[PATCH v7 1/3] mm: preserve PG_dropbehind flag during folio split

[Jeff Layton <jlayton@kernel.org>]

__split_folio_to_order() copies page flags from the original folio to
newly created sub-folios using an explicit allowlist, but PG_dropbehind
is not included. When a large folio with PG_dropbehind set is split,
only the head sub-folio retains the flag; all tail sub-folios silently
lose it and will not be reclaimed eagerly after writeback completes.

Add PG_dropbehind to the flag copy mask so that the drop-behind hint
is preserved across folio splits.

Fixes: a323281cdfec ("mm: add PG_dropbehind folio flag")
Assisted-by: Claude:claude-opus-4-6
Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 mm/huge_memory.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 970e077019b7..e01917b14d1a 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3642,6 +3642,7 @@ static void __split_folio_to_order(struct folio *folio, int old_order,
 				 (1L << PG_arch_3) |
 #endif
 				 (1L << PG_dirty) |
+				 (1L << PG_dropbehind) |
 				 LRU_GEN_MASK | LRU_REFS_MASK));
 
 		if (handle_hwpoison &&

-- 
2.54.0