kmem_buckets_create() clones kmalloc_caches[KMALLOC_NORMAL]. kmalloc_slab() figures out the kmalloc type the caller asks for, but then ignored it whenever a bucket set was in use, returning a normal cache regardless. That breaks an allocation that needs other pages: a GFP_DMA allocation would not get memory from ZONE_DMA, and a __GFP_RECLAIMABLE one would miss the reclaimable caches. None of the current users do this, so there is no problem, but it makes adding new users fragile. For example, skb data[1] needs to handle GFP_DMA (rarely).
Send those allocations to the general caches instead, so nothing breaks and regular allocations remain isolated in the set. Accounted allocations stay in the set: memcg charges each object on its own, in any cache, so a bucket cache serves them as well as kmalloc-cg-* does. Built and tests pass with ARCH=x86_64 defconfig with GCC 16.2.0, with CONFIG_SLAB_BUCKETS as y and n, and with CONFIG_MEMCG as y, n, and y with "cgroup.memory=nokmem". Assisted-by: LLM Link: https://lore.kernel.org/all/[email protected]/ [1] Signed-off-by: Kees Cook <[email protected]> --- mm/slab.h | 19 +++++++++++-- lib/tests/slub_kunit.c | 63 ++++++++++++++++++++++++++++++++++++++++++ mm/slab_common.c | 5 ++++ 3 files changed, 85 insertions(+), 2 deletions(-) diff --git a/mm/slab.h b/mm/slab.h index 8fd6835e4235..af39a4e47c9e 100644 --- a/mm/slab.h +++ b/mm/slab.h @@ -421,6 +421,22 @@ static inline unsigned int size_index_elem(unsigned int bytes) return (bytes - 1) / 8; } +/* + * Which set of buckets to use for the given kmalloc_cache_type. A bucket set + * mirrors the KMALLOC_NORMAL caches, and also serves accounted allocations: + * memcg charges each object on its own, in any cache. Types that need + * different pages (DMA, reclaimable) or no obj_exts fall back to the + * general caches. + */ +static inline kmem_buckets * +kmalloc_choose_bucket(kmem_buckets *bucket, enum kmalloc_cache_type type) +{ + if (bucket && (type <= KMALLOC_PARTITION_END || type == KMALLOC_CGROUP)) + return bucket; + + return &kmalloc_caches[type]; +} + /* * Find the kmem_cache structure that serves a given size of * allocation @@ -438,8 +454,7 @@ kmalloc_slab(size_t size, kmem_buckets *b, gfp_t flags, kmalloc_token_t token, if (alloc_flags & SLAB_ALLOC_NO_OBJ_EXT) type = KMALLOC_NO_OBJ_EXT; - if (!b) - b = &kmalloc_caches[type]; + b = kmalloc_choose_bucket(b, type); if (size <= 192) index = kmalloc_size_index[size_index_elem(size)]; else diff --git a/lib/tests/slub_kunit.c b/lib/tests/slub_kunit.c index a2a15a49c5d7..1e6fcbbf8409 100644 --- a/lib/tests/slub_kunit.c +++ b/lib/tests/slub_kunit.c @@ -697,6 +697,68 @@ static void test_kmem_buckets_destroy(struct kunit *test) KUNIT_EXPECT_EQ(test, 2, slab_errors); } +/* + * A bucket set mirrors the normal kmalloc caches, which can serve accounted + * allocations too, so those stay in the set. An allocation that needs other + * pages (DMA or reclaimable) has to come from the general caches. Check that + * it does, rather than being served a cache that does not satisfy what the + * flags asked for. + */ +static void test_kmem_buckets_type_fallback(struct kunit *test) +{ + struct kmem_cache *c; + kmem_buckets *b; + void *p; + + if (!IS_ENABLED(CONFIG_SLAB_BUCKETS)) + kunit_skip(test, "needs CONFIG_SLAB_BUCKETS"); + + b = kmem_buckets_create("test_buckets", 0, INT_MAX); + KUNIT_ASSERT_BUCKETS_CREATED(test, b); + + /* A plain allocation stays isolated in the bucket set. */ + p = kmem_buckets_alloc(b, 128, GFP_KERNEL); + KUNIT_ASSERT_NOT_NULL(test, p); + c = cache_of(p); + kfree(p); + KUNIT_ASSERT_NOT_NULL(test, c); + + KUNIT_EXPECT_TRUE_MSG(test, strstarts(c->name, "test_buckets-"), + "expected a bucket cache, got %s", c->name); + + /* One that needs ZONE_DMA cannot, so it falls back. */ + if (IS_ENABLED(CONFIG_ZONE_DMA)) { + p = kmem_buckets_alloc(b, 128, GFP_KERNEL | GFP_DMA); + KUNIT_ASSERT_NOT_NULL(test, p); + c = cache_of(p); + kfree(p); + KUNIT_ASSERT_NOT_NULL(test, c); + + KUNIT_EXPECT_TRUE_MSG(test, strstarts(c->name, "dma-kmalloc-"), + "expected a DMA cache, got %s", c->name); + } + + /* Nor can one that is reclaimable. */ + p = kmem_buckets_alloc(b, 128, GFP_KERNEL | __GFP_RECLAIMABLE); + KUNIT_ASSERT_NOT_NULL(test, p); + c = cache_of(p); + kfree(p); + KUNIT_ASSERT_NOT_NULL(test, c); + + KUNIT_EXPECT_TRUE_MSG(test, strstarts(c->name, "kmalloc-rcl-"), + "expected a reclaimable cache, got %s", c->name); + + /* An accounted allocation stays in the set; memcg charges it there. */ + p = kmem_buckets_alloc(b, 128, GFP_KERNEL | __GFP_ACCOUNT); + KUNIT_ASSERT_NOT_NULL(test, p); + c = cache_of(p); + kfree(p); + KUNIT_ASSERT_NOT_NULL(test, c); + + KUNIT_EXPECT_TRUE_MSG(test, strstarts(c->name, "test_buckets-"), + "expected a bucket cache, got %s", c->name); +} + static struct kunit_case test_cases[] = { KUNIT_CASE(test_clobber_zone), @@ -723,6 +785,7 @@ static struct kunit_case test_cases[] = { KUNIT_CASE(test_kmem_buckets_alignment), KUNIT_CASE(test_kmem_buckets_disabled), KUNIT_CASE(test_kmem_buckets_destroy), + KUNIT_CASE(test_kmem_buckets_type_fallback), {} }; diff --git a/mm/slab_common.c b/mm/slab_common.c index fb1dd15953a7..6fa02b4ff775 100644 --- a/mm/slab_common.c +++ b/mm/slab_common.c @@ -420,6 +420,11 @@ static struct kmem_cache *kmem_buckets_cache __ro_after_init; * @usersize: How many bytes, starting at @useroffset, may be copied * to/from userspace. * + * Accounted (__GFP_ACCOUNT) allocations are served by the set like any + * other. Allocations that need DMA or reclaimable memory are served by the + * general kmalloc caches instead, without the set's isolation or usercopy + * region. + * * Context: Cannot be called within an interrupt, but can be interrupted. * * Return: a pointer to the cache on success, NULL on failure. When -- 2.55.0
