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

Lance Yang
2026-05-06T05:24:42Z

On Wed, Apr 29, 2026 at 11:29:12AM -0400, 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>
>---
> 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

Yeah, try_to_unmap_one() also already documents the required arch
guarantee for a clean cached TLB entry after the PTE is cleared.

/*
* We clear the PTE but do not flush so potentially
* a remote CPU could still be writing to the folio.
* If the entry was previously clean then the
* architecture must guarantee that a clear->dirty
* transition on a cached TLB entry is written through
* and traps if the PTE is unmapped.
*/

Lesson learned :)

>+ * 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;
>+ }

LGTM.
Reviewed-by: Lance Yang <lance.yang@linux.dev>