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

[Zi Yan]

On Wed Oct 7, 2026 at 12:10 AM EDT, Kyle Zeng wrote:
> 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@xxxxxxxxxxxxxxx
> Assisted-by: LLM
> Signed-off-by: Kyle Zeng <kylebot@xxxxxxxxxx>
> ---
> 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(-)
>
LGTM. Thanks.

Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>


-- 
Best Regards,
Yan, Zi