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Howlett" , Nico Pache , Ryan Roberts , Usama Arif , "Matthew Wilcox (Oracle)" , Sasha Levin Subject: [PATCH 6.12 275/670] mm/khugepaged: flush deferred unmaps before dropping a failed folio Date: Fri, 13 Nov 2026 10:47:19 -0500 Message-ID: <20261113153821.211266881@linuxfoundation.org> X-Mailer: git-send-email 2.56.0 In-Reply-To: <20261113153811.177190860@linuxfoundation.org> References: <20261113153811.177190860@linuxfoundation.org> User-Agent: quilt/0.69 X-stable: review X-Patchwork-Hint: ignore X-Mailing-List: stable@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 6.12-stable review patch. If anyone has any objections, please let me know. ------------------ From: Kyle Zeng [ Upstream commit 854e5ff8b3ce2a776586cabebaeae645c992345b ] collapse_file() can fail its reference-count or dirty-folio check after unmapping with TTU_BATCH_FLUSH. Both paths put back the isolated folio, then unlock it and drop the lookup reference before reaching the common try_to_unmap_flush(). The page-cache reference does not keep the folio stable once the lock is released. Another collapse can replace and free it. With its PTEs already gone, that collapse cannot flush the first task's per-task TLB batch, and retract_page_tables() skips short or unaligned VMAs. A CPU can therefore retain a user translation to the freed folio. This has been reproduced with unprivileged MADV_COLLAPSE on a memfd. Flush at out_unlock while the lookup reference and folio lock are still held. The common flush continues to cover the accumulated pagelist on both success and rollback, preserving batching on successful collapses. Link: https://lore.kernel.org/20261007041001.43181-1-kylebot@openai.com Fixes: 6d9df8a5889c ("mm/thp: collapse_file() do try_to_unmap(TTU_BATCH_FLUSH)") Signed-off-by: Kyle Zeng Signed-off-by: Andrew Morton Acked-by: David Hildenbrand (Arm) Reviewed-by: Zi Yan Reviewed-by: Baolin Wang Acked-by: Hugh Dickins Assisted-by: LLM Cc: Barry Song Cc: Dev Jain Cc: Kiryl Shutsemau Cc: Lance Yang Cc: "Liam R. Howlett" Cc: Nico Pache Cc: Ryan Roberts Cc: Usama Arif Cc: Matthew Wilcox (Oracle) Cc: Signed-off-by: Sasha Levin --- mm/khugepaged.c | 10 +++++++--- 1 file changed, 7 insertions(+), 3 deletions(-) diff --git a/mm/khugepaged.c b/mm/khugepaged.c index 7a54ce0f126bc..1250d878d7242 100644 --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -2018,6 +2018,11 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr, index += folio_nr_pages(folio); continue; out_unlock: + /* + * The folio may have been unmapped with TTU_BATCH_FLUSH. + * Flush before releasing the lock and our last reference. + */ + try_to_unmap_flush(); folio_unlock(folio); folio_put(folio); goto xa_unlocked; @@ -2028,9 +2033,8 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr, xa_unlocked: /* - * If collapse is successful, flush must be done now before copying. - * If collapse is unsuccessful, does flush actually need to be done? - * Do it anyway, to clear the state. + * Flush before copying the folios, or releasing them in rollback. + * This is a no-op if out_unlock already flushed the batch. */ try_to_unmap_flush(); -- 2.53.0