[PATCH 1/1] mm: Fix folio_pte_batch() overcount with zero PTEs

["Petr Vaněk" <arkamar@atlas.cz>]

folio_pte_batch() could overcount the number of contiguous PTEs when
pte_advance_pfn() returns a zero-valued PTE and the following PTE in
memory also happens to be zero. The loop doesn't break in such a case
because pte_same() returns true, and the batch size is advanced by one
more than it should be.

To fix this, bail out early if a non-present PTE is encountered,
preventing the invalid comparison.

This issue started to appear after commit 10ebac4f95e7 ("mm/memory:
optimize unmap/zap with PTE-mapped THP") and was discovered via git
bisect.

Fixes: 10ebac4f95e7 ("mm/memory: optimize unmap/zap with PTE-mapped THP")
Cc: stable@vger.kernel.org
Signed-off-by: Petr Vaněk <arkamar@atlas.cz>
---
 mm/internal.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/internal.h b/mm/internal.h
index e9695baa5922..c181fe2bac9d 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -279,6 +279,8 @@ static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
 			dirty = !!pte_dirty(pte);
 		pte = __pte_batch_clear_ignored(pte, flags);
 
+		if (!pte_present(pte))
+			break;
 		if (!pte_same(pte, expected_pte))
 			break;
 
-- 
2.48.1