Re: [PATCH v5 02/14] mm/khugepaged: add folio dirty check after try_to_unmap()

Nico Pache
2026-05-04T03:53:29Z

On 4/29/26 9:29 AM, Zi Yan wrote:
> This check ensures the correctness of read-only PMD folio collapse
> after it is enabled for all FSes supporting PMD pagecache folios and
> replaces READ_ONLY_THP_FOR_FS.
>
> READ_ONLY_THP_FOR_FS only supports read-only fd and uses mapping->nr_thps
> and inode->i_writecount to prevent any write to read-only to-be-collapsed
> folios. In upcoming commits, READ_ONLY_THP_FOR_FS will be removed and the
> aforementioned mechanism will go away too. To ensure khugepaged functions
> as expected after the changes, skip if any folio is dirty after
> try_to_unmap(), since a dirty folio at that point means this read-only
> folio can get writes between try_to_unmap() and try_to_unmap_flush() via
> cached TLB entries and khugepaged does not support writable pagecache folio
> collapse yet.
>
> Signed-off-by: Zi Yan <ziy@nvidia.com>
> Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> Acked-by: David Hildenbrand (Arm) <david@kernel.org>

LGTM

Reviewed-by: Nico Pache <npache@redhat.com>

> ---
> mm/khugepaged.c | 28 ++++++++++++++++++++++++----
> 1 file changed, 24 insertions(+), 4 deletions(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 6808f2b48d864..71209a72195ab 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -2327,8 +2327,7 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,
> }
> } else if (folio_test_dirty(folio)) {
> /*
> - * khugepaged only works on read-only fd,
> - * so this page is dirty because it hasn't
> + * This page is dirty because it hasn't
> * been flushed since first write. There
> * won't be new dirty pages.
> *
> @@ -2386,8 +2385,8 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,
> if (!is_shmem && (folio_test_dirty(folio) ||
> folio_test_writeback(folio))) {
> /*
> - * khugepaged only works on read-only fd, so this
> - * folio is dirty because it hasn't been flushed
> + * khugepaged only works on clean file-backed folios,
> + * so this folio is dirty because it hasn't been flushed
> * since first write.
> */
> result = SCAN_PAGE_DIRTY_OR_WRITEBACK;
> @@ -2431,6 +2430,27 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,
> goto out_unlock;
> }
>
> + /*
> + * At this point, the folio is locked and unmapped. If the PTE
> + * was dirty, try_to_unmap() has transferred the dirty bit to
> + * the folio and we must not collapse it into a clean
> + * file-backed folio.
> + *
> + * If the folio is clean here, no one can write it until we
> + * drop the folio lock. A write through a stale TLB entry came
> + * from a clean PTE and must fault because the PTE has been
> + * cleared; the fault path has to take the folio lock before
> + * installing a writable mapping. Buffered write paths also
> + * have to take the folio lock before modifying file contents
> + * without a mapping, typically via write_begin_get_folio().
> + */
> + if (!is_shmem && folio_test_dirty(folio)) {
> + result = SCAN_PAGE_DIRTY_OR_WRITEBACK;
> + xas_unlock_irq(&xas);
> + folio_putback_lru(folio);
> + goto out_unlock;
> + }
> +
> /*
> * Accumulate the folios that are being collapsed.
> */