[PATCH] mm/shmem: Use page_mapping() to detect page cache for uffd continue

[Peter Xu]

mfill_atomic_install_pte() checks page->mapping to detect whether one page
is used in the page cache.  However as pointed out by Matthew, the page can
logically be a tail page rather than always the head in the case of uffd
minor mode with UFFDIO_CONTINUE.  It means we could wrongly install one pte
with shmem thp tail page assuming it's an anonymous page.

It's not that clear even for anonymous page, since normally anonymous pages
also have page->mapping being setup with the anon vma. It's safe here only
because the only such caller to mfill_atomic_install_pte() is always
passing in a newly allocated page (mcopy_atomic_pte()), whose page->mapping
is not yet setup.  However that's not extremely obvious either.

For either of above, use page_mapping() instead.

And this should be stable material.

Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Axel Rasmussen <axelrasmussen@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Reported-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
---
 mm/userfaultfd.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 3d0fef3980b3..650ab6cfd5f4 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -64,7 +64,7 @@ int mfill_atomic_install_pte(struct mm_struct *dst_mm, pmd_t *dst_pmd,
 	pte_t _dst_pte, *dst_pte;
 	bool writable = dst_vma->vm_flags & VM_WRITE;
 	bool vm_shared = dst_vma->vm_flags & VM_SHARED;
-	bool page_in_cache = page->mapping;
+	bool page_in_cache = page_mapping(page);
 	spinlock_t *ptl;
 	struct inode *inode;
 	pgoff_t offset, max_off;
-- 
2.37.3


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