[PATCH v3] mm: filemap: retain mapped dropbehind folios

[Wenjie Qi]

From: Wenjie Qi <qiwenjie@xxxxxxxxxx>

Fault-around can map ready dropbehind folios without going through the
normal page-cache lookup that clears dropbehind. A mapping represents a
competing cached user, so retain the folio instead of forcibly unmapping it
when writeback completes.

For a mapped folio, folio_unmap_invalidate() can call
unmap_mapping_folio(), which takes i_mmap_rwsem and may sleep. Retaining
mapped folios avoids this path when folio_end_dropbehind() runs in
non-preemptible task context.

Unmapped dropbehind folios continue through the existing invalidation path.

Fixes: fb7d3bc41493 ("mm/filemap: drop streaming/uncached pages when writeback completes")
Signed-off-by: Wenjie Qi <qiwenjie@xxxxxxxxxx>
Reviewed-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
Reviewed-by: Tal Zussman <tz2294@xxxxxxxxxxxx>
---
Changes since v2:
- Explain why retaining mapped folios avoids a sleepable unmap path.
- Add the Fixes and Reviewed-by trailers.

 mm/filemap.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 6afec636881f..00fd89cf6f55 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1616,7 +1616,7 @@ static void filemap_end_dropbehind(struct folio *folio)
 		return;
 	if (!folio_test_clear_dropbehind(folio))
 		return;
-	if (mapping)
+	if (mapping && !folio_mapped(folio))
 		folio_unmap_invalidate(mapping, folio, 0);
 }
 
-- 
2.43.0