[PATCH v3] mm/slub: prevent pfmemalloc objects from entering the barn

[hu.shengming]

From: Shengming Hu <hu.shengming@xxxxxxxxxx>

kmem_cache_return_sheaf() may refill a partially consumed sheaf before
placing it in the barn. Without an explicit restriction, this refill may
draw objects from pfmemalloc slabs and consume emergency reserves.

Add __GFP_NOMEMALLOC so that returned sheaves are refilled only from
non-pfmemalloc slabs. Also add __GFP_NOWARN, as suggested by Hao Li,
because this refill is a best-effort attempt and failure is acceptable.
If the refill fails, flush and free the sheaf instead.

Fixes: 3c1ea5c5019f ("slab: sheaf prefilling for guaranteed allocations")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Shengming Hu <hu.shengming@xxxxxxxxxx>
---
Changes in v3:
- Replace “normal memory” with the more precise “non-pfmemalloc slabs”,
  and add Fixes: and Cc: stable@xxxxxxxxxxxxxxx, as suggested by Harry.
- Link to v2: https://lore.kernel.org/all/20260720211127124sYtTNbvsxCXN9_pDh-pwt@xxxxxxxxxx/

Changes in v2:
- Add __GFP_NOWARN as suggested by Hao.
- Link to v1: https://lore.kernel.org/all/20260719113701797vZ3R8NxiqYt8dEfeUxtON@xxxxxxxxxx/

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

diff --git a/mm/slub.c b/mm/slub.c
index 53b4976d3831..357b7522c817 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -5123,7 +5123,7 @@ void kmem_cache_return_sheaf(struct kmem_cache *s, gfp_t gfp,
 	 * simply flush and free it.
 	 */
 	if (!barn || data_race(barn->nr_full) >= MAX_FULL_SHEAVES ||
-	    refill_sheaf(s, sheaf, gfp)) {
+	    refill_sheaf(s, sheaf, gfp | __GFP_NOMEMALLOC | __GFP_NOWARN)) {
 		sheaf_flush_unused(s, sheaf);
 		free_empty_sheaf(s, sheaf);
 		return;
-- 
2.25.1