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 --- 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();