[PATCH RFC 11/13] mm/huge_memory: clean up after-split folio freeing in __folio_split

[Kairui Song via B4 Relay]

From: Kairui Song <kasong@xxxxxxxxxxx>

Replace free_folio_and_swap_cache() with an explicit folio_free_swap()
and folio_put() in the after-split loop.  free_folio_and_swap_cache()
unlocks the folio, then free_swap_cache() must trylock it again and
re-check folio_mapped() before freeing the swap cache entries; if the
trylock loses a race, the entries are left behind even though the folio
reference is dropped.  The sub folios are still locked and unmapped
here, so just directly call folio_free_swap() directly under the lock,
unlock and drop the reference.  This makes the swap cache freeing
deterministic and the reference drop explicit.

Signed-off-by: Kairui Song <kasong@xxxxxxxxxxx>
---
 mm/huge_memory.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index b80d0db63225..39c91c8e5bc8 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -4304,14 +4304,16 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
 		if (new_folio == page_folio(lock_at))
 			continue;
 
-		folio_unlock(new_folio);
 		/*
 		 * Subpages whose mapping has been zapped may be freed
 		 * earlier, but freeing them requires taking the
 		 * lru_lock, so we defer put_page() on tail pages until
 		 * after the split completes.
 		 */
-		free_folio_and_swap_cache(new_folio);
+		if (is_swapcache)
+			folio_free_swap(new_folio);
+		folio_unlock(new_folio);
+		folio_put(new_folio);
 	}
 
 out:

-- 
2.55.0