The patch titled
Subject: mm/userfaultfd: rely on vma->vm_page_prot in uffd_wp_range()
has been added to the -mm mm-unstable branch. Its filename is
mm-userfaultfd-rely-on-vma-vm_page_prot-in-uffd_wp_range.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-userfaultfd-rely-on-vma-vm_page_prot-in-uffd_wp_range.patch
This patch will later appear in the mm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: David Hildenbrand <david@xxxxxxxxxx>
Subject: mm/userfaultfd: rely on vma->vm_page_prot in uffd_wp_range()
Date: Fri, 23 Dec 2022 16:56:15 +0100
Patch series "mm: uffd-wp + change_protection() cleanups".
Cleanup page protection handling in uffd-wp when calling
change_protection() and improve unprotecting uffd=wp in private mappings,
trying to set PTEs writable again if possible just like we do during
mprotect() when upgrading write permissions. Make the change_protection()
interface harder to get wrong :)
I consider both pages primarily cleanups, although patch #1 fixes a corner
case with uffd-wp and softdirty tracking for shmem. @Peter, please let me
know if we should flag patch #1 as pure cleanup -- I have no idea how
important softdirty tracking on shmem is.
This patch (of 2):
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.
Link: https://lkml.kernel.org/r/20221223155616.297723-1-david@xxxxxxxxxx
Link: https://lkml.kernel.org/r/20221223155616.297723-2-david@xxxxxxxxxx
Fixes: b1f9e876862d ("mm/uffd: enable write protection for shmem & hugetlbfs")
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Nadav Amit <nadav.amit@xxxxxxxxx>
Cc: Peter Xu <peterx@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
--- a/mm/userfaultfd.c~mm-userfaultfd-rely-on-vma-vm_page_prot-in-uffd_wp_range
+++ a/mm/userfaultfd.c
@@ -713,17 +713,25 @@ ssize_t mcopy_continue(struct mm_struct
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);
}
_
Patches currently in -mm which might be from david@xxxxxxxxxx are
mm-hugetlb-fix-pte-marker-handling-in-hugetlb_change_protection.patch
mm-hugetlb-fix-uffd-wp-handling-for-migration-entries-in-hugetlb_change_protection.patch
mm-userfaultfd-enable-writenotify-while-userfaultfd-wp-is-enabled-for-a-vma.patch
mm-userfaultfd-rely-on-vma-vm_page_prot-in-uffd_wp_range.patch
mm-mprotect-drop-pgprot_t-parameter-from-change_protection.patch
