[PATCH 17/46] mm/khugepaged: preserve dirty bits when collapsing PTE-mapped THPs

[David Hildenbrand (Arm)]

From: Chris Mason <mason@xxxxxxxxxx>

try_collapse_pte_mapped_thp() clears PTEs without transferring their dirty
bits to the folio. A shmem read fault can map a clean folio writable, so
a later store only dirties the PTE. Losing that dirty bit allows reclaim
to discard modified data.

The assumption that regular files are read-only is also obsolete since
commit 1b6444331bebb ("mm/khugepaged: enable clean pagecache folio collapse
for writable files"). In any case, transferring PTE dirty bits to the folio
when removing mappings (and while the folio is still mapped) is the right
thing to do.

Use get_and_clear_ptes() to collect the dirty bits and mark the folio
dirty before unmapping it. Update the comment.

This was reproduced with shmem using MADV_COLLAPSE followed by
MADV_PAGEOUT.

Fix generated by an LLM and sent by Chris Mason in private.

Fixes: 27e1f8273113 ("khugepaged: enable collapse pmd for pte-mapped THP")
Fixes: 1b6444331bebb ("mm/khugepaged: enable clean pagecache folio collapse for writable files")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Chris Mason <mason@xxxxxxxxxx>
[ david: adjust comment; simplify description; cover file-thp as well ]
Signed-off-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
---
 mm/khugepaged.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 04d9bb21ff524..dd17c3dae8006 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1995,9 +1995,10 @@ static enum scan_result try_collapse_pte_mapped_thp(struct mm_struct *mm, unsign
 		/*
 		 * Must clear entry, or a racing truncate may re-remove it.
 		 * TLB flush can be left until pmdp_collapse_flush() does it.
-		 * PTE dirty? Shmem page is already dirty; file is read-only.
 		 */
-		clear_ptes(mm, addr, pte, nr_batch_ptes);
+		ptent = get_and_clear_ptes(mm, addr, pte, nr_batch_ptes);
+		if (pte_dirty(ptent))
+			folio_mark_dirty(folio);
 		folio_remove_rmap_ptes(folio, page, nr_batch_ptes, vma);
 		nr_mapped_ptes += nr_batch_ptes;
 	}

-- 
2.43.0