[PATCH v2] mm/khugepaged: flush deferred unmaps before dropping a failed folio

[Kyle Zeng <kylebot@openai.com>]

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.

Fixes: 6d9df8a5889c ("mm/thp: collapse_file() do try_to_unmap(TTU_BATCH_FLUSH)")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Kyle Zeng <kylebot@openai.com>
---
Changes in v2:
- Use Assisted-by: LLM.
- Explain that the common flush is a no-op after out_unlock flushes.

 mm/khugepaged.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 75639298efc2..e1a5890818ad 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -2478,6 +2478,11 @@ static enum scan_result 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;
@@ -2488,9 +2493,8 @@ static enum scan_result 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();