From: Jesper Dangaard Brouer brouer@redhat.com
mainline inclusion from mainline-v6.5-rc1 commit d0bf7d5759c1d89fb013aa41cca5832e00b9632a category: feature bugzilla: https://gitee.com/openeuler/kernel/issues/IB05J2
Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?i...
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Allow API users of kmem_cache_create to specify that they don't want any slab merge or aliasing (with similar sized objects). Use this in kfence_test.
The SKB (sk_buff) kmem_cache slab is critical for network performance. Network stack uses kmem_cache_{alloc,free}_bulk APIs to gain performance by amortising the alloc/free cost.
For the bulk API to perform efficiently the slub fragmentation need to be low. Especially for the SLUB allocator, the efficiency of bulk free API depend on objects belonging to the same slab (page).
When running different network performance microbenchmarks, I started to notice that performance was reduced (slightly) when machines had longer uptimes. I believe the cause was 'skbuff_head_cache' got aliased/merged into the general slub for 256 bytes sized objects (with my kernel config, without CONFIG_HARDENED_USERCOPY).
For SKB kmem_cache network stack have reasons for not merging, but it varies depending on kernel config (e.g. CONFIG_HARDENED_USERCOPY). We want to explicitly set SLAB_NO_MERGE for this kmem_cache.
Another use case for the flag has been described by David Sterba [1]:
This can be used for more fine grained control over the caches or for debugging builds where separate slabs can verify that no objects leak.
The slab_nomerge boot option is too coarse and would need to be enabled on all testing hosts. There are some other ways how to disable merging, e.g. a slab constructor but this disables poisoning besides that it adds additional overhead. Other flags are internal and may have other semantics.
A concrete example what motivates the flag. During 'btrfs balance' slab top reported huge increase in caches like
1330095 1330095 100% 0.10K 34105 39 136420K Acpi-ParseExt 1734684 1734684 100% 0.14K 61953 28 247812K pid_namespace 8244036 6873075 83% 0.11K 229001 36 916004K khugepaged_mm_slot
which was confusing and that it's because of slab merging was not the first idea. After rebooting with slab_nomerge all the caches were from btrfs_ namespace as expected.
[1] https://lore.kernel.org/all/20230524101748.30714-1-dsterba@suse.com/
[ vbabka@suse.cz: rename to SLAB_NO_MERGE, change the flag value to the one proposed by David so it does not collide with internal SLAB/SLUB flags, write a comment for the flag, expand changelog, drop the skbuff part to be handled spearately ]
Link: https://lore.kernel.org/all/167396280045.539803.7540459812377220500.stgit@fi... Reported-by: David Sterba dsterba@suse.com Signed-off-by: Jesper Dangaard Brouer brouer@redhat.com Signed-off-by: Vlastimil Babka vbabka@suse.cz Acked-by: Jesper Dangaard Brouer brouer@redhat.com Acked-by: Roman Gushchin roman.gushchin@linux.dev Conflicts: mm/slab.h mm/slab_common.c [Conflicts come from the lack of some SLAB_* flags in 5.10.] Signed-off-by: GONG, Ruiqi gongruiqi1@huawei.com --- include/linux/slab.h | 12 ++++++++++++ mm/kfence/kfence_test.c | 7 +++---- mm/slab.h | 5 +++-- mm/slab_common.c | 2 +- 4 files changed, 19 insertions(+), 7 deletions(-)
diff --git a/include/linux/slab.h b/include/linux/slab.h index b27f7372f365..58686a11f998 100644 --- a/include/linux/slab.h +++ b/include/linux/slab.h @@ -93,6 +93,18 @@ /* Avoid kmemleak tracing */ #define SLAB_NOLEAKTRACE ((slab_flags_t __force)0x00800000U)
+/* + * Prevent merging with compatible kmem caches. This flag should be used + * cautiously. Valid use cases: + * + * - caches created for self-tests (e.g. kunit) + * - general caches created and used by a subsystem, only when a + * (subsystem-specific) debug option is enabled + * - performance critical caches, should be very rare and consulted with slab + * maintainers, and not used together with CONFIG_SLUB_TINY + */ +#define SLAB_NO_MERGE ((slab_flags_t __force)0x01000000U) + /* Fault injection mark */ #ifdef CONFIG_FAILSLAB # define SLAB_FAILSLAB ((slab_flags_t __force)0x02000000U) diff --git a/mm/kfence/kfence_test.c b/mm/kfence/kfence_test.c index 0acbc7365412..08a3c1e793c1 100644 --- a/mm/kfence/kfence_test.c +++ b/mm/kfence/kfence_test.c @@ -186,11 +186,10 @@ static size_t setup_test_cache(struct kunit *test, size_t size, slab_flags_t fla kunit_info(test, "%s: size=%zu, ctor=%ps\n", __func__, size, ctor);
/* - * Use SLAB_NOLEAKTRACE to prevent merging with existing caches. Any - * other flag in SLAB_NEVER_MERGE also works. Use SLAB_ACCOUNT to - * allocate via memcg, if enabled. + * Use SLAB_NO_MERGE to prevent merging with existing caches. + * Use SLAB_ACCOUNT to allocate via memcg, if enabled. */ - flags |= SLAB_NOLEAKTRACE | SLAB_ACCOUNT; + flags |= SLAB_NO_MERGE | SLAB_ACCOUNT; test_cache = kmem_cache_create("test", size, 1, flags, ctor); KUNIT_ASSERT_TRUE_MSG(test, test_cache, "could not create cache");
diff --git a/mm/slab.h b/mm/slab.h index 8414c345127b..594f8b0c65b9 100644 --- a/mm/slab.h +++ b/mm/slab.h @@ -142,10 +142,10 @@ static inline slab_flags_t kmem_cache_flags(unsigned int object_size, #if defined(CONFIG_SLAB) #define SLAB_CACHE_FLAGS (SLAB_MEM_SPREAD | SLAB_NOLEAKTRACE | \ SLAB_RECLAIM_ACCOUNT | SLAB_TEMPORARY | \ - SLAB_ACCOUNT) + SLAB_ACCOUNT | SLAB_NO_MERGE) #elif defined(CONFIG_SLUB) #define SLAB_CACHE_FLAGS (SLAB_NOLEAKTRACE | SLAB_RECLAIM_ACCOUNT | \ - SLAB_TEMPORARY | SLAB_ACCOUNT) + SLAB_TEMPORARY | SLAB_ACCOUNT | SLAB_NO_MERGE) #else #define SLAB_CACHE_FLAGS (SLAB_NOLEAKTRACE) #endif @@ -164,6 +164,7 @@ static inline slab_flags_t kmem_cache_flags(unsigned int object_size, SLAB_NOLEAKTRACE | \ SLAB_RECLAIM_ACCOUNT | \ SLAB_TEMPORARY | \ + SLAB_NO_MERGE | \ SLAB_ACCOUNT)
bool __kmem_cache_empty(struct kmem_cache *); diff --git a/mm/slab_common.c b/mm/slab_common.c index 5f1a1c38a815..1b8857d07e06 100644 --- a/mm/slab_common.c +++ b/mm/slab_common.c @@ -54,7 +54,7 @@ static DECLARE_WORK(slab_caches_to_rcu_destroy_work, */ #define SLAB_NEVER_MERGE (SLAB_RED_ZONE | SLAB_POISON | SLAB_STORE_USER | \ SLAB_TRACE | SLAB_TYPESAFE_BY_RCU | SLAB_NOLEAKTRACE | \ - SLAB_FAILSLAB | SLAB_KASAN) + SLAB_FAILSLAB | SLAB_NO_MERGE | SLAB_KASAN)
#define SLAB_MERGE_SAME (SLAB_RECLAIM_ACCOUNT | SLAB_CACHE_DMA | \ SLAB_CACHE_DMA32 | SLAB_ACCOUNT)