[PATCH v4] mm/slub: use empty sheaf helpers for oversized sheaves

[hu.shengming]

From: Shengming Hu <hu.shengming@xxxxxxxxxx>

Oversized prefilled sheaves are allocated separately, but after they are
flushed they are empty sheaves as well. Release them through
free_empty_sheaf() instead of calling kfree() directly.

Use __alloc_empty_sheaf() for oversized prefilled sheaves as well, so
that they are allocated through the same helper as regular empty sheaves
and released through the matching helper.

Since oversized sheaves are now allocated and freed through the empty
sheaf helpers, SHEAF_ALLOC and SHEAF_FREE also account for oversized
sheaves. Update the stat comments accordingly.

This keeps the oversized and pfmemalloc prefill paths consistent after
flushing.

Signed-off-by: Shengming Hu <hu.shengming@xxxxxxxxxx>
---
Changes in v2:
- Rework the change as suggested by Harry.
- Teach __alloc_empty_sheaf() to initialize capacity and pfmemalloc.
- Allocate oversized prefilled sheaves through __alloc_empty_sheaf().
- Free flushed oversized and pfmemalloc sheaves through free_empty_sheaf().
- Link to v1: https://lore.kernel.org/all/20260521195015105Y4zvKHj0TfPZEujixy9Vo@xxxxxxxxxx/

Changes in v3:
- Address Hao's comments:
- Drop the redundant `pfmemalloc` initialization in
  __alloc_empty_sheaf().
- Keep initializing `capacity` and `pfmemalloc` in the normal-sized
  prefill path, since the sheaf may be reused rather than freshly
  allocated.
- Link to v2: https://lore.kernel.org/all/20260522145900248m-nBcy07_SCDk2ATDWfmg@xxxxxxxxxx/

Changes in v4:
- Address comments from Hao and Harry:
- Keep capacity initialization in the oversized prefill path.
- Update SHEAF_ALLOC and SHEAF_FREE comments to mention oversized sheaves.
- Restore the oversized sheaf pfmemalloc comment.
- Link to v3: https://lore.kernel.org/all/20260525211000387LYqTHmxYL900XIB8qwV3h@xxxxxxxxxx/

---
 mm/slub.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 04692a6f9128..08812de7a8b5 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -362,8 +362,8 @@ enum stat_item {
 	CMPXCHG_DOUBLE_FAIL,	/* Failures of slab freelist update */
 	SHEAF_FLUSH,		/* Objects flushed from a sheaf */
 	SHEAF_REFILL,		/* Objects refilled to a sheaf */
-	SHEAF_ALLOC,		/* Allocation of an empty sheaf */
-	SHEAF_FREE,		/* Freeing of an empty sheaf */
+	SHEAF_ALLOC,		/* Allocation of an empty sheaf including oversized ones */
+	SHEAF_FREE,		/* Freeing of an empty sheaf including oversized ones */
 	BARN_GET,		/* Got full sheaf from barn */
 	BARN_GET_FAIL,		/* Failed to get full sheaf from barn */
 	BARN_PUT,		/* Put full sheaf to barn */
@@ -5015,12 +5015,11 @@ kmem_cache_prefill_sheaf(struct kmem_cache *s, gfp_t gfp, unsigned int size)

 	if (unlikely(size > s->sheaf_capacity)) {

-		sheaf = kzalloc_flex(*sheaf, objects, size, gfp);
+		sheaf = __alloc_empty_sheaf(s, gfp, size);
 		if (!sheaf)
 			return NULL;

 		stat(s, SHEAF_PREFILL_OVERSIZE);
-		sheaf->cache = s;
 		sheaf->capacity = size;

 		/*
@@ -5029,7 +5028,7 @@ kmem_cache_prefill_sheaf(struct kmem_cache *s, gfp_t gfp, unsigned int size)
 		 */
 		if (!__kmem_cache_alloc_bulk(s, gfp, size,
 					     &sheaf->objects[0])) {
-			kfree(sheaf);
+			free_empty_sheaf(s, sheaf);
 			return NULL;
 		}

@@ -5097,7 +5096,7 @@ void kmem_cache_return_sheaf(struct kmem_cache *s, gfp_t gfp,
 	if (unlikely((sheaf->capacity != s->sheaf_capacity)
 		     || sheaf->pfmemalloc)) {
 		sheaf_flush_unused(s, sheaf);
-		kfree(sheaf);
+		free_empty_sheaf(s, sheaf);
 		return;
 	}

-- 
2.25.1