improve the kmem_cache_alloc_bulk API v2

improve the kmem_cache_alloc_bulk API v2

[Christoph Hellwig]

Hi all,

kmem_cache_alloc_bulk has a very unintuitive and undocumented return
value convention.  Fix that and add documentation.

Changes since v1:
 - fix compile in drm and test_meminit
 - document that 0-sized allocations fail

Diffstat:
 drivers/gpu/drm/msm/msm_iommu.c       |    6 +--
 drivers/gpu/drm/panthor/panthor_mmu.c |   13 +++----
 include/linux/slab.h                  |    6 ++-
 io_uring/io_uring.c                   |   23 +++++--------
 lib/test_meminit.c                    |   23 ++++++-------
 mm/kasan/kasan_test_c.c               |    5 +-
 mm/kfence/kfence_test.c               |    9 ++---
 mm/slub.c                             |   59 +++++++++++++++++++---------------
 net/bpf/test_run.c                    |    7 +---
 net/core/skbuff.c                     |   24 +++++++------
 tools/include/linux/slab.h            |    2 -
 tools/testing/shared/linux.c          |   19 ++++------
 12 files changed, 97 insertions(+), 99 deletions(-)

[PATCH] mm/slab: improve kmem_cache_alloc_bulk

[Christoph Hellwig]

The kmem_cache_alloc_bulk return value is weird.  It returns the number
of allocated objects, but that must always be 0 or the requested number
based on the implementations and the handling in the callers, but that
assumption is not actually documented anywhere, which confuses automated
review tools.

Fix this by returning a bool if the allocation succeeded and adding a
kerneldoc comment explaining the API.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Alexander Lobakin <aleksander.lobakin@intel.com> # skbuff
---
 drivers/gpu/drm/msm/msm_iommu.c       |  6 +--
 drivers/gpu/drm/panthor/panthor_mmu.c | 13 +++---
 include/linux/slab.h                  |  6 ++-
 io_uring/io_uring.c                   | 23 ++++-------
 lib/test_meminit.c                    | 23 +++++------
 mm/kasan/kasan_test_c.c               |  5 +--
 mm/kfence/kfence_test.c               |  9 ++--
 mm/slub.c                             | 59 +++++++++++++++------------
 net/bpf/test_run.c                    |  7 ++--
 net/core/skbuff.c                     | 24 ++++++-----
 tools/include/linux/slab.h            |  2 +-
 tools/testing/shared/linux.c          | 19 ++++-----
 12 files changed, 97 insertions(+), 99 deletions(-)

diff --git a/drivers/gpu/drm/msm/msm_iommu.c b/drivers/gpu/drm/msm/msm_iommu.c
index 058c71c82cf5..533104d71f6c 100644
--- a/drivers/gpu/drm/msm/msm_iommu.c
+++ b/drivers/gpu/drm/msm/msm_iommu.c
@@ -330,17 +330,15 @@ static int
 msm_iommu_pagetable_prealloc_allocate(struct msm_mmu *mmu, struct msm_mmu_prealloc *p)
 {
 	struct kmem_cache *pt_cache = get_pt_cache(mmu);
-	int ret;
 
 	p->pages = kvmalloc_objs(*p->pages, p->count);
 	if (!p->pages)
 		return -ENOMEM;
 
-	ret = kmem_cache_alloc_bulk(pt_cache, GFP_KERNEL, p->count, p->pages);
-	if (ret != p->count) {
+	if (!kmem_cache_alloc_bulk(pt_cache, GFP_KERNEL, p->count, p->pages)) {
 		kfree(p->pages);
 		p->pages = NULL;
-		p->count = ret;
+		p->count = 0;
 		return -ENOMEM;
 	}
 
diff --git a/drivers/gpu/drm/panthor/panthor_mmu.c b/drivers/gpu/drm/panthor/panthor_mmu.c
index 75d98dad7b1d..b12c641af46c 100644
--- a/drivers/gpu/drm/panthor/panthor_mmu.c
+++ b/drivers/gpu/drm/panthor/panthor_mmu.c
@@ -1274,13 +1274,13 @@ static int panthor_vm_prepare_map_op_ctx(struct panthor_vm_op_ctx *op_ctx,
 		goto err_cleanup;
 	}
 
-	ret = kmem_cache_alloc_bulk(pt_cache, GFP_KERNEL, pt_count,
-				    op_ctx->rsvd_page_tables.pages);
-	op_ctx->rsvd_page_tables.count = ret;
-	if (ret != pt_count) {
+	if (!kmem_cache_alloc_bulk(pt_cache, GFP_KERNEL, pt_count,
+			op_ctx->rsvd_page_tables.pages)) {
+		op_ctx->rsvd_page_tables.count = 0;
 		ret = -ENOMEM;
 		goto err_cleanup;
 	}
+	op_ctx->rsvd_page_tables.count = pt_count;
 
 	/* Insert BO into the extobj list last, when we know nothing can fail. */
 	dma_resv_lock(panthor_vm_resv(vm), NULL);
@@ -1328,9 +1328,8 @@ static int panthor_vm_prepare_unmap_op_ctx(struct panthor_vm_op_ctx *op_ctx,
 			goto err_cleanup;
 		}
 
-		ret = kmem_cache_alloc_bulk(pt_cache, GFP_KERNEL, pt_count,
-					    op_ctx->rsvd_page_tables.pages);
-		if (ret != pt_count) {
+		if (!kmem_cache_alloc_bulk(pt_cache, GFP_KERNEL, pt_count,
+				op_ctx->rsvd_page_tables.pages)) {
 			ret = -ENOMEM;
 			goto err_cleanup;
 		}
diff --git a/include/linux/slab.h b/include/linux/slab.h
index 2b5ab488e96b..6a7b452d43a0 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -815,8 +815,10 @@ kmem_buckets *kmem_buckets_create(const char *name, slab_flags_t flags,
  */
 void kmem_cache_free_bulk(struct kmem_cache *s, size_t size, void **p);
 
-int kmem_cache_alloc_bulk_noprof(struct kmem_cache *s, gfp_t flags, size_t size, void **p);
-#define kmem_cache_alloc_bulk(...)	alloc_hooks(kmem_cache_alloc_bulk_noprof(__VA_ARGS__))
+bool kmem_cache_alloc_bulk_noprof(struct kmem_cache *s, gfp_t flags,
+		size_t size, void **p);
+#define kmem_cache_alloc_bulk(...) \
+	alloc_hooks(kmem_cache_alloc_bulk_noprof(__VA_ARGS__))
 
 static __always_inline void kfree_bulk(size_t size, void **p)
 {
diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index 103b6c88f252..bf847ca823f7 100644
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -978,29 +978,24 @@ __cold bool __io_alloc_req_refill(struct io_ring_ctx *ctx)
 {
 	gfp_t gfp = GFP_KERNEL | __GFP_NOWARN | __GFP_ZERO;
 	void *reqs[IO_REQ_ALLOC_BATCH];
-	int ret;
-
-	ret = kmem_cache_alloc_bulk(req_cachep, gfp, ARRAY_SIZE(reqs), reqs);
+	int nr_reqs = ARRAY_SIZE(reqs);
 
 	/*
-	 * Bulk alloc is all-or-nothing. If we fail to get a batch,
-	 * retry single alloc to be on the safe side.
+	 * Bulk alloc is all-or-nothing. If we fail to get a batch, retry a
+	 * single allocation to be on the safe side.
 	 */
-	if (unlikely(ret <= 0)) {
+	if (!kmem_cache_alloc_bulk(req_cachep, gfp, nr_reqs, reqs)) {
 		reqs[0] = kmem_cache_alloc(req_cachep, gfp);
 		if (!reqs[0])
 			return false;
-		ret = 1;
+		nr_reqs = 1;
 	}
 
-	percpu_ref_get_many(&ctx->refs, ret);
-	ctx->nr_req_allocated += ret;
-
-	while (ret--) {
-		struct io_kiocb *req = reqs[ret];
+	percpu_ref_get_many(&ctx->refs, nr_reqs);
+	ctx->nr_req_allocated += nr_reqs;
 
-		io_req_add_to_cache(req, ctx);
-	}
+	while (nr_reqs--)
+		io_req_add_to_cache(reqs[nr_reqs], ctx);
 	return true;
 }
 
diff --git a/lib/test_meminit.c b/lib/test_meminit.c
index d028a6552cd6..68c3b9da090e 100644
--- a/lib/test_meminit.c
+++ b/lib/test_meminit.c
@@ -229,16 +229,14 @@ static int __init do_kmem_cache_size(size_t size, bool want_ctor,
 	for (iter = 0; iter < 10; iter++) {
 		/* Do a test of bulk allocations */
 		if (!want_rcu && !want_ctor) {
-			int ret;
-
-			ret = kmem_cache_alloc_bulk(c, alloc_mask, BULK_SIZE, bulk_array);
-			if (!ret) {
+			if (!kmem_cache_alloc_bulk(c, alloc_mask, BULK_SIZE,
+					bulk_array)) {
 				fail = true;
 			} else {
 				int i;
-				for (i = 0; i < ret; i++)
+				for (i = 0; i < BULK_SIZE; i++)
 					fail |= check_buf(bulk_array[i], size, want_ctor, want_rcu, want_zero);
-				kmem_cache_free_bulk(c, ret, bulk_array);
+				kmem_cache_free_bulk(c, BULK_SIZE, bulk_array);
 			}
 		}
 
@@ -348,23 +346,24 @@ static int __init do_kmem_cache_size_bulk(int size, int *total_failures)
 {
 	struct kmem_cache *c;
 	int i, iter, maxiter = 1024;
-	int num, bytes;
+	int bytes;
 	bool fail = false;
 	void *objects[10];
 
 	c = kmem_cache_create("test_cache", size, size, 0, NULL);
 	for (iter = 0; (iter < maxiter) && !fail; iter++) {
-		num = kmem_cache_alloc_bulk(c, GFP_KERNEL, ARRAY_SIZE(objects),
-					    objects);
-		for (i = 0; i < num; i++) {
+		if (!kmem_cache_alloc_bulk(c, GFP_KERNEL, ARRAY_SIZE(objects),
+				objects))
+			continue;
+
+		for (i = 0; i < ARRAY_SIZE(objects); i++) {
 			bytes = count_nonzero_bytes(objects[i], size);
 			if (bytes)
 				fail = true;
 			fill_with_garbage(objects[i], size);
 		}
 
-		if (num)
-			kmem_cache_free_bulk(c, num, objects);
+		kmem_cache_free_bulk(c, ARRAY_SIZE(objects), objects);
 	}
 	kmem_cache_destroy(c);
 	*total_failures += fail;
diff --git a/mm/kasan/kasan_test_c.c b/mm/kasan/kasan_test_c.c
index 3f4ed29178b3..b9e167ed5be3 100644
--- a/mm/kasan/kasan_test_c.c
+++ b/mm/kasan/kasan_test_c.c
@@ -1225,14 +1225,13 @@ static void kmem_cache_bulk(struct kunit *test)
 	struct kmem_cache *cache;
 	size_t size = 200;
 	char *p[10];
-	bool ret;
 	int i;
 
 	cache = kmem_cache_create("test_cache", size, 0, 0, NULL);
 	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, cache);
 
-	ret = kmem_cache_alloc_bulk(cache, GFP_KERNEL, ARRAY_SIZE(p), (void **)&p);
-	if (!ret) {
+	if (!kmem_cache_alloc_bulk(cache, GFP_KERNEL, ARRAY_SIZE(p),
+			(void **)&p)) {
 		kunit_err(test, "Allocation failed: %s\n", __func__);
 		kmem_cache_destroy(cache);
 		return;
diff --git a/mm/kfence/kfence_test.c b/mm/kfence/kfence_test.c
index 10424cd25e5a..c472e66e7242 100644
--- a/mm/kfence/kfence_test.c
+++ b/mm/kfence/kfence_test.c
@@ -761,9 +761,10 @@ static void test_memcache_alloc_bulk(struct kunit *test)
 	timeout = jiffies + msecs_to_jiffies(100 * kfence_sample_interval);
 	do {
 		void *objects[100];
-		int i, num = kmem_cache_alloc_bulk(test_cache, GFP_ATOMIC, ARRAY_SIZE(objects),
-						   objects);
-		if (!num)
+		int i;
+
+		if (!kmem_cache_alloc_bulk(test_cache, GFP_ATOMIC,
+				ARRAY_SIZE(objects), objects))
 			continue;
 		for (i = 0; i < ARRAY_SIZE(objects); i++) {
 			if (is_kfence_address(objects[i])) {
@@ -771,7 +772,7 @@ static void test_memcache_alloc_bulk(struct kunit *test)
 				break;
 			}
 		}
-		kmem_cache_free_bulk(test_cache, num, objects);
+		kmem_cache_free_bulk(test_cache, ARRAY_SIZE(objects), objects);
 		/*
 		 * kmem_cache_alloc_bulk() disables interrupts, and calling it
 		 * in a tight loop may not give KFENCE a chance to switch the
diff --git a/mm/slub.c b/mm/slub.c
index a2bf3756ca7d..57b2af707e95 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -4981,8 +4981,8 @@ static int __prefill_sheaf_pfmemalloc(struct kmem_cache *s,
 	return ret;
 }
 
-static int __kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags,
-				   size_t size, void **p);
+static bool __kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags,
+		size_t size, void **p);
 
 /*
  * returns a sheaf that has at least the requested size
@@ -5154,9 +5154,8 @@ int kmem_cache_refill_sheaf(struct kmem_cache *s, gfp_t gfp,
 			return __prefill_sheaf_pfmemalloc(s, sheaf, gfp);
 
 		if (!__kmem_cache_alloc_bulk(s, gfp, sheaf->capacity - sheaf->size,
-					     &sheaf->objects[sheaf->size])) {
+					     &sheaf->objects[sheaf->size]))
 			return -ENOMEM;
-		}
 		sheaf->size = sheaf->capacity;
 
 		return 0;
@@ -7289,9 +7288,8 @@ refill_objects(struct kmem_cache *s, void **p, gfp_t gfp, unsigned int min,
 	return refilled;
 }
 
-static inline
-int __kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags, size_t size,
-			    void **p)
+static bool __kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags,
+		size_t size, void **p)
 {
 	int i;
 
@@ -7312,30 +7310,43 @@ int __kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags, size_t size,
 		stat_add(s, ALLOC_SLOWPATH, i);
 	}
 
-	return i;
+	return true;
 
 error:
 	__kmem_cache_free_bulk(s, i, p);
-	return 0;
-
+	return false;
 }
 
-/*
- * Note that interrupts must be enabled when calling this function and gfp
- * flags must allow spinning.
+/**
+ * kmem_cache_alloc_bulk - Allocate multiple objects
+ * @s:		The cache to allocate from
+ * @flags:	GFP_* flags. See kmalloc().
+ * @size:	Number of objects to allocate
+ * @p:		Array of allocated objects
+ *
+ * Allocate @size objects from @s and places them into @p.  @size must be larger
+ * than 0.
+ *
+ * Interrupts must be enabled when calling this function and @flags must allow
+ * spinning.
+ *
+ * Unlike alloc_pages_bulk(), this function does not check for already allocated
+ * objects in @p, and thus the caller does not need to zero it.
+ *
+ * Return: %true if the allocation succeeded, or %false if it failed.
  */
-int kmem_cache_alloc_bulk_noprof(struct kmem_cache *s, gfp_t flags, size_t size,
-				 void **p)
+bool kmem_cache_alloc_bulk_noprof(struct kmem_cache *s, gfp_t flags,
+		size_t size, void **p)
 {
 	unsigned int i = 0;
 	void *kfence_obj;
 
 	if (!size)
-		return 0;
+		return false;
 
 	s = slab_pre_alloc_hook(s, flags);
 	if (unlikely(!s))
-		return 0;
+		return false;
 
 	/*
 	 * to make things simpler, only assume at most once kfence allocated
@@ -7352,18 +7363,18 @@ int kmem_cache_alloc_bulk_noprof(struct kmem_cache *s, gfp_t flags, size_t size,
 	}
 
 	i = alloc_from_pcs_bulk(s, flags, size, p);
-
 	if (i < size) {
 		/*
 		 * If we ran out of memory, don't bother with freeing back to
 		 * the percpu sheaves, we have bigger problems.
 		 */
-		if (unlikely(__kmem_cache_alloc_bulk(s, flags, size - i, p + i) == 0)) {
+		if (unlikely(!__kmem_cache_alloc_bulk(s, flags, size - i,
+				p + i))) {
 			if (i > 0)
 				__kmem_cache_free_bulk(s, i, p);
 			if (kfence_obj)
 				__kfence_free(kfence_obj);
-			return 0;
+			return false;
 		}
 	}
 
@@ -7382,12 +7393,8 @@ int kmem_cache_alloc_bulk_noprof(struct kmem_cache *s, gfp_t flags, size_t size,
 	 * memcg and kmem_cache debug support and memory initialization.
 	 * Done outside of the IRQ disabled fastpath loop.
 	 */
-	if (unlikely(!slab_post_alloc_hook(s, NULL, flags, size, p,
-		    slab_want_init_on_alloc(flags, s), s->object_size))) {
-		return 0;
-	}
-
-	return size;
+	return likely(slab_post_alloc_hook(s, NULL, flags, size, p,
+			slab_want_init_on_alloc(flags, s), s->object_size));
 }
 EXPORT_SYMBOL(kmem_cache_alloc_bulk_noprof);
 
diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
index 2bc04feadfab..99ab9ddb05e3 100644
--- a/net/bpf/test_run.c
+++ b/net/bpf/test_run.c
@@ -243,12 +243,11 @@ static int xdp_recv_frames(struct xdp_frame **frames, int nframes,
 			   struct net_device *dev)
 {
 	gfp_t gfp = __GFP_ZERO | GFP_ATOMIC;
-	int i, n;
+	int i;
 	LIST_HEAD(list);
 
-	n = kmem_cache_alloc_bulk(net_hotdata.skbuff_cache, gfp, nframes,
-				  (void **)skbs);
-	if (unlikely(n == 0)) {
+	if (!kmem_cache_alloc_bulk(net_hotdata.skbuff_cache, gfp, nframes,
+				   (void **)skbs)) {
 		for (i = 0; i < nframes; i++)
 			xdp_return_frame(frames[i]);
 		return -ENOMEM;
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 44ac121cfccb..73045b688385 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -288,11 +288,11 @@ static inline struct sk_buff *napi_skb_cache_get(bool alloc)
 
 	local_lock_nested_bh(&napi_alloc_cache.bh_lock);
 	if (unlikely(!nc->skb_count)) {
-		if (alloc)
-			nc->skb_count = kmem_cache_alloc_bulk(net_hotdata.skbuff_cache,
-						GFP_ATOMIC | __GFP_NOWARN,
-						NAPI_SKB_CACHE_BULK,
-						nc->skb_cache);
+		if (alloc && kmem_cache_alloc_bulk(net_hotdata.skbuff_cache,
+						   GFP_ATOMIC | __GFP_NOWARN,
+						   NAPI_SKB_CACHE_BULK,
+						   nc->skb_cache))
+			nc->skb_count = NAPI_SKB_CACHE_BULK;
 		if (unlikely(!nc->skb_count)) {
 			local_unlock_nested_bh(&napi_alloc_cache.bh_lock);
 			return NULL;
@@ -353,16 +353,18 @@ u32 napi_skb_cache_get_bulk(void **skbs, u32 n)
 
 	/* No enough cached skbs. Try refilling the cache first */
 	bulk = min(NAPI_SKB_CACHE_SIZE - nc->skb_count, NAPI_SKB_CACHE_BULK);
-	nc->skb_count += kmem_cache_alloc_bulk(net_hotdata.skbuff_cache,
-					       GFP_ATOMIC | __GFP_NOWARN, bulk,
-					       &nc->skb_cache[nc->skb_count]);
+	if (kmem_cache_alloc_bulk(net_hotdata.skbuff_cache,
+				  GFP_ATOMIC | __GFP_NOWARN, bulk,
+				  &nc->skb_cache[nc->skb_count]))
+		nc->skb_count += bulk;
 	if (likely(nc->skb_count >= n))
 		goto get;
 
 	/* Still not enough. Bulk-allocate the missing part directly, zeroed */
-	n -= kmem_cache_alloc_bulk(net_hotdata.skbuff_cache,
-				   GFP_ATOMIC | __GFP_ZERO | __GFP_NOWARN,
-				   n - nc->skb_count, &skbs[nc->skb_count]);
+	if (kmem_cache_alloc_bulk(net_hotdata.skbuff_cache,
+				  GFP_ATOMIC | __GFP_ZERO | __GFP_NOWARN,
+				  n - nc->skb_count, &skbs[nc->skb_count]))
+		n = nc->skb_count;
 	if (likely(nc->skb_count >= n))
 		goto get;
 
diff --git a/tools/include/linux/slab.h b/tools/include/linux/slab.h
index 6d8e9413d5a4..2e63c2e726aa 100644
--- a/tools/include/linux/slab.h
+++ b/tools/include/linux/slab.h
@@ -183,7 +183,7 @@ __kmem_cache_create(const char *name, unsigned int size, unsigned int align,
 		default: __kmem_cache_create)(__name, __object_size, __args, __VA_ARGS__)
 
 void kmem_cache_free_bulk(struct kmem_cache *cachep, size_t size, void **list);
-int kmem_cache_alloc_bulk(struct kmem_cache *cachep, gfp_t gfp, size_t size,
+bool kmem_cache_alloc_bulk(struct kmem_cache *cachep, gfp_t gfp, size_t size,
 			  void **list);
 struct slab_sheaf *
 kmem_cache_prefill_sheaf(struct kmem_cache *s, gfp_t gfp, unsigned int size);
diff --git a/tools/testing/shared/linux.c b/tools/testing/shared/linux.c
index 8c7257155958..e9c3bc9b3272 100644
--- a/tools/testing/shared/linux.c
+++ b/tools/testing/shared/linux.c
@@ -154,7 +154,7 @@ void kmem_cache_shrink(struct kmem_cache *cachep)
 {
 }
 
-int kmem_cache_alloc_bulk(struct kmem_cache *cachep, gfp_t gfp, size_t size,
+bool kmem_cache_alloc_bulk(struct kmem_cache *cachep, gfp_t gfp, size_t size,
 			  void **p)
 {
 	size_t i;
@@ -213,7 +213,7 @@ int kmem_cache_alloc_bulk(struct kmem_cache *cachep, gfp_t gfp, size_t size,
 		pthread_mutex_unlock(&cachep->lock);
 		if (cachep->callback)
 			cachep->exec_callback = true;
-		return 0;
+		return false;
 	}
 
 	for (i = 0; i < size; i++) {
@@ -224,7 +224,7 @@ int kmem_cache_alloc_bulk(struct kmem_cache *cachep, gfp_t gfp, size_t size,
 			printf("Allocating %p from slab\n", p[i]);
 	}
 
-	return size;
+	return true;
 }
 
 struct kmem_cache *
@@ -271,8 +271,8 @@ kmem_cache_prefill_sheaf(struct kmem_cache *s, gfp_t gfp, unsigned int size)
 
 	sheaf->cache = s;
 	sheaf->capacity = capacity;
-	sheaf->size = kmem_cache_alloc_bulk(s, gfp, size, sheaf->objects);
-	if (!sheaf->size) {
+	sheaf->size = size;
+	if (!kmem_cache_alloc_bulk(s, gfp, size, sheaf->objects)) {
 		free(sheaf);
 		return NULL;
 	}
@@ -284,7 +284,6 @@ int kmem_cache_refill_sheaf(struct kmem_cache *s, gfp_t gfp,
 		 struct slab_sheaf **sheafp, unsigned int size)
 {
 	struct slab_sheaf *sheaf = *sheafp;
-	int refill;
 
 	if (sheaf->size >= size)
 		return 0;
@@ -299,12 +298,10 @@ int kmem_cache_refill_sheaf(struct kmem_cache *s, gfp_t gfp,
 		return 0;
 	}
 
-	refill = kmem_cache_alloc_bulk(s, gfp, size - sheaf->size,
-				       &sheaf->objects[sheaf->size]);
-	if (!refill)
+	if (!kmem_cache_alloc_bulk(s, gfp, size - sheaf->size,
+			&sheaf->objects[sheaf->size]))
 		return -ENOMEM;
-
-	sheaf->size += refill;
+	sheaf->size += (size - sheaf->size);
 	return 0;
 }
 
-- 
2.53.0

Re: [PATCH] mm/slab: improve kmem_cache_alloc_bulk

[Vlastimil Babka (SUSE)]

On 5/28/26 11:34, Christoph Hellwig wrote:
> The kmem_cache_alloc_bulk return value is weird.  It returns the number
> of allocated objects, but that must always be 0 or the requested number
> based on the implementations and the handling in the callers, but that
> assumption is not actually documented anywhere, which confuses automated
> review tools.
> 
> Fix this by returning a bool if the allocation succeeded and adding a
> kerneldoc comment explaining the API.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> Reviewed-by: Alexander Lobakin <aleksander.lobakin@intel.com> # skbuff
> ---
>  drivers/gpu/drm/msm/msm_iommu.c       |  6 +--
>  drivers/gpu/drm/panthor/panthor_mmu.c | 13 +++---
>  include/linux/slab.h                  |  6 ++-
>  io_uring/io_uring.c                   | 23 ++++-------
>  lib/test_meminit.c                    | 23 +++++------
>  mm/kasan/kasan_test_c.c               |  5 +--
>  mm/kfence/kfence_test.c               |  9 ++--
>  mm/slub.c                             | 59 +++++++++++++++------------
>  net/bpf/test_run.c                    |  7 ++--
>  net/core/skbuff.c                     | 24 ++++++-----
>  tools/include/linux/slab.h            |  2 +-
>  tools/testing/shared/linux.c          | 19 ++++-----
>  12 files changed, 97 insertions(+), 99 deletions(-)

Thanks, I applied it to slab/for-7.2/alloc_bulk and merged to slab/for-next
(it's still yankable in case of issues)

Did some fixups below (the comment was stale prior to the patch; restored
unlikely(), simplified one line).

A test merge into yesterday's -next found a conflict in drivers/gpu/drm/
panthor/panthor_mmu.c. Commit 1013bf53650e ("drm/panthor: Split
panthor_vm_prepare_map_op_ctx() to prepare for reclaim") moved the changed
codeto a new function panthor_vm_op_ctx_prealloc_pts().
But it's solvable so no need for a complicated coordination I think.

diff --git a/mm/slub.c b/mm/slub.c
index 6caf6f3ceeed..711df528c9a6 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -7372,10 +7372,7 @@ bool kmem_cache_alloc_bulk_noprof(struct kmem_cache *s, gfp_t flags,
        }
 
 out:
-       /*
-        * memcg and kmem_cache debug support and memory initialization.
-        * Done outside of the IRQ disabled fastpath loop.
-        */
+       /* memcg and kmem_cache debug support and memory initialization */
        return likely(slab_post_alloc_hook(s, NULL, flags, size, p,
                        slab_want_init_on_alloc(flags, s), s->object_size));
 }
diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
index 99ab9ddb05e3..dbf0d8eae8d8 100644
--- a/net/bpf/test_run.c
+++ b/net/bpf/test_run.c
@@ -246,8 +246,8 @@ static int xdp_recv_frames(struct xdp_frame **frames, int nframes,
        int i;
        LIST_HEAD(list);
 
-       if (!kmem_cache_alloc_bulk(net_hotdata.skbuff_cache, gfp, nframes,
-                                  (void **)skbs)) {
+       if (unlikely(!kmem_cache_alloc_bulk(net_hotdata.skbuff_cache, gfp,
+                                           nframes, (void **)skbs))) {
                for (i = 0; i < nframes; i++)
                        xdp_return_frame(frames[i]);
                return -ENOMEM;
diff --git a/tools/testing/shared/linux.c b/tools/testing/shared/linux.c
index e9c3bc9b3272..e0a0693df08f 100644
--- a/tools/testing/shared/linux.c
+++ b/tools/testing/shared/linux.c
@@ -301,7 +301,7 @@ int kmem_cache_refill_sheaf(struct kmem_cache *s, gfp_t gfp,
        if (!kmem_cache_alloc_bulk(s, gfp, size - sheaf->size,
                        &sheaf->objects[sheaf->size]))
                return -ENOMEM;
-       sheaf->size += (size - sheaf->size);
+       sheaf->size = size;
        return 0;
 }

Re: [PATCH] mm/slab: improve kmem_cache_alloc_bulk

[Christoph Hellwig]

On Fri, May 29, 2026 at 01:54:48PM +0200, Vlastimil Babka (SUSE) wrote:
> Thanks, I applied it to slab/for-7.2/alloc_bulk and merged to slab/for-next
> (it's still yankable in case of issues)
> 
> Did some fixups below (the comment was stale prior to the patch; restored
> unlikely(), simplified one line).
> 
> A test merge into yesterday's -next found a conflict in drivers/gpu/drm/
> panthor/panthor_mmu.c. Commit 1013bf53650e ("drm/panthor: Split
> panthor_vm_prepare_map_op_ctx() to prepare for reclaim") moved the changed
> codeto a new function panthor_vm_op_ctx_prealloc_pts().
> But it's solvable so no need for a complicated coordination I think.

Ok, thanks.  The two Sashiko complains also look like they had merrits,
but I won't get to looking into them until Monday.

Your fixups look good to me.

Re: [PATCH] mm/slab: improve kmem_cache_alloc_bulk

[Harry Yoo]

[-- Attachment #1.1: Type: text/plain, Size: 1372 bytes --]



On 5/29/26 10:50 PM, Christoph Hellwig wrote:
> On Fri, May 29, 2026 at 01:54:48PM +0200, Vlastimil Babka (SUSE) wrote:
>> Thanks, I applied it to slab/for-7.2/alloc_bulk and merged to slab/for-next
>> (it's still yankable in case of issues)
>>
>> Did some fixups below (the comment was stale prior to the patch; restored
>> unlikely(), simplified one line).
>>
>> A test merge into yesterday's -next found a conflict in drivers/gpu/drm/
>> panthor/panthor_mmu.c. Commit 1013bf53650e ("drm/panthor: Split
>> panthor_vm_prepare_map_op_ctx() to prepare for reclaim") moved the changed
>> codeto a new function panthor_vm_op_ctx_prealloc_pts().
>> But it's solvable so no need for a complicated coordination I think.
> 
> Ok, thanks.  The two Sashiko complains also look like they had merrits,
> but I won't get to looking into them until Monday.
The review:
https://sashiko.dev/#/patchset/20260528093437.2519248-2-hch%40lst.de

So there is a user who might call kmem_cache_alloc_bulk() with size = 0
(although the comment says @size must be larger than 0!) and
kmem_cache_alloc_bulk() returning 0 was considered a success in that case.

Either fixing kmem_cache_alloc_bulk() (and the comment) or fixing the
user sounds fine to me.

And yeah freeing an object via kfree() allocated via kvmalloc is a bug...

-- 
Cheers,
Harry / Hyeonggon


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msm_iommu_pagetable_prealloc_allocate, was: Re: [PATCH] mm/slab: improve kmem_cache_alloc_bulk

[Christoph Hellwig]

On Mon, Jun 01, 2026 at 03:39:06PM +0900, Harry Yoo wrote:
> > Ok, thanks.  The two Sashiko complains also look like they had merrits,
> > but I won't get to looking into them until Monday.
> The review:
> https://sashiko.dev/#/patchset/20260528093437.2519248-2-hch%40lst.de
> 
> So there is a user who might call kmem_cache_alloc_bulk() with size = 0
> (although the comment says @size must be larger than 0!) and
> kmem_cache_alloc_bulk() returning 0 was considered a success in that case.
> 
> Either fixing kmem_cache_alloc_bulk() (and the comment) or fixing the
> user sounds fine to me.
> 
> And yeah freeing an object via kfree() allocated via kvmalloc is a bug...

I think the right fix here is to check p->count somewhere in the msm
code and never get here.  And the kfree/kvfree fix of course.

І.e. nothing to change in this patch, but a headsup for the MSM
maintainers.

Re: [PATCH] mm/slab: improve kmem_cache_alloc_bulk

[Vlastimil Babka (SUSE)]

On 6/1/26 08:39, Harry Yoo wrote:
> 
> 
> On 5/29/26 10:50 PM, Christoph Hellwig wrote:
>> On Fri, May 29, 2026 at 01:54:48PM +0200, Vlastimil Babka (SUSE) wrote:
>>> Thanks, I applied it to slab/for-7.2/alloc_bulk and merged to slab/for-next
>>> (it's still yankable in case of issues)
>>>
>>> Did some fixups below (the comment was stale prior to the patch; restored
>>> unlikely(), simplified one line).
>>>
>>> A test merge into yesterday's -next found a conflict in drivers/gpu/drm/
>>> panthor/panthor_mmu.c. Commit 1013bf53650e ("drm/panthor: Split
>>> panthor_vm_prepare_map_op_ctx() to prepare for reclaim") moved the changed
>>> codeto a new function panthor_vm_op_ctx_prealloc_pts().
>>> But it's solvable so no need for a complicated coordination I think.
>> 
>> Ok, thanks.  The two Sashiko complains also look like they had merrits,
>> but I won't get to looking into them until Monday.
> The review:
> https://sashiko.dev/#/patchset/20260528093437.2519248-2-hch%40lst.de
> 
> So there is a user who might call kmem_cache_alloc_bulk() with size = 0

I don't know if it can really happen there, but maybe DRM folks can tell us.

> (although the comment says @size must be larger than 0!) and

The comment is however new, the caller existed when there was no comment and
the return value 0 when asked for 0 was working there.

> kmem_cache_alloc_bulk() returning 0 was considered a success in that case.
> 
> Either fixing kmem_cache_alloc_bulk() (and the comment) or fixing the
> user sounds fine to me.

Would it be wrong if we just returned true for size of 0? Would something
else break?

> And yeah freeing an object via kfree() allocated via kvmalloc is a bug...

Yep.

Re: [PATCH] mm/slab: improve kmem_cache_alloc_bulk

[Christoph Hellwig]

On Mon, Jun 01, 2026 at 10:16:30AM +0200, Vlastimil Babka (SUSE) wrote:
> > kmem_cache_alloc_bulk() returning 0 was considered a success in that case.
> > 
> > Either fixing kmem_cache_alloc_bulk() (and the comment) or fixing the
> > user sounds fine to me.
> 
> Would it be wrong if we just returned true for size of 0? Would something
> else break?

I don't think it is wrong per se, but it feels like the wrong kind of
API.  I.e. I don't think the MSM caller actually wants this, as they'd
also do a zero-sized kvmalloc.

Re: [PATCH] mm/slab: improve kmem_cache_alloc_bulk

[Vlastimil Babka (SUSE)]

On 6/1/26 13:38, Christoph Hellwig wrote:
> On Mon, Jun 01, 2026 at 10:16:30AM +0200, Vlastimil Babka (SUSE) wrote:
>> > kmem_cache_alloc_bulk() returning 0 was considered a success in that case.
>> > 
>> > Either fixing kmem_cache_alloc_bulk() (and the comment) or fixing the
>> > user sounds fine to me.
>> 
>> Would it be wrong if we just returned true for size of 0? Would something
>> else break?
> 
> I don't think it is wrong per se, but it feels like the wrong kind of
> API.  I.e. I don't think the MSM caller actually wants this, as they'd
> also do a zero-sized kvmalloc.

If p->count is 0 then indeed there's a zero-sized kvmalloc so p->pages ==
ZERO_SIZE_PTR but then nothing breaks because nothing tries to dereference it?

msm_iommu_pagetable_prealloc_cleanup() has a "if (p->count > 0)" branch so
it seems it's considered possible. But then the rest of the functions also
seems working fine, i.e. kmem_cache_free_bulk() of zero size does nothing,
kvfree() of ZERO_SIZE_PTR does nothing.

It seems to me kmem_cache_alloc_bulk() returning true for size == 0 fits
naturally in this world and is less likely to result in a gotcha?

Re: [PATCH] mm/slab: improve kmem_cache_alloc_bulk

[Rob Clark]

On Mon, Jun 1, 2026 at 5:50 AM Vlastimil Babka (SUSE) <vbabka@kernel.org> wrote:
>
> On 6/1/26 13:38, Christoph Hellwig wrote:
> > On Mon, Jun 01, 2026 at 10:16:30AM +0200, Vlastimil Babka (SUSE) wrote:
> >> > kmem_cache_alloc_bulk() returning 0 was considered a success in that case.
> >> >
> >> > Either fixing kmem_cache_alloc_bulk() (and the comment) or fixing the
> >> > user sounds fine to me.
> >>
> >> Would it be wrong if we just returned true for size of 0? Would something
> >> else break?
> >
> > I don't think it is wrong per se, but it feels like the wrong kind of
> > API.  I.e. I don't think the MSM caller actually wants this, as they'd
> > also do a zero-sized kvmalloc.
>
> If p->count is 0 then indeed there's a zero-sized kvmalloc so p->pages ==
> ZERO_SIZE_PTR but then nothing breaks because nothing tries to dereference it?
>
> msm_iommu_pagetable_prealloc_cleanup() has a "if (p->count > 0)" branch so
> it seems it's considered possible. But then the rest of the functions also
> seems working fine, i.e. kmem_cache_free_bulk() of zero size does nothing,
> kvfree() of ZERO_SIZE_PTR does nothing.
>
> It seems to me kmem_cache_alloc_bulk() returning true for size == 0 fits
> naturally in this world and is less likely to result in a gotcha?

I think I was probably expecting kvmalloc(0) => NULL ... but it
happened to work out before

Adding an "if (!p->count) return 0;" at the top of
msm_iommu_pagetable_prealloc_allocate() seems like the thing to do..
if you want, I can send that patch (but traveling this week... so
let's see what I can do)

BR,
-R

Re: [PATCH] mm/slab: improve kmem_cache_alloc_bulk

[Rob Clark]

On Mon, Jun 1, 2026 at 6:32 AM Rob Clark <rob.clark@oss.qualcomm.com> wrote:
>
> On Mon, Jun 1, 2026 at 5:50 AM Vlastimil Babka (SUSE) <vbabka@kernel.org> wrote:
> >
> > On 6/1/26 13:38, Christoph Hellwig wrote:
> > > On Mon, Jun 01, 2026 at 10:16:30AM +0200, Vlastimil Babka (SUSE) wrote:
> > >> > kmem_cache_alloc_bulk() returning 0 was considered a success in that case.
> > >> >
> > >> > Either fixing kmem_cache_alloc_bulk() (and the comment) or fixing the
> > >> > user sounds fine to me.
> > >>
> > >> Would it be wrong if we just returned true for size of 0? Would something
> > >> else break?
> > >
> > > I don't think it is wrong per se, but it feels like the wrong kind of
> > > API.  I.e. I don't think the MSM caller actually wants this, as they'd
> > > also do a zero-sized kvmalloc.
> >
> > If p->count is 0 then indeed there's a zero-sized kvmalloc so p->pages ==
> > ZERO_SIZE_PTR but then nothing breaks because nothing tries to dereference it?
> >
> > msm_iommu_pagetable_prealloc_cleanup() has a "if (p->count > 0)" branch so
> > it seems it's considered possible. But then the rest of the functions also
> > seems working fine, i.e. kmem_cache_free_bulk() of zero size does nothing,
> > kvfree() of ZERO_SIZE_PTR does nothing.
> >
> > It seems to me kmem_cache_alloc_bulk() returning true for size == 0 fits
> > naturally in this world and is less likely to result in a gotcha?
>
> I think I was probably expecting kvmalloc(0) => NULL ... but it
> happened to work out before
>
> Adding an "if (!p->count) return 0;" at the top of
> msm_iommu_pagetable_prealloc_allocate() seems like the thing to do..
> if you want, I can send that patch (but traveling this week... so
> let's see what I can do)

Aaaaaand.. sending patch from hotel wifi doesn't seem to be a thing
that works.. but I've tested the following w/ deqp-vk cts, and works
as expected

----------------

diff --git a/drivers/gpu/drm/msm/msm_iommu.c b/drivers/gpu/drm/msm/msm_iommu.c
index 7d449e5202c5..ef744d154bbe 100644
--- a/drivers/gpu/drm/msm/msm_iommu.c
+++ b/drivers/gpu/drm/msm/msm_iommu.c
@@ -332,13 +332,16 @@ msm_iommu_pagetable_prealloc_allocate(struct
msm_mmu *mmu, struct msm_mmu_preall
        struct kmem_cache *pt_cache = get_pt_cache(mmu);
        int ret;

+       if (!p->count)
+               return 0;
+
        p->pages = kvmalloc_objs(*p->pages, p->count);
        if (!p->pages)
                return -ENOMEM;

        ret = kmem_cache_alloc_bulk(pt_cache, GFP_KERNEL, p->count, p->pages);
        if (ret != p->count) {
-               kfree(p->pages);
+               kvfree(p->pages);
                p->pages = NULL;
                p->count = ret;
                return -ENOMEM;

Re: [PATCH] mm/slab: improve kmem_cache_alloc_bulk

[Harry Yoo]

[-- Attachment #1.1: Type: text/plain, Size: 1959 bytes --]



On 6/1/26 5:16 PM, Vlastimil Babka (SUSE) wrote:
> On 6/1/26 08:39, Harry Yoo wrote:
>>
>>
>> On 5/29/26 10:50 PM, Christoph Hellwig wrote:
>>> On Fri, May 29, 2026 at 01:54:48PM +0200, Vlastimil Babka (SUSE) wrote:
>>>> Thanks, I applied it to slab/for-7.2/alloc_bulk and merged to slab/for-next
>>>> (it's still yankable in case of issues)
>>>>
>>>> Did some fixups below (the comment was stale prior to the patch; restored
>>>> unlikely(), simplified one line).
>>>>
>>>> A test merge into yesterday's -next found a conflict in drivers/gpu/drm/
>>>> panthor/panthor_mmu.c. Commit 1013bf53650e ("drm/panthor: Split
>>>> panthor_vm_prepare_map_op_ctx() to prepare for reclaim") moved the changed
>>>> codeto a new function panthor_vm_op_ctx_prealloc_pts().
>>>> But it's solvable so no need for a complicated coordination I think.
>>>
>>> Ok, thanks.  The two Sashiko complains also look like they had merrits,
>>> but I won't get to looking into them until Monday.
>> The review:
>> https://sashiko.dev/#/patchset/20260528093437.2519248-2-hch%40lst.de
>>
>> So there is a user who might call kmem_cache_alloc_bulk() with size = 0
> 
> I don't know if it can really happen there, but maybe DRM folks can tell us.

Yeah beyond my area :)

>> (although the comment says @size must be larger than 0!) and
> 
> The comment is however new, the caller existed when there was no comment and
> the return value 0 when asked for 0 was working there.

Haha, you're right. Didn't notice that...

>> kmem_cache_alloc_bulk() returning 0 was considered a success in that case.
>>
>> Either fixing kmem_cache_alloc_bulk() (and the comment) or fixing the
>> user sounds fine to me.
> 
> Would it be wrong if we just returned true for size of 0? Would something
> else break?

I think it won't break as nobody should be accessing an array of size 0,
and it matches the previous behavior.

-- 
Cheers,
Harry / Hyeonggon


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