| Message ID | 20221223155616.297723-2-david@redhat.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | mm: uffd-wp + change_protection() cleanups | expand |
On 23.12.22 16:56, David Hildenbrand wrote: > uffd_wp_range() currently calculates page protection manually using > vm_get_page_prot(). This will ignore any other reason for active > writenotify: one mechanism applicable to shmem is softdirty tracking. > > For example, the following sequence > > 1) Write to mapped shmem page > 2) Clear softdirty > 3) Register uffd-wp covering the mapped page > 4) Unregister uffd-wp covering the mapped page > 5) Write to page again > > will not set the modified page softdirty, because uffd_wp_range() will > ignore that writenotify is required for softdirty tracking and simply map > the page writable again using change_protection(). Similarly, instead of > unregistering, protecting followed by un-protecting the page using > uffd-wp would result in the same situation. > > Now that we enable writenotify whenever enabling uffd-wp on a VMA, > vma->vm_page_prot will already properly reflect our requirements: the > default is to write-protect all PTEs. However, for shared mappings we > would now not remap the PTEs writable if possible when unprotecting, just > like for private mappings (COW). To compensate, set > MM_CP_TRY_CHANGE_WRITABLE just like mprotect() does to try mapping > individual PTEs writable. > > For private mappings, this change implies that we will now always try > setting PTEs writable when un-protecting, just like when upgrading write > permissions using mprotect(), which is an improvement. > > For shared mappings, we will only set PTEs writable if > can_change_pte_writable()/can_change_pmd_writable() indicates that it's > ok. For ordinary shmem, this will be the case when PTEs are dirty, which > should usually be the case -- otherwise we could special-case shmem in > can_change_pte_writable()/can_change_pmd_writable() easily, because > shmem itself doesn't require writenotify. > > Note that hugetlb does not yet implement MM_CP_TRY_CHANGE_WRITABLE, so we > won't try setting PTEs writable when unprotecting or when unregistering > uffd-wp. This can be added later on top by implementing > MM_CP_TRY_CHANGE_WRITABLE. > > While commit ffd05793963a ("userfaultfd: wp: support write protection for > userfault vma range") introduced that code, it should only be applicable > to uffd-wp on shared mappings -- shmem (hugetlb does not support softdirty > tracking). I don't think this corner cases justifies to cc stable. Let's > just handle it correctly and prepare for change_protection() cleanups. > > Fixes: b1f9e876862d ("mm/uffd: enable write protection for shmem & hugetlbfs") > Signed-off-by: David Hildenbrand <david@redhat.com> > --- > mm/userfaultfd.c | 18 +++++++++++++----- > 1 file changed, 13 insertions(+), 5 deletions(-) > > diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c > index 0499907b6f1a..351e8d6b398b 100644 > --- a/mm/userfaultfd.c > +++ b/mm/userfaultfd.c > @@ -727,17 +727,25 @@ ssize_t mcopy_continue(struct mm_struct *dst_mm, unsigned long start, > void uffd_wp_range(struct mm_struct *dst_mm, struct vm_area_struct *dst_vma, > unsigned long start, unsigned long len, bool enable_wp) > { > + unsigned int mm_cp_flags; > struct mmu_gather tlb; > - pgprot_t newprot; > > if (enable_wp) > - newprot = vm_get_page_prot(dst_vma->vm_flags & ~(VM_WRITE)); > + mm_cp_flags = MM_CP_UFFD_WP; > else > - newprot = vm_get_page_prot(dst_vma->vm_flags); > + mm_cp_flags = MM_CP_UFFD_WP_RESOLVE; > > + /* > + * vma->vm_page_prot already reflects that uffd-wp is enabled for this > + * VMA (see userfaultfd_set_vm_flags()) and that all PTEs are supposed > + * to be write-protected as default whenever protection changes. > + * Try upgrading write permissions manually. > + */ > + if (vma_wants_manual_pte_write_upgrade(dst_vma)) > + mm_cp_flags |= MM_CP_TRY_CHANGE_WRITABLE; > tlb_gather_mmu(&tlb, dst_mm); > - change_protection(&tlb, dst_vma, start, start + len, newprot, > - enable_wp ? MM_CP_UFFD_WP : MM_CP_UFFD_WP_RESOLVE); > + change_protection(&tlb, dst_vma, start, start + len, vma->vm_page_prot, > + mm_cp_flags); > tlb_finish_mmu(&tlb); > } > The following optimization makes sense: From 779b36768328d99dbcc96fbba7be8b50536ce350 Mon Sep 17 00:00:00 2001 From: David Hildenbrand <david@redhat.com> Date: Sat, 24 Dec 2022 15:02:36 +0100 Subject: [PATCH] fixup: mm/userfaultfd: enable writenotify while userfaultfd-wp is enabled for a VMA No need for additional harmless checks if we're wr-protecting either way. Signed-off-by: David Hildenbrand <david@redhat.com> --- mm/userfaultfd.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c index be7ee9d82e72..1ac1de527719 100644 --- a/mm/userfaultfd.c +++ b/mm/userfaultfd.c @@ -741,7 +741,7 @@ void uffd_wp_range(struct mm_struct *dst_mm, struct vm_area_struct *dst_vma, * to be write-protected as default whenever protection changes. * Try upgrading write permissions manually. */ - if (vma_wants_manual_pte_write_upgrade(dst_vma)) + if (!enable_wp && vma_wants_manual_pte_write_upgrade(dst_vma)) mm_cp_flags |= MM_CP_TRY_CHANGE_WRITABLE; tlb_gather_mmu(&tlb, dst_mm); change_protection(&tlb, dst_vma, start, start + len, mm_cp_flags);
diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c index 0499907b6f1a..351e8d6b398b 100644 --- a/mm/userfaultfd.c +++ b/mm/userfaultfd.c @@ -727,17 +727,25 @@ ssize_t mcopy_continue(struct mm_struct *dst_mm, unsigned long start, void uffd_wp_range(struct mm_struct *dst_mm, struct vm_area_struct *dst_vma, unsigned long start, unsigned long len, bool enable_wp) { + unsigned int mm_cp_flags; struct mmu_gather tlb; - pgprot_t newprot; if (enable_wp) - newprot = vm_get_page_prot(dst_vma->vm_flags & ~(VM_WRITE)); + mm_cp_flags = MM_CP_UFFD_WP; else - newprot = vm_get_page_prot(dst_vma->vm_flags); + mm_cp_flags = MM_CP_UFFD_WP_RESOLVE; + /* + * vma->vm_page_prot already reflects that uffd-wp is enabled for this + * VMA (see userfaultfd_set_vm_flags()) and that all PTEs are supposed + * to be write-protected as default whenever protection changes. + * Try upgrading write permissions manually. + */ + if (vma_wants_manual_pte_write_upgrade(dst_vma)) + mm_cp_flags |= MM_CP_TRY_CHANGE_WRITABLE; tlb_gather_mmu(&tlb, dst_mm); - change_protection(&tlb, dst_vma, start, start + len, newprot, - enable_wp ? MM_CP_UFFD_WP : MM_CP_UFFD_WP_RESOLVE); + change_protection(&tlb, dst_vma, start, start + len, vma->vm_page_prot, + mm_cp_flags); tlb_finish_mmu(&tlb); }
uffd_wp_range() currently calculates page protection manually using vm_get_page_prot(). This will ignore any other reason for active writenotify: one mechanism applicable to shmem is softdirty tracking. For example, the following sequence 1) Write to mapped shmem page 2) Clear softdirty 3) Register uffd-wp covering the mapped page 4) Unregister uffd-wp covering the mapped page 5) Write to page again will not set the modified page softdirty, because uffd_wp_range() will ignore that writenotify is required for softdirty tracking and simply map the page writable again using change_protection(). Similarly, instead of unregistering, protecting followed by un-protecting the page using uffd-wp would result in the same situation. Now that we enable writenotify whenever enabling uffd-wp on a VMA, vma->vm_page_prot will already properly reflect our requirements: the default is to write-protect all PTEs. However, for shared mappings we would now not remap the PTEs writable if possible when unprotecting, just like for private mappings (COW). To compensate, set MM_CP_TRY_CHANGE_WRITABLE just like mprotect() does to try mapping individual PTEs writable. For private mappings, this change implies that we will now always try setting PTEs writable when un-protecting, just like when upgrading write permissions using mprotect(), which is an improvement. For shared mappings, we will only set PTEs writable if can_change_pte_writable()/can_change_pmd_writable() indicates that it's ok. For ordinary shmem, this will be the case when PTEs are dirty, which should usually be the case -- otherwise we could special-case shmem in can_change_pte_writable()/can_change_pmd_writable() easily, because shmem itself doesn't require writenotify. Note that hugetlb does not yet implement MM_CP_TRY_CHANGE_WRITABLE, so we won't try setting PTEs writable when unprotecting or when unregistering uffd-wp. This can be added later on top by implementing MM_CP_TRY_CHANGE_WRITABLE. While commit ffd05793963a ("userfaultfd: wp: support write protection for userfault vma range") introduced that code, it should only be applicable to uffd-wp on shared mappings -- shmem (hugetlb does not support softdirty tracking). I don't think this corner cases justifies to cc stable. Let's just handle it correctly and prepare for change_protection() cleanups. Fixes: b1f9e876862d ("mm/uffd: enable write protection for shmem & hugetlbfs") Signed-off-by: David Hildenbrand <david@redhat.com> --- mm/userfaultfd.c | 18 +++++++++++++----- 1 file changed, 13 insertions(+), 5 deletions(-)