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charset=UTF-8 Content-Transfer-Encoding: 8bit X-Stat-Signature: zjcucm69nhk4p87ucm8x6idu9xq8e81u X-Rspamd-Queue-Id: 6574EA0012 X-Rspam-User: X-Rspamd-Server: rspam05 X-HE-Tag: 1745937672-381948 X-HE-Meta: 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 UYeoJwrz I4a797oqf0TMosLwBiliK3Mjt4t6mcRaLV54rrwVxiXgnFoa/1qryPY9J+3Kgvo6s+8zpa5/ag1jYQ8yDiza5tXE1xKR7Wws3UfryAB9TGA7YPnNGmqkaoWWyOzgHVubSZlxYnKxTyHBTCUVhQ8slH/gu5Do6JGIFCny1rGAZt+dbYHY42TLGNulYzQL6tnFc5S3Bmi25bAktM4pUimyXPpYIBWbfsv8fMhbj9cTnbK7j3DU= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 29/04/2025 15:29, David Hildenbrand wrote: > On 29.04.25 16:22, Petr Vaněk wrote: >> 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 >> --- >>   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; > > How could pte_same() suddenly match on a present and non-present PTE. > > Something with XEN is really problematic here. > We are inside a lazy MMU region (arch_enter_lazy_mmu_mode()) at this point, which I believe XEN uses. If a PTE was written then read back while in lazy mode you could get a stale value. See https://lore.kernel.org/all/912c7a32-b39c-494f-a29c-4865cd92aeba@agordeev.local/ for an example bug.