Re: [PATCH v1 1/2] mm/userfaultfd: rely on vma->vm_page_prot in uffd_wp_range()
From: David Hildenbrand
Date: Sat Dec 24 2022 - 12:00:44 EST
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@xxxxxxxxxx>
---
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@xxxxxxxxxx>
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@xxxxxxxxxx>
---
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);
--
2.38.1
--
Thanks,
David / dhildenb