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mm/slab: stop allocating objcg pointers when unnecessary

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Start using the slab_needs_objcg() helper to calculate slabobj_ext size.
Caches that we know to never need objcg pointers (currently
KMALLOC_NORMAL caches) will thus stop wasting memory on them when memory
allocation profiling is enabled.

For things to work properly, we need to also add slab_needs_objcg()
checks to mem_cgroup_from_obj_slab() and memcg_slab_free_hook(), because
when obj_exts array exists for a slab only due to mem_alloc profiling,
we would otherwise attempt to access a non-existing objcg pointer in
that slab.

In slab_obj_ext_[set_]objcg() add debug warnings if called on a slab
where slab_needs_objcg() is false.

Reviewed-by: Hao Li <hao.li@linux.dev>
Link: https://patch.msgid.link/20260727-b4-objext_split-v3-12-c29ef0f1f257@kernel.org
Reviewed-by: Harry Yoo <harry@kernel.org>
Signed-off-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
This commit is contained in:
Vlastimil Babka (SUSE) 2026-07-27 14:54:06 +02:00
parent 7def2e8549
commit d4404b0f5b
3 changed files with 21 additions and 3 deletions
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3
mm/memcontrol.c
View File

@ -2871,6 +2871,9 @@ struct mem_cgroup *mem_cgroup_from_obj_slab(struct slab *slab, void *p)
if (!obj_exts)
return NULL;
if (!slab_needs_objcg(slab))
return NULL;
get_slab_obj_exts(obj_exts);
obj_ext = slab_obj_ext(slab->slab_cache, slab, obj_exts, p);
objcg = slab_obj_ext_objcg(slab, obj_ext);

18
mm/slab.h
View File

@ -615,7 +615,7 @@ static inline size_t cache_obj_ext_size(struct kmem_cache *s)
{
size_t sz = 0;
if (IS_ENABLED(CONFIG_MEMCG))
if (cache_needs_objcg(s))
sz += 1;
if (slab_obj_ext_has_codetag())
@ -626,7 +626,15 @@ static inline size_t cache_obj_ext_size(struct kmem_cache *s)
static inline size_t slab_obj_ext_size(struct slab *slab)
{
return cache_obj_ext_size(slab->slab_cache);
size_t sz = 0;
if (slab_needs_objcg(slab))
sz += 1;
if (slab_obj_ext_has_codetag())
sz += 1;
return sizeof(struct slabobj_ext) * sz;
}
#ifdef CONFIG_SLAB_OBJ_EXT
@ -738,6 +746,8 @@ slab_obj_ext(struct kmem_cache *s, struct slab *slab, unsigned long obj_exts,
static inline struct obj_cgroup *
slab_obj_ext_objcg(struct slab *slab, struct slabobj_ext *obj_ext)
{
VM_WARN_ON_ONCE(!slab_needs_objcg(slab));
/* if objcg exists, it comes first, so we don't need to do anything */
return obj_ext->_objcg;
}
@ -746,6 +756,8 @@ static inline void
slab_obj_ext_set_objcg(struct slab *slab, struct slabobj_ext *obj_ext,
struct obj_cgroup *objcg)
{
VM_WARN_ON_ONCE(!slab_needs_objcg(slab));
/* if objcg exists, it comes first, so we don't need to do anything */
obj_ext->_objcg = objcg;
}
@ -757,7 +769,7 @@ slab_obj_ext_codetag_ref(struct slab *slab, struct slabobj_ext *obj_ext)
{
VM_WARN_ON_ONCE(!slab_obj_ext_has_codetag());
if (IS_ENABLED(CONFIG_MEMCG))
if (slab_needs_objcg(slab))
obj_ext += 1;
return &obj_ext->_ctref;

3
mm/slub.c
View File

@ -2513,6 +2513,9 @@ void memcg_slab_free_hook(struct kmem_cache *s, struct slab *slab, void **p,
if (likely(!obj_exts))
return;
if (!slab_needs_objcg(slab))
return;
get_slab_obj_exts(obj_exts);
__memcg_slab_free_hook(s, slab, p, objects, obj_exts);
put_slab_obj_exts(obj_exts);