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From: Hao Li To: "Vlastimil Babka (SUSE)" Cc: Harry Yoo , Suren Baghdasaryan , Shakeel Butt , Alexander Potapenko , Marco Elver , Andrew Morton , Christoph Lameter , David Rientjes , Roman Gushchin , linux-mm@kvack.org, linux-kernel@vger.kernel.org, cgroups@vger.kernel.org Subject: Re: [PATCH v2 12/13] mm/slab: add cache_ and slab_needs_objcg() helpers Message-ID: References: <20260720-b4-objext_split-v2-0-2fa7c6f60dbe@kernel.org> <20260720-b4-objext_split-v2-12-2fa7c6f60dbe@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260720-b4-objext_split-v2-12-2fa7c6f60dbe@kernel.org> X-Migadu-Flow: FLOW_OUT X-Rspam-User: X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: 7E97920003 X-Stat-Signature: qqnj3uh8mq6ykhy6z5ng8diac1cgh6qd X-HE-Tag: 1784886122-623759 X-HE-Meta: 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 NIH7GxcR cpXKbH48ecn0RjEEdGYbgaeT2zQjBw2p1YyV+5xN0USxxcdo9fY4mayGxxgHBzB6PysTmY2yJ3lgypxLcBVlyJrRKOLnN7EkHOIdcO0B26SXdUnz5M1tsdVM0Ykiesa6ssDft/CQoHV+MaIj0O/6gVojGGY4S01QqVYMbO/S/n/u2syQNUfPQXFdLCgrhCLKjp1GYmI/K8UyNB0YaPQ3fWHQO2a6fmdleNx4QMFj+jLA/3k2QqGhke2Ed+b2L78hfwH+jKXffebwXwKUDI6VwQM7lzw== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, Jul 20, 2026 at 04:16:26PM +0200, Vlastimil Babka (SUSE) wrote: > Slabs of some caches never need the objcg part of struct slabobj_ext. > Introduce helpers to query this for a cache or a slab. > > Introduce SLAB_MAY_ACCOUNT flag that is currently only internal and only > is_kmalloc_normal() caches don't have the flag. it seems this could be "only type == KMALLOC_NORMAL caches don't have this flag". > > For named caches we currently can't derive SLAB_MAY_ACCOUNT from > SLAB_ACCOUNT because some caches might be created without SLAB_ACCOUNT > and then used both with and without __GFP_ACCOUNT concurrently, > allocating obj_ext arrays on demand. So just add the SLAB_MAY_ACCOUNT > to all kmem caches, unless kmem accounting is disabled. > > This can be improved later by finding out all caches used with > __GFP_ACCOUNT, adding the SLAB_MAY_ACCOUNT flag to them explicitly and > then ignoring __GFP_ACCOUNT for all other caches (with possible > warning). > > To make the evaluation of slab_needs_objcg() faster in the allocation > and free fast paths, add a obj_exts_needs_objcg flag into slab itself. > This optimization is only available on 64bit architectures where free > bits are available for the flag. > > Signed-off-by: Vlastimil Babka (SUSE) > --- > include/linux/slab.h | 3 +++ > mm/kfence/core.c | 3 +++ > mm/slab.h | 31 +++++++++++++++++++++++++++++-- > mm/slab_common.c | 25 ++++++++++++++++++++----- > mm/slub.c | 6 +++++- > 5 files changed, 60 insertions(+), 8 deletions(-) > > diff --git a/include/linux/slab.h b/include/linux/slab.h > index 3a14df50766a..f9132902c5a6 100644 > --- a/include/linux/slab.h > +++ b/include/linux/slab.h > @@ -45,6 +45,7 @@ enum _slab_flag_bits { > #endif > #ifdef CONFIG_MEMCG > _SLAB_ACCOUNT, > + _SLAB_MAY_ACCOUNT, > #endif > #ifdef CONFIG_KASAN_GENERIC > _SLAB_KASAN, > @@ -204,8 +205,10 @@ enum _slab_flag_bits { > */ > #ifdef CONFIG_MEMCG > # define SLAB_ACCOUNT __SLAB_FLAG_BIT(_SLAB_ACCOUNT) > +# define SLAB_MAY_ACCOUNT __SLAB_FLAG_BIT(_SLAB_MAY_ACCOUNT) > #else > # define SLAB_ACCOUNT __SLAB_FLAG_UNUSED > +# define SLAB_MAY_ACCOUNT __SLAB_FLAG_UNUSED > #endif > > #ifdef CONFIG_KASAN_GENERIC > diff --git a/mm/kfence/core.c b/mm/kfence/core.c > index 05b5482c8150..897ecf2594fb 100644 > --- a/mm/kfence/core.c > +++ b/mm/kfence/core.c > @@ -640,6 +640,9 @@ static unsigned long kfence_init_pool(void) > struct slab *slab = page_slab(page); > slab->obj_exts = (unsigned long)&kfence_metadata_init[i / 2 - 1].obj_exts | > MEMCG_DATA_OBJEXTS; > +#ifdef CONFIG_64BIT > + slab->obj_exts_needs_objcg = 1; > +#endif > #endif > } > > diff --git a/mm/slab.h b/mm/slab.h > index c5f37a46434e..65b44902c06e 100644 > --- a/mm/slab.h > +++ b/mm/slab.h > @@ -81,10 +81,11 @@ struct freelist_counters { > #ifdef CONFIG_64BIT > /* > * Some optimizations use free bits in 'counters' field > - * to save memory. If these free bits are not available, > - * such optimizations are disabled. > + * to save memory or CPU. If these free bits are not > + * available, such optimizations are disabled. > */ > unsigned obj_exts_in_object:1; > + unsigned obj_exts_needs_objcg:1; > #endif > }; > }; > @@ -584,6 +585,32 @@ static inline bool slab_obj_ext_has_codetag(void) > } > #endif > > +#ifdef CONFIG_MEMCG > +static inline bool cache_needs_objcg(struct kmem_cache *cache) > +{ > + return (cache->flags & SLAB_MAY_ACCOUNT); > +} > + > +static inline bool slab_needs_objcg(struct slab *slab) > +{ > +#ifdef CONFIG_64BIT > + return slab->obj_exts_needs_objcg; > +#else > + return cache_needs_objcg(slab->slab_cache); > +#endif > +} > +#else > +static inline bool cache_needs_objcg(struct kmem_cache *cache) > +{ > + return false; > +} > + > +static inline bool slab_needs_objcg(struct slab *slab) > +{ > + return false; > +} > +#endif > + > static inline size_t cache_obj_ext_size(struct kmem_cache *s) > { > size_t sz = 0; > diff --git a/mm/slab_common.c b/mm/slab_common.c > index 03ecac12cd86..4e983e250338 100644 > --- a/mm/slab_common.c > +++ b/mm/slab_common.c > @@ -52,7 +52,7 @@ struct kmem_cache *kmem_cache; > SLAB_OBJ_EXT_IN_OBJ) > > #define SLAB_MERGE_SAME (SLAB_RECLAIM_ACCOUNT | SLAB_CACHE_DMA | \ > - SLAB_CACHE_DMA32 | SLAB_ACCOUNT) > + SLAB_CACHE_DMA32 | SLAB_ACCOUNT | SLAB_MAY_ACCOUNT) > > /* > * Merge control. If this is set then no merging of slab caches will occur. > @@ -359,6 +359,13 @@ struct kmem_cache *__kmem_cache_create_args(const char *name, > goto out_unlock; > } > > + /* > + * For now we assume any cache can be used with __GFP_ACCOUNT and thus > + * may need to store objcg pointers for objects > + */ > + if (!mem_cgroup_kmem_disabled()) > + flags |= SLAB_MAY_ACCOUNT; > + > /* Fail closed on bad usersize of useroffset values. */ > if (!IS_ENABLED(CONFIG_HARDENED_USERCOPY) || > WARN_ON(!args->usersize && args->useroffset) || > @@ -984,11 +991,19 @@ new_kmalloc_cache(int idx, enum kmalloc_cache_type type) > #endif > > /* > - * If CONFIG_MEMCG is enabled, disable cache merging for > - * KMALLOC_NORMAL caches. > + * If memcg_kmem is enabled and this is a KMALLOC_NORMAL cache and not > + * aliased with any other type, make sure it's never merged with any other > + * cache. > + * > + * In other cases the kmalloc cache may end up being used for a > + * __GFP_ACCOUNT allocation so mark it as such > */ > - if (IS_ENABLED(CONFIG_MEMCG) && (type == KMALLOC_NORMAL)) > - flags |= SLAB_NO_MERGE; > + if (!mem_cgroup_kmem_disabled()) { > + if (type == KMALLOC_NORMAL && KMALLOC_RECLAIM != KMALLOC_NORMAL) With only those checks, the dedicated KMALLOC_NO_OBJ_EXT cache will also have the SLAB_MAY_ACCOUNT flag, which might be a bit misleading. is it worth explicitly excluding SLAB_MAY_ACCOUNT for dedicated KMALLOC_NO_OBJ_EXT cache? > + flags |= SLAB_NO_MERGE; > + else > + flags |= SLAB_MAY_ACCOUNT; > + } > > if (minalign > ARCH_KMALLOC_MINALIGN) { > aligned_size = ALIGN(aligned_size, minalign); > diff --git a/mm/slub.c b/mm/slub.c > index d78d3e50c877..30435e2509ea 100644 > --- a/mm/slub.c > +++ b/mm/slub.c > @@ -2557,7 +2557,7 @@ bool memcg_slab_post_charge(void *p, gfp_t flags) > * of slab_obj_exts being allocated from the same slab and thus the slab > * becoming effectively unfreeable. > */ > - if (is_kmalloc_normal(s)) > + if (!cache_needs_objcg(s)) > return true; > > /* Ignore already charged objects. */ > @@ -3435,6 +3435,10 @@ static struct slab *allocate_slab(struct kmem_cache *s, gfp_t flags, > > slab->objects = oo_objects(oo); > > +#ifdef CONFIG_64BIT > + if (cache_needs_objcg(s)) > + slab->obj_exts_needs_objcg = 1; > +#endif > slab->slab_cache = s; > > kasan_poison_slab(slab); > > -- > 2.55.0 > -- Thanks, Hao