the kernel, bypassing the userspace page tables, so please mark > it as dirty now so that it will be properly written back later". Part > of that is that it marks the page as recently used, which has an > effect on LRU pageout behavior, I think - as far as I understand, that > is why populate_vma_page_range() uses FOLL_TOUCH. > > If you look at __get_user_pages(), you can see that it is split up > into two major parts: faultin_page() for creating PTEs, and > follow_page_mask() for grabbing pages from PTEs. faultin_page() > ignores FOLL_TOUCH completely; only follow_page_mask() uses it. > > In a way I guess maybe you do want the "mark as recently accessed" > part that FOLL_TOUCH would give you without FOLL_WRITE? But I think > you very much don't want the dirtying that FOLL_TOUCH|FOLL_WRITE leads > to. Maybe the ideal approach would be to add a new FOLL flag to say "I > only want to mark as recently used, I don't want to dirty". Or maybe > it's enough to just leave out the FOLL_TOUCH entirely, I don't know. Any thoughts why populate_vma_page_range() does it?
On 30.03.21 18:30, David Hildenbrand wrote: > On 30.03.21 18:21, Jann Horn wrote: >> On Tue, Mar 30, 2021 at 5:01 PM David Hildenbrand <david@redhat.com> wrote: >>>>> +long faultin_vma_page_range(struct vm_area_struct *vma, unsigned long start, >>>>> + unsigned long end, bool write, int *locked) >>>>> +{ >>>>> + struct mm_struct *mm = vma->vm_mm; >>>>> + unsigned long nr_pages = (end - start) / PAGE_SIZE; >>>>> + int gup_flags; >>>>> + >>>>> + VM_BUG_ON(!PAGE_ALIGNED(start)); >>>>> + VM_BUG_ON(!PAGE_ALIGNED(end)); >>>>> + VM_BUG_ON_VMA(start < vma->vm_start, vma); >>>>> + VM_BUG_ON_VMA(end > vma->vm_end, vma); >>>>> + mmap_assert_locked(mm); >>>>> + >>>>> + /* >>>>> + * FOLL_HWPOISON: Return -EHWPOISON instead of -EFAULT when we hit >>>>> + * a poisoned page. >>>>> + * FOLL_POPULATE: Always populate memory with VM_LOCKONFAULT. >>>>> + * !FOLL_FORCE: Require proper access permissions. >>>>> + */ >>>>> + gup_flags = FOLL_TOUCH | FOLL_POPULATE | FOLL_MLOCK | FOLL_HWPOISON; >>>>> + if (write) >>>>> + gup_flags |= FOLL_WRITE; >>>>> + >>>>> + /* >>>>> + * See check_vma_flags(): Will return -EFAULT on incompatible mappings >>>>> + * or with insufficient permissions. >>>>> + */ >>>>> + return __get_user_pages(mm, start, nr_pages, gup_flags, >>>>> + NULL, NULL, locked); >>>> >>>> You mentioned in the commit message that you don't want to actually >>>> dirty all the file pages and force writeback; but doesn't >>>> POPULATE_WRITE still do exactly that? In follow_page_pte(), if >>>> FOLL_TOUCH and FOLL_WRITE are set, we mark the page as dirty: >>> >>> Well, I mention that POPULATE_READ explicitly doesn't do that. I >>> primarily set it because populate_vma_page_range() also sets it. >>> >>> Is it safe to *not* set it? IOW, fault something writable into a page >>> table (where the CPU could dirty it without additional page faults) >>> without marking it accessed? For me, this made logically sense. Thus I >>> also understood why populate_vma_page_range() set it. >> >> FOLL_TOUCH doesn't have anything to do with installing the PTE - it >> essentially means "the caller of get_user_pages wants to read/write >> the contents of the returned page, so please do the same things you >> would do if userspace was accessing the page". So in particular, if >> you look up a page via get_user_pages() with FOLL_WRITE|FOLL_TOUCH, >> that tells the MM subsystem "I will be writing into this page directly >> from the kernel, bypassing the userspace page tables, so please mark >> it as dirty now so that it will be properly written back later". Part >> of that is that it marks the page as recently used, which has an >> effect on LRU pageout behavior, I think - as far as I understand, that >> is why populate_vma_page_range() uses FOLL_TOUCH. >> >> If you look at __get_user_pages(), you can see that it is split up >> into two major parts: faultin_page() for creating PTEs, and >> follow_page_mask() for grabbing pages from PTEs. faultin_page() >> ignores FOLL_TOUCH completely; only follow_page_mask() uses it. >> >> In a way I guess maybe you do want the "mark as recently accessed" >> part that FOLL_TOUCH would give you without FOLL_WRITE? But I think >> you very much don't want the dirtying that FOLL_TOUCH|FOLL_WRITE leads >> to. Maybe the ideal approach would be to add a new FOLL flag to say "I >> only want to mark as recently used, I don't want to dirty". Or maybe >> it's enough to just leave out the FOLL_TOUCH entirely, I don't know. > > Any thoughts why populate_vma_page_range() does it? Sorry, I missed the explanation above - thanks!
On 30.03.21 18:31, David Hildenbrand wrote: > On 30.03.21 18:30, David Hildenbrand wrote: >> On 30.03.21 18:21, Jann Horn wrote: >>> On Tue, Mar 30, 2021 at 5:01 PM David Hildenbrand <david@redhat.com> wrote: >>>>>> +long faultin_vma_page_range(struct vm_area_struct *vma, unsigned long start, >>>>>> + unsigned long end, bool write, int *locked) >>>>>> +{ >>>>>> + struct mm_struct *mm = vma->vm_mm; >>>>>> + unsigned long nr_pages = (end - start) / PAGE_SIZE; >>>>>> + int gup_flags; >>>>>> + >>>>>> + VM_BUG_ON(!PAGE_ALIGNED(start)); >>>>>> + VM_BUG_ON(!PAGE_ALIGNED(end)); >>>>>> + VM_BUG_ON_VMA(start < vma->vm_start, vma); >>>>>> + VM_BUG_ON_VMA(end > vma->vm_end, vma); >>>>>> + mmap_assert_locked(mm); >>>>>> + >>>>>> + /* >>>>>> + * FOLL_HWPOISON: Return -EHWPOISON instead of -EFAULT when we hit >>>>>> + * a poisoned page. >>>>>> + * FOLL_POPULATE: Always populate memory with VM_LOCKONFAULT. >>>>>> + * !FOLL_FORCE: Require proper access permissions. >>>>>> + */ >>>>>> + gup_flags = FOLL_TOUCH | FOLL_POPULATE | FOLL_MLOCK | FOLL_HWPOISON; >>>>>> + if (write) >>>>>> + gup_flags |= FOLL_WRITE; >>>>>> + >>>>>> + /* >>>>>> + * See check_vma_flags(): Will return -EFAULT on incompatible mappings >>>>>> + * or with insufficient permissions. >>>>>> + */ >>>>>> + return __get_user_pages(mm, start, nr_pages, gup_flags, >>>>>> + NULL, NULL, locked); >>>>> >>>>> You mentioned in the commit message that you don't want to actually >>>>> dirty all the file pages and force writeback; but doesn't >>>>> POPULATE_WRITE still do exactly that? In follow_page_pte(), if >>>>> FOLL_TOUCH and FOLL_WRITE are set, we mark the page as dirty: >>>> >>>> Well, I mention that POPULATE_READ explicitly doesn't do that. I >>>> primarily set it because populate_vma_page_range() also sets it. >>>> >>>> Is it safe to *not* set it? IOW, fault something writable into a page >>>> table (where the CPU could dirty it without additional page faults) >>>> without marking it accessed? For me, this made logically sense. Thus I >>>> also understood why populate_vma_page_range() set it. >>> >>> FOLL_TOUCH doesn't have anything to do with installing the PTE - it >>> essentially means "the caller of get_user_pages wants to read/write >>> the contents of the returned page, so please do the same things you >>> would do if userspace was accessing the page". So in particular, if >>> you look up a page via get_user_pages() with FOLL_WRITE|FOLL_TOUCH, >>> that tells the MM subsystem "I will be writing into this page directly >>> from the kernel, bypassing the userspace page tables, so please mark >>> it as dirty now so that it will be properly written back later". Part >>> of that is that it marks the page as recently used, which has an >>> effect on LRU pageout behavior, I think - as far as I understand, that >>> is why populate_vma_page_range() uses FOLL_TOUCH. >>> >>> If you look at __get_user_pages(), you can see that it is split up >>> into two major parts: faultin_page() for creating PTEs, and >>> follow_page_mask() for grabbing pages from PTEs. faultin_page() >>> ignores FOLL_TOUCH completely; only follow_page_mask() uses it. >>> >>> In a way I guess maybe you do want the "mark as recently accessed" >>> part that FOLL_TOUCH would give you without FOLL_WRITE? But I think >>> you very much don't want the dirtying that FOLL_TOUCH|FOLL_WRITE leads >>> to. Maybe the ideal approach would be to add a new FOLL flag to say "I >>> only want to mark as recently used, I don't want to dirty". Or maybe >>> it's enough to just leave out the FOLL_TOUCH entirely, I don't know. >> >> Any thoughts why populate_vma_page_range() does it? > > Sorry, I missed the explanation above - thanks! Looking into the details, adjusting the FOLL_TOUCH logic won't make too much of a difference for MADV_POPULATE_WRITE I guess. AFAIKs, the biggest impact of FOLL_TOUCH is actually with FOLL_FORCE - which we are not using, but populate_vma_page_range() is. If a page was not faulted in yet, faultin_page(FOLL_WRITE)->handle_mm_fault(FAULT_FLAG_WRITE) will already mark the PTE/PMD/... dirty and accessed. One example is handle_pte_fault(). We will mark the page accessed again via FOLL_TOUCH, which doesn't seem to be strictly required. If the page was already faulted in, we have three cases: 1. Page faulted in writable. The page should already be dirty (otherwise we would be in trouble I guess). We will mark it accessed. 2. Page faulted in readable. handle_mm_fault() will fault it in writable and set the page dirty. 3. Page faulted in readable and we have FOLL_FORCE. We mark the page dirty and accessed. So doing a MADV_POPULATE_WRITE, whereby we prefault page tables writable, doesn't seem to fly without marking the pages dirty. That's one reason why I included MADV_POPULATE_READ. We could a) Drop FOLL_TOUCH. We are not marking the page accessed, which would mean it gets evicted rather earlier than later. b) Introduce FOLL_ACCESSED which won't do the dirtying. But then, the pages are already dirty as explained above, so there isn't a real observable change. c) Keep it as is: Mark the page accessed and dirty. As it's already dirty, that does not seem to be a real issue. Am I missing something obvious? Thanks!
On 07.04.21 12:31, David Hildenbrand wrote: > On 30.03.21 18:31, David Hildenbrand wrote: >> On 30.03.21 18:30, David Hildenbrand wrote: >>> On 30.03.21 18:21, Jann Horn wrote: >>>> On Tue, Mar 30, 2021 at 5:01 PM David Hildenbrand <david@redhat.com> wrote: >>>>>>> +long faultin_vma_page_range(struct vm_area_struct *vma, unsigned long start, >>>>>>> + unsigned long end, bool write, int *locked) >>>>>>> +{ >>>>>>> + struct mm_struct *mm = vma->vm_mm; >>>>>>> + unsigned long nr_pages = (end - start) / PAGE_SIZE; >>>>>>> + int gup_flags; >>>>>>> + >>>>>>> + VM_BUG_ON(!PAGE_ALIGNED(start)); >>>>>>> + VM_BUG_ON(!PAGE_ALIGNED(end)); >>>>>>> + VM_BUG_ON_VMA(start < vma->vm_start, vma); >>>>>>> + VM_BUG_ON_VMA(end > vma->vm_end, vma); >>>>>>> + mmap_assert_locked(mm); >>>>>>> + >>>>>>> + /* >>>>>>> + * FOLL_HWPOISON: Return -EHWPOISON instead of -EFAULT when we hit >>>>>>> + * a poisoned page. >>>>>>> + * FOLL_POPULATE: Always populate memory with VM_LOCKONFAULT. >>>>>>> + * !FOLL_FORCE: Require proper access permissions. >>>>>>> + */ >>>>>>> + gup_flags = FOLL_TOUCH | FOLL_POPULATE | FOLL_MLOCK | FOLL_HWPOISON; >>>>>>> + if (write) >>>>>>> + gup_flags |= FOLL_WRITE; >>>>>>> + >>>>>>> + /* >>>>>>> + * See check_vma_flags(): Will return -EFAULT on incompatible mappings >>>>>>> + * or with insufficient permissions. >>>>>>> + */ >>>>>>> + return __get_user_pages(mm, start, nr_pages, gup_flags, >>>>>>> + NULL, NULL, locked); >>>>>> >>>>>> You mentioned in the commit message that you don't want to actually >>>>>> dirty all the file pages and force writeback; but doesn't >>>>>> POPULATE_WRITE still do exactly that? In follow_page_pte(), if >>>>>> FOLL_TOUCH and FOLL_WRITE are set, we mark the page as dirty: >>>>> >>>>> Well, I mention that POPULATE_READ explicitly doesn't do that. I >>>>> primarily set it because populate_vma_page_range() also sets it. >>>>> >>>>> Is it safe to *not* set it? IOW, fault something writable into a page >>>>> table (where the CPU could dirty it without additional page faults) >>>>> without marking it accessed? For me, this made logically sense. Thus I >>>>> also understood why populate_vma_page_range() set it. >>>> >>>> FOLL_TOUCH doesn't have anything to do with installing the PTE - it >>>> essentially means "the caller of get_user_pages wants to read/write >>>> the contents of the returned page, so please do the same things you >>>> would do if userspace was accessing the page". So in particular, if >>>> you look up a page via get_user_pages() with FOLL_WRITE|FOLL_TOUCH, >>>> that tells the MM subsystem "I will be writing into this page directly >>>> from the kernel, bypassing the userspace page tables, so please mark >>>> it as dirty now so that it will be properly written back later". Part >>>> of that is that it marks the page as recently used, which has an >>>> effect on LRU pageout behavior, I think - as far as I understand, that >>>> is why populate_vma_page_range() uses FOLL_TOUCH. >>>> >>>> If you look at __get_user_pages(), you can see that it is split up >>>> into two major parts: faultin_page() for creating PTEs, and >>>> follow_page_mask() for grabbing pages from PTEs. faultin_page() >>>> ignores FOLL_TOUCH completely; only follow_page_mask() uses it. >>>> >>>> In a way I guess maybe you do want the "mark as recently accessed" >>>> part that FOLL_TOUCH would give you without FOLL_WRITE? But I think >>>> you very much don't want the dirtying that FOLL_TOUCH|FOLL_WRITE leads >>>> to. Maybe the ideal approach would be to add a new FOLL flag to say "I >>>> only want to mark as recently used, I don't want to dirty". Or maybe >>>> it's enough to just leave out the FOLL_TOUCH entirely, I don't know. >>> >>> Any thoughts why populate_vma_page_range() does it? >> >> Sorry, I missed the explanation above - thanks! > > Looking into the details, adjusting the FOLL_TOUCH logic won't make too > much of a difference for MADV_POPULATE_WRITE I guess. AFAIKs, the > biggest impact of FOLL_TOUCH is actually with FOLL_FORCE - which we are > not using, but populate_vma_page_range() is. > > > If a page was not faulted in yet, > faultin_page(FOLL_WRITE)->handle_mm_fault(FAULT_FLAG_WRITE) will already > mark the PTE/PMD/... dirty and accessed. One example is > handle_pte_fault(). We will mark the page accessed again via FOLL_TOUCH, > which doesn't seem to be strictly required. > > > If the page was already faulted in, we have three cases: > > 1. Page faulted in writable. The page should already be dirty (otherwise > we would be in trouble I guess). We will mark it accessed. > > 2. Page faulted in readable. handle_mm_fault() will fault it in writable > and set the page dirty. > > 3. Page faulted in readable and we have FOLL_FORCE. We mark the page > dirty and accessed. > > > So doing a MADV_POPULATE_WRITE, whereby we prefault page tables > writable, doesn't seem to fly without marking the pages dirty. That's > one reason why I included MADV_POPULATE_READ. > > We could > > a) Drop FOLL_TOUCH. We are not marking the page accessed, which would > mean it gets evicted rather earlier than later. > > b) Introduce FOLL_ACCESSED which won't do the dirtying. But then, the > pages are already dirty as explained above, so there isn't a real > observable change. > > c) Keep it as is: Mark the page accessed and dirty. As it's already > dirty, that does not seem to be a real issue. > > Am I missing something obvious? Thanks! > I did some more digging. I think there are cases for shared mappings where we can have pte_write() but not pte_dirty(). One example seems to be mm/memory.c:copy_present_pte() , used during fork. IIUC, this means that the child process can write to these pages, but won't mark the PTEs dirty -- as there won't be a write fault. I'd assume we'd need pte_mkclean(pte_wrprotect(pte)), but I'm fairly new to that code. (Similarly, we do an pte_mkold() without revoking any protection, meaning we won't catch read accesses.) Maybe the logic here is that if the PTE was writable in some parent, it was also dirty in some parent (at least in the one originally mapping it writable). When evicting/writeback'ing file pages, we'll have go over the rmap and zap all entries of any page tables either way; it's sufficient if one PTE entry is dirty. I can spot that even zap_pte_range() will sync the dirty flag back to the page. Am I right or is there some other magic going on? :)
diff --git a/arch/alpha/include/uapi/asm/mman.h b/arch/alpha/include/uapi/asm/mman.h index a18ec7f63888..56b4ee5a6c9e 100644 --- a/arch/alpha/include/uapi/asm/mman.h +++ b/arch/alpha/include/uapi/asm/mman.h @@ -71,6 +71,9 @@ #define MADV_COLD 20 /* deactivate these pages */ #define MADV_PAGEOUT 21 /* reclaim these pages */ +#define MADV_POPULATE_READ 22 /* populate (prefault) page tables readable */ +#define MADV_POPULATE_WRITE 23 /* populate (prefault) page tables writable */ + /* compatibility flags */ #define MAP_FILE 0 diff --git a/arch/mips/include/uapi/asm/mman.h b/arch/mips/include/uapi/asm/mman.h index 57dc2ac4f8bd..40b210c65a5a 100644 --- a/arch/mips/include/uapi/asm/mman.h +++ b/arch/mips/include/uapi/asm/mman.h @@ -98,6 +98,9 @@ #define MADV_COLD 20 /* deactivate these pages */ #define MADV_PAGEOUT 21 /* reclaim these pages */ +#define MADV_POPULATE_READ 22 /* populate (prefault) page tables readable */ +#define MADV_POPULATE_WRITE 23 /* populate (prefault) page tables writable */ + /* compatibility flags */ #define MAP_FILE 0 diff --git a/arch/parisc/include/uapi/asm/mman.h b/arch/parisc/include/uapi/asm/mman.h index ab78cba446ed..9e3c010c0f61 100644 --- a/arch/parisc/include/uapi/asm/mman.h +++ b/arch/parisc/include/uapi/asm/mman.h @@ -52,6 +52,9 @@ #define MADV_COLD 20 /* deactivate these pages */ #define MADV_PAGEOUT 21 /* reclaim these pages */ +#define MADV_POPULATE_READ 22 /* populate (prefault) page tables readable */ +#define MADV_POPULATE_WRITE 23 /* populate (prefault) page tables writable */ + #define MADV_MERGEABLE 65 /* KSM may merge identical pages */ #define MADV_UNMERGEABLE 66 /* KSM may not merge identical pages */ diff --git a/arch/xtensa/include/uapi/asm/mman.h b/arch/xtensa/include/uapi/asm/mman.h index e5e643752947..b3a22095371b 100644 --- a/arch/xtensa/include/uapi/asm/mman.h +++ b/arch/xtensa/include/uapi/asm/mman.h @@ -106,6 +106,9 @@ #define MADV_COLD 20 /* deactivate these pages */ #define MADV_PAGEOUT 21 /* reclaim these pages */ +#define MADV_POPULATE_READ 22 /* populate (prefault) page tables readable */ +#define MADV_POPULATE_WRITE 23 /* populate (prefault) page tables writable */ + /* compatibility flags */ #define MAP_FILE 0 diff --git a/include/uapi/asm-generic/mman-common.h b/include/uapi/asm-generic/mman-common.h index f94f65d429be..1567a3294c3d 100644 --- a/include/uapi/asm-generic/mman-common.h +++ b/include/uapi/asm-generic/mman-common.h @@ -72,6 +72,9 @@ #define MADV_COLD 20 /* deactivate these pages */ #define MADV_PAGEOUT 21 /* reclaim these pages */ +#define MADV_POPULATE_READ 22 /* populate (prefault) page tables readable */ +#define MADV_POPULATE_WRITE 23 /* populate (prefault) page tables writable */ + /* compatibility flags */ #define MAP_FILE 0 diff --git a/mm/gup.c b/mm/gup.c index e40579624f10..80fad8578066 100644 --- a/mm/gup.c +++ b/mm/gup.c @@ -1403,6 +1403,60 @@ long populate_vma_page_range(struct vm_area_struct *vma, NULL, NULL, locked); } +/* + * faultin_vma_page_range() - populate (prefault) page tables inside the + * given VMA range readable/writable + * + * This takes care of mlocking the pages, too, if VM_LOCKED is set. + * + * @vma: target vma + * @start: start address + * @end: end address + * @write: whether to prefault readable or writable + * @locked: whether the mmap_lock is still held + * + * Returns either number of processed pages in the vma, or a negative error + * code on error (see __get_user_pages()). + * + * vma->vm_mm->mmap_lock must be held. The range must be page-aligned and + * covered by the VMA. + * + * If @locked is NULL, it may be held for read or write and will be unperturbed. + * + * If @locked is non-NULL, it must held for read only and may be released. If + * it's released, *@locked will be set to 0. + */ +long faultin_vma_page_range(struct vm_area_struct *vma, unsigned long start, + unsigned long end, bool write, int *locked) +{ + struct mm_struct *mm = vma->vm_mm; + unsigned long nr_pages = (end - start) / PAGE_SIZE; + int gup_flags; + + VM_BUG_ON(!PAGE_ALIGNED(start)); + VM_BUG_ON(!PAGE_ALIGNED(end)); + VM_BUG_ON_VMA(start < vma->vm_start, vma); + VM_BUG_ON_VMA(end > vma->vm_end, vma); + mmap_assert_locked(mm); + + /* + * FOLL_HWPOISON: Return -EHWPOISON instead of -EFAULT when we hit + * a poisoned page. + * FOLL_POPULATE: Always populate memory with VM_LOCKONFAULT. + * !FOLL_FORCE: Require proper access permissions. + */ + gup_flags = FOLL_TOUCH | FOLL_POPULATE | FOLL_MLOCK | FOLL_HWPOISON; + if (write) + gup_flags |= FOLL_WRITE; + + /* + * See check_vma_flags(): Will return -EFAULT on incompatible mappings + * or with insufficient permissions. + */ + return __get_user_pages(mm, start, nr_pages, gup_flags, + NULL, NULL, locked); +} + /* * __mm_populate - populate and/or mlock pages within a range of address space. * diff --git a/mm/internal.h b/mm/internal.h index 3f22c4ceb7b5..ee398696380f 100644 --- a/mm/internal.h +++ b/mm/internal.h @@ -335,6 +335,9 @@ void __vma_unlink_list(struct mm_struct *mm, struct vm_area_struct *vma); #ifdef CONFIG_MMU extern long populate_vma_page_range(struct vm_area_struct *vma, unsigned long start, unsigned long end, int *locked); +extern long faultin_vma_page_range(struct vm_area_struct *vma, + unsigned long start, unsigned long end, + bool write, int *locked); extern void munlock_vma_pages_range(struct vm_area_struct *vma, unsigned long start, unsigned long end); static inline void munlock_vma_pages_all(struct vm_area_struct *vma) diff --git a/mm/madvise.c b/mm/madvise.c index 01fef79ac761..857460873f7a 100644 --- a/mm/madvise.c +++ b/mm/madvise.c @@ -53,6 +53,8 @@ static int madvise_need_mmap_write(int behavior) case MADV_COLD: case MADV_PAGEOUT: case MADV_FREE: + case MADV_POPULATE_READ: + case MADV_POPULATE_WRITE: return 0; default: /* be safe, default to 1. list exceptions explicitly */ @@ -822,6 +824,64 @@ static long madvise_dontneed_free(struct vm_area_struct *vma, return -EINVAL; } +static long madvise_populate(struct vm_area_struct *vma, + struct vm_area_struct **prev, + unsigned long start, unsigned long end, + int behavior) +{ + const bool write = behavior == MADV_POPULATE_WRITE; + struct mm_struct *mm = vma->vm_mm; + unsigned long tmp_end; + int locked = 1; + long pages; + + *prev = vma; + + while (start < end) { + /* + * We might have temporarily dropped the lock. For example, + * our VMA might have been split. + */ + if (!vma || start >= vma->vm_end) { + vma = find_vma(mm, start); + if (!vma || start < vma->vm_start) + return -ENOMEM; + } + + tmp_end = min_t(unsigned long, end, vma->vm_end); + /* Populate (prefault) page tables readable/writable. */ + pages = faultin_vma_page_range(vma, start, tmp_end, write, + &locked); + if (!locked) { + mmap_read_lock(mm); + locked = 1; + *prev = NULL; + vma = NULL; + } + if (pages < 0) { + switch (pages) { + case -EINTR: + return -EINTR; + case -EFAULT: /* Incompatible mappings / permissions. */ + return -EINVAL; + case -EHWPOISON: + return -EHWPOISON; + case -EBUSY: + case -EAGAIN: + continue; + default: + pr_warn_once("%s: unhandled return value: %ld\n", + __func__, pages); + fallthrough; + case -ENOMEM: + return -ENOMEM; + } + } + start += pages * PAGE_SIZE; + } + return 0; +} + /* * Application wants to free up the pages and associated backing store. * This is effectively punching a hole into the middle of a file. @@ -935,6 +995,9 @@ madvise_vma(struct vm_area_struct *vma, struct vm_area_struct **prev, case MADV_FREE: case MADV_DONTNEED: return madvise_dontneed_free(vma, prev, start, end, behavior); + case MADV_POPULATE_READ: + case MADV_POPULATE_WRITE: + return madvise_populate(vma, prev, start, end, behavior); default: return madvise_behavior(vma, prev, start, end, behavior); } @@ -955,6 +1018,8 @@ madvise_behavior_valid(int behavior) case MADV_FREE: case MADV_COLD: case MADV_PAGEOUT: + case MADV_POPULATE_READ: + case MADV_POPULATE_WRITE: #ifdef CONFIG_KSM case MADV_MERGEABLE: case MADV_UNMERGEABLE: @@ -1042,6 +1107,10 @@ process_madvise_behavior_valid(int behavior) * easily if memory pressure hanppens. * MADV_PAGEOUT - the application is not expected to use this memory soon, * page out the pages in this range immediately. + * MADV_POPULATE_READ - populate (prefault) page tables readable by + * triggering read faults if required + * MADV_POPULATE_WRITE - populate (prefault) page tables writable by + * triggering write faults if required * * return values: * zero - success
I. Background: Sparse Memory Mappings When we manage sparse memory mappings dynamically in user space - also sometimes involving MAP_NORESERVE - we want to dynamically populate/ discard memory inside such a sparse memory region. Example users are hypervisors (especially implementing memory ballooning or similar technologies like virtio-mem) and memory allocators. In addition, we want to fail in a nice way (instead of generating SIGBUS) if populating does not succeed because we are out of backend memory (which can happen easily with file-based mappings, especially tmpfs and hugetlbfs). While MADV_DONTNEED, MADV_REMOVE and FALLOC_FL_PUNCH_HOLE allow for reliably discarding memory, there is no generic approach to populate page tables and preallocate memory. Although mmap() supports MAP_POPULATE, it is not applicable to the concept of sparse memory mappings, where we want to do populate/discard dynamically and avoid expensive/problematic remappings. In addition, we never actually report errors during the final populate phase - it is best-effort only. fallocate() can be used to preallocate file-based memory and fail in a safe way. However, it cannot really be used for any private mappings on anonymous files via memfd due to COW semantics. In addition, fallocate() does not actually populate page tables, so we still always get pagefaults on first access - which is sometimes undesired (i.e., real-time workloads) and requires real prefaulting of page tables, not just a preallocation of backend storage. There might be interesting use cases for sparse memory regions along with mlockall(MCL_ONFAULT) which fallocate() cannot satisfy as it does not prefault page tables. II. On preallcoation/prefaulting from user space Because we don't have a proper interface, what applications (like QEMU and databases) end up doing is touching (i.e., reading+writing one byte to not overwrite existing data) all individual pages. However, that approach 1) Can result in wear on storage backing, because we end up writing and thereby dirtying each page --- i.e., disks or pmem. 2) Can result in mmap_sem contention when prefaulting via multiple threads. 3) Requires expensive signal handling, especially to catch SIGBUS in case of hugetlbfs/shmem/file-backed memory. For example, this is problematic in hypervisors like QEMU where SIGBUS handlers might already be used by other subsystems concurrently to e.g, handle hardware errors. "Simply" doing preallocation concurrently from other thread is not that easy. III. On MADV_WILLNEED Extending MADV_WILLNEED is not an option because 1. It would change the semantics: "Expect access in the near future." and "might be a good idea to read some pages" vs. "Definitely populate/ preallocate all memory and definitely fail on errors.". 2. Existing users (like virtio-balloon in QEMU when deflating the balloon) don't want populate/prealloc semantics. They treat this rather as a hint to give a little performance boost without too much overhead - and don't expect that a lot of memory might get consumed or a lot of time might be spent. IV. MADV_POPULATE_READ and MADV_POPULATE_WRITE Let's introduce MADV_POPULATE_READ and MADV_POPULATE_WRITE with the following semantics: 1. MADV_POPULATE_READ can be used to preallocate backend memory and prefault page tables just like manually reading each individual page. This will not break any COW mappings -- e.g., it will populate the shared zeropage when applicable. 2. If MADV_POPULATE_READ succeeds, all page tables have been populated (prefaulted) readable once. 3. MADV_POPULATE_WRITE can be used to preallocate backend memory and prefault page tables just like manually writing (or reading+writing) each individual page. This will break any COW mappings -- e.g., the shared zeropage is never populated. 4. If MADV_POPULATE_WRITE succeeds, all page tables have been populated (prefaulted) writable once. 5. MADV_POPULATE_READ and MADV_POPULATE_WRITE cannot be applied to special mappings marked with VM_PFNMAP and VM_IO. Also, proper access permissions (e.g., PROT_READ, PROT_WRITE) are required. If any such mapping is encountered, madvise() fails with -EINVAL. 6. If MADV_POPULATE_READ or MADV_POPULATE_WRITE fails, some page tables might have been populated. In that case, madvise() fails with -ENOMEM. 7. MADV_POPULATE_READ and MADV_POPULATE_WRITE will return -EHWPOISON when encountering a HW poisoned page in the range. 8. Similar to MAP_POPULATE, MADV_POPULATE_READ and MADV_POPULATE_WRITE cannot protect from the OOM (Out Of Memory) handler killing the process. While the use case for MADV_POPULATE_WRITE is fairly obvious (i.e., preallocate memory and prefault page tables for VMs), there are valid use cases for MADV_POPULATE_READ: 1. Efficiently populate page tables with zero pages (i.e., shared zeropage). This is necessary when using userfaultfd() WP (Write-Protect to properly catch all modifications within a mapping: for write-protection to be effective for a virtual address, there has to be a page already mapped -- even if it's the shared zeropage. 2. Pre-read a whole mapping from backend storage without marking it dirty, such that eviction won't have to write it back. If no backend memory has been allocated yet, allocate the backend memory. Helpful when preallocating/prefaulting a file stored on disk without having to writeback each and every page on eviction. Although sparse memory mappings are the primary use case, this will also be useful for ordinary preallocations where MAP_POPULATE is not desired especially in QEMU, where users can trigger preallocation of guest RAM after the mapping was created. Looking at the history, MADV_POPULATE was already proposed in 2013 [1], however, the main motivation back than was performance improvements (which should also still be the case, but it is a secondary concern). V. Single-threaded performance comparison There is a performance benefit when using POPULATE_READ / POPULATE_WRITE already when only using a single thread to do prefaulting/preallocation. As we have less pagefaults for huge pages, the performance benefit is negligible with small mappings. Using fallocate() to preallocate shared files is the fastest approach, however as discussed, we get pagefaults at runtime on actual access which might or might not be relevant depending on the actual use case. Average across 10 iterations each: ================================================== 2 MiB MAP_PRIVATE: ************************************************** Anon 4 KiB : Read : 0.117 ms Anon 4 KiB : Write : 0.240 ms Anon 4 KiB : Read+Write : 0.386 ms Anon 4 KiB : POPULATE_READ : 0.063 ms Anon 4 KiB : POPULATE_WRITE : 0.163 ms Memfd 4 KiB : Read : 0.077 ms Memfd 4 KiB : Write : 0.375 ms Memfd 4 KiB : Read+Write : 0.464 ms Memfd 4 KiB : POPULATE_READ : 0.080 ms Memfd 4 KiB : POPULATE_WRITE : 0.301 ms Memfd 2 MiB : Read : 0.042 ms Memfd 2 MiB : Write : 0.032 ms Memfd 2 MiB : Read+Write : 0.032 ms Memfd 2 MiB : POPULATE_READ : 0.031 ms Memfd 2 MiB : POPULATE_WRITE : 0.032 ms tmpfs : Read : 0.086 ms tmpfs : Write : 0.351 ms tmpfs : Read+Write : 0.427 ms tmpfs : POPULATE_READ : 0.041 ms tmpfs : POPULATE_WRITE : 0.298 ms file : Read : 0.077 ms file : Write : 0.368 ms file : Read+Write : 0.466 ms file : POPULATE_READ : 0.079 ms file : POPULATE_WRITE : 0.303 ms ************************************************** 2 MiB MAP_SHARED: ************************************************** Memfd 4 KiB : Read : 0.418 ms Memfd 4 KiB : Write : 0.367 ms Memfd 4 KiB : Read+Write : 0.428 ms Memfd 4 KiB : POPULATE_READ : 0.347 ms Memfd 4 KiB : POPULATE_WRITE : 0.286 ms Memfd 4 KiB : FALLOCATE : 0.140 ms Memfd 2 MiB : Read : 0.031 ms Memfd 2 MiB : Write : 0.030 ms Memfd 2 MiB : Read+Write : 0.030 ms Memfd 2 MiB : POPULATE_READ : 0.030 ms Memfd 2 MiB : POPULATE_WRITE : 0.030 ms Memfd 2 MiB : FALLOCATE : 0.030 ms tmpfs : Read : 0.434 ms tmpfs : Write : 0.367 ms tmpfs : Read+Write : 0.435 ms tmpfs : POPULATE_READ : 0.349 ms tmpfs : POPULATE_WRITE : 0.291 ms tmpfs : FALLOCATE : 0.144 ms file : Read : 0.423 ms file : Write : 0.367 ms file : Read+Write : 0.432 ms file : POPULATE_READ : 0.351 ms file : POPULATE_WRITE : 0.290 ms file : FALLOCATE : 0.144 ms hugetlbfs : Read : 0.032 ms hugetlbfs : Write : 0.030 ms hugetlbfs : Read+Write : 0.031 ms hugetlbfs : POPULATE_READ : 0.030 ms hugetlbfs : POPULATE_WRITE : 0.030 ms hugetlbfs : FALLOCATE : 0.030 ms ************************************************** 4096 MiB MAP_PRIVATE: ************************************************** Anon 4 KiB : Read : 237.099 ms Anon 4 KiB : Write : 708.062 ms Anon 4 KiB : Read+Write : 1057.147 ms Anon 4 KiB : POPULATE_READ : 124.942 ms Anon 4 KiB : POPULATE_WRITE : 575.082 ms Memfd 4 KiB : Read : 237.593 ms Memfd 4 KiB : Write : 984.245 ms Memfd 4 KiB : Read+Write : 1149.859 ms Memfd 4 KiB : POPULATE_READ : 166.066 ms Memfd 4 KiB : POPULATE_WRITE : 856.914 ms Memfd 2 MiB : Read : 352.202 ms Memfd 2 MiB : Write : 352.029 ms Memfd 2 MiB : Read+Write : 352.198 ms Memfd 2 MiB : POPULATE_READ : 351.033 ms Memfd 2 MiB : POPULATE_WRITE : 351.181 ms tmpfs : Read : 230.796 ms tmpfs : Write : 936.138 ms tmpfs : Read+Write : 1065.565 ms tmpfs : POPULATE_READ : 80.823 ms tmpfs : POPULATE_WRITE : 803.829 ms file : Read : 231.055 ms file : Write : 980.575 ms file : Read+Write : 1208.742 ms file : POPULATE_READ : 167.808 ms file : POPULATE_WRITE : 859.270 ms ************************************************** 4096 MiB MAP_SHARED: ************************************************** Memfd 4 KiB : Read : 1095.979 ms Memfd 4 KiB : Write : 958.777 ms Memfd 4 KiB : Read+Write : 1120.127 ms Memfd 4 KiB : POPULATE_READ : 937.689 ms Memfd 4 KiB : POPULATE_WRITE : 811.594 ms Memfd 4 KiB : FALLOCATE : 309.438 ms Memfd 2 MiB : Read : 353.045 ms Memfd 2 MiB : Write : 353.356 ms Memfd 2 MiB : Read+Write : 352.829 ms Memfd 2 MiB : POPULATE_READ : 351.954 ms Memfd 2 MiB : POPULATE_WRITE : 351.840 ms Memfd 2 MiB : FALLOCATE : 351.274 ms tmpfs : Read : 1096.222 ms tmpfs : Write : 980.651 ms tmpfs : Read+Write : 1114.757 ms tmpfs : POPULATE_READ : 939.181 ms tmpfs : POPULATE_WRITE : 817.255 ms tmpfs : FALLOCATE : 312.521 ms file : Read : 1112.135 ms file : Write : 967.688 ms file : Read+Write : 1111.620 ms file : POPULATE_READ : 951.175 ms file : POPULATE_WRITE : 818.380 ms file : FALLOCATE : 313.008 ms hugetlbfs : Read : 353.710 ms hugetlbfs : Write : 353.309 ms hugetlbfs : Read+Write : 353.280 ms hugetlbfs : POPULATE_READ : 353.138 ms hugetlbfs : POPULATE_WRITE : 352.620 ms hugetlbfs : FALLOCATE : 352.204 ms ************************************************** [1] https://lkml.org/lkml/2013/6/27/698 Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Arnd Bergmann <arnd@arndb.de> Cc: Michal Hocko <mhocko@suse.com> Cc: Oscar Salvador <osalvador@suse.de> Cc: Matthew Wilcox (Oracle) <willy@infradead.org> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Minchan Kim <minchan@kernel.org> Cc: Jann Horn <jannh@google.com> Cc: Jason Gunthorpe <jgg@ziepe.ca> Cc: Dave Hansen <dave.hansen@intel.com> Cc: Hugh Dickins <hughd@google.com> Cc: Rik van Riel <riel@surriel.com> Cc: Michael S. Tsirkin <mst@redhat.com> Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com> Cc: Vlastimil Babka <vbabka@suse.cz> Cc: Richard Henderson <rth@twiddle.net> Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru> Cc: Matt Turner <mattst88@gmail.com> Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de> Cc: "James E.J. Bottomley" <James.Bottomley@HansenPartnership.com> Cc: Helge Deller <deller@gmx.de> Cc: Chris Zankel <chris@zankel.net> Cc: Max Filippov <jcmvbkbc@gmail.com> Cc: Mike Kravetz <mike.kravetz@oracle.com> Cc: Peter Xu <peterx@redhat.com> Cc: Rolf Eike Beer <eike-kernel@sf-tec.de> Cc: linux-alpha@vger.kernel.org Cc: linux-mips@vger.kernel.org Cc: linux-parisc@vger.kernel.org Cc: linux-xtensa@linux-xtensa.org Cc: linux-arch@vger.kernel.org Cc: Linux API <linux-api@vger.kernel.org> Signed-off-by: David Hildenbrand <david@redhat.com> --- arch/alpha/include/uapi/asm/mman.h | 3 ++ arch/mips/include/uapi/asm/mman.h | 3 ++ arch/parisc/include/uapi/asm/mman.h | 3 ++ arch/xtensa/include/uapi/asm/mman.h | 3 ++ include/uapi/asm-generic/mman-common.h | 3 ++ mm/gup.c | 54 ++++++++++++++++++++ mm/internal.h | 3 ++ mm/madvise.c | 69 ++++++++++++++++++++++++++ 8 files changed, 141 insertions(+)