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slab: replace cache_from_obj() with inline checks

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Eric Dumazet has noticed cache_from_obj() is not inlined with clang and
suggested splitting it into two functions, where the smaller inlined one
assumes the fastpath is !CONFIG_SLAB_FREELIST_HARDENED. However most
distros enable it these days and so this would likely add a function
call to the object free fastpaths.

Instead take a step back and consider that cache_from_obj() is a relict
from when memcgs created their separate kmem_cache copies, as the
outdated comment in build_detached_freelist() reminds us.

Meanwhile hardening/debugging had reused cache_from_obj() to validate
that the freed object really belongs to a slab from the cache we think
we are freeing from.

In build_detached_freelist() simply remove this, because it did not
handle the NULL result from cache_from_obj() failure properly, nor
validate objects (for the NULL slab->slab_cache pointer) when called via
kfree_bulk(). If anyone is motivated to implement it properly, it should
be possible in a similar way to kmem_cache_free().

In kmem_cache_free(), do the hardening/debugging checks directly so they
are inlined by definition and virt_to_slab(obj) is performed just once.
In case they failed, call a newly introduced warn_free_bad_obj() that
performs the warnings outside of the fastpath, and leak the object.

As an intentional change, leak the object when slab->slab_cache differs
from the cache given to kmem_cache_free(). Previously we would only leak
when the object is not in a valid slab page or the slab->slab_cache
pointer is NULL, and otherwise trust the slab->slab_cache over the
kmem_cache_free() argument. But if those differ, it means something went
wrong enough that it's best not to continue freeing.

As a result the fastpath should be inlined in all configs and the
warnings are moved away.

Reported-by: Eric Dumazet <edumazet@google.com>
Closes: https://lore.kernel.org/all/20260115130642.3419324-1-edumazet@google.com/
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
Reviewed-by: Hao Li <hao.li@linux.dev>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
This commit is contained in:
Vlastimil Babka 2026-01-21 07:57:10 +01:00
parent 99a3e3a1cf
commit d5dc831eb3
1 changed files with 33 additions and 23 deletions
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56
mm/slub.c
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@ -6742,30 +6742,26 @@ void ___cache_free(struct kmem_cache *cache, void *x, unsigned long addr)
}
#endif
static inline struct kmem_cache *virt_to_cache(const void *obj)
static noinline void warn_free_bad_obj(struct kmem_cache *s, void *obj)
{
struct kmem_cache *cachep;
struct slab *slab;
slab = virt_to_slab(obj);
if (WARN_ONCE(!slab, "%s: Object is not a Slab page!\n", __func__))
return NULL;
return slab->slab_cache;
}
if (WARN_ONCE(!slab,
"kmem_cache_free(%s, %p): object is not in a slab page\n",
s->name, obj))
return;
static inline struct kmem_cache *cache_from_obj(struct kmem_cache *s, void *x)
{
struct kmem_cache *cachep;
cachep = slab->slab_cache;
if (!IS_ENABLED(CONFIG_SLAB_FREELIST_HARDENED) &&
!kmem_cache_debug_flags(s, SLAB_CONSISTENCY_CHECKS))
return s;
cachep = virt_to_cache(x);
if (WARN(cachep && cachep != s,
"%s: Wrong slab cache. %s but object is from %s\n",
__func__, s->name, cachep->name))
print_tracking(cachep, x);
return cachep;
if (WARN_ONCE(cachep != s,
"kmem_cache_free(%s, %p): object belongs to different cache %s\n",
s->name, obj, cachep ? cachep->name : "(NULL)")) {
if (cachep)
print_tracking(cachep, obj);
return;
}
}
/**
@ -6778,11 +6774,25 @@ static inline struct kmem_cache *cache_from_obj(struct kmem_cache *s, void *x)
*/
void kmem_cache_free(struct kmem_cache *s, void *x)
{
s = cache_from_obj(s, x);
if (!s)
return;
struct slab *slab;
slab = virt_to_slab(x);
if (IS_ENABLED(CONFIG_SLAB_FREELIST_HARDENED) ||
kmem_cache_debug_flags(s, SLAB_CONSISTENCY_CHECKS)) {
/*
* Intentionally leak the object in these cases, because it
* would be too dangerous to continue.
*/
if (unlikely(!slab || (slab->slab_cache != s))) {
warn_free_bad_obj(s, x);
return;
}
}
trace_kmem_cache_free(_RET_IP_, x, s);
slab_free(s, virt_to_slab(x), x, _RET_IP_);
slab_free(s, slab, x, _RET_IP_);
}
EXPORT_SYMBOL(kmem_cache_free);
@ -7309,7 +7319,7 @@ int build_detached_freelist(struct kmem_cache *s, size_t size,
df->s = slab->slab_cache;
} else {
df->slab = slab;
df->s = cache_from_obj(s, object); /* Support for memcg */
df->s = s;
}
/* Start new detached freelist */