[PATCH v2 1/9] mm: fix eager write-bit restore for write-notified shared mappings

[Carlos López]

change_protection() applies vma->vm_page_prot, which lacks
the write bit for CoW and write-notified mappings. To avoid
needless write faults, callers set MM_CP_TRY_CHANGE_WRITABLE when
vma_wants_manual_pte_write_upgrade() returns true, so that entries that
can be written without a fault, such as dirty PTEs in a shared mapping,
are made writable right away.

For shared mappings, that function passes vma->vm_page_prot to
vma_wants_writenotify(). Once writenotify is enabled, this protection
lacks the write bit, while the vma_pgprot_modify() result it is compared
against adds it back. The two never match, so vma_wants_writenotify()
returns false before checking for soft-dirty, uffd-wp and writeback.

As a result, write-notified shmem mappings never get
MM_CP_TRY_CHANGE_WRITABLE, and their dirty PTEs stay read-only after a
protection change. For example, commit 931298e103c2 ("mm/userfaultfd:
rely on vma->vm_page_prot in uffd_wp_range()") made uffd-wp unprotect
set MM_CP_TRY_CHANGE_WRITABLE so that dirty shmem PTEs become writable
again, but this never takes effect. Unprotecting N dirty shmem pages
still leads to N faults that only set the write bit.

This dates back to the commit in the Fixes tag, which moved the
equivalent check in mprotect_fixup() before vma_set_page_prot().

Fix this by passing the protection recomputed from the VMA flags, as
vma_set_page_prot() does. Note that this is a performance-only fix, as
the write bit is restored on the next page fault anyway.

Fixes: 64e455079e1b ("mm: softdirty: enable write notifications on VMAs after VM_SOFTDIRTY cleared")
Signed-off-by: Carlos López <clopez at suse.de>
---
 mm/vma.h | 23 ++++++++++++++---------
 1 file changed, 14 insertions(+), 9 deletions(-)

diff --git a/mm/vma.h b/mm/vma.h
index 3fc3b47a954b..e83700e248e4 100644
--- a/mm/vma.h
+++ b/mm/vma.h
@@ -589,6 +589,14 @@ int split_vma(struct vma_iterator *vmi, struct vm_area_struct *vma,
 	      unsigned long addr, int new_below);
 #endif
 
+#ifdef CONFIG_MMU
+static inline pgprot_t vma_pgprot_modify(pgprot_t oldprot, vma_flags_t vma_flags)
+{
+	const pgprot_t prot = vma_flags_to_page_prot(vma_flags);
+
+	return pgprot_modify(oldprot, prot);
+}
+
 static inline bool vma_wants_manual_pte_write_upgrade(struct vm_area_struct *vma)
 {
 	/*
@@ -596,19 +604,16 @@ static inline bool vma_wants_manual_pte_write_upgrade(struct vm_area_struct *vma
 	 * if we can't do that automatically for all PTEs in a mapping. For
 	 * private mappings, that's always the case when we have write
 	 * permissions as we properly have to handle COW.
+	 *
+	 * vma->vm_page_prot lacks the write bit if writenotify is already
+	 * enabled, so pass it with the bits restored, as vma_set_page_prot()
+	 * does.
 	 */
 	if (vma->vm_flags & VM_SHARED)
-		return vma_wants_writenotify(vma, vma->vm_page_prot);
+		return vma_wants_writenotify(vma,
+		       vma_pgprot_modify(vma->vm_page_prot, vma->flags));
 	return !!(vma->vm_flags & VM_WRITE);
 }
-
-#ifdef CONFIG_MMU
-static inline pgprot_t vma_pgprot_modify(pgprot_t oldprot, vma_flags_t vma_flags)
-{
-	const pgprot_t prot = vma_flags_to_page_prot(vma_flags);
-
-	return pgprot_modify(oldprot, prot);
-}
 #endif
 
 static inline struct vm_area_struct *vma_prev_limit(struct vma_iterator *vmi,
-- 
2.51.0