Commit d5dc831e authored Jan 21, 2026 by Vlastimil Babka

slab: replace cache_from_obj() with inline checks



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outdated comment in build_detached_freelist() reminds us.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>

parent 99a3e3a1

mm/slub.c

+33 −23

Original line number	Diff line number	Diff line
		@@ -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;
		}

		static inline struct kmem_cache cache_from_obj(struct kmem_cache s, void *x)
		{
		struct kmem_cache *cachep;
		if (WARN_ONCE(!slab,
		"kmem_cache_free(%s, %p): object is not in a slab page\n",
		s->name, obj))
		return;

		if (!IS_ENABLED(CONFIG_SLAB_FREELIST_HARDENED) &&
		!kmem_cache_debug_flags(s, SLAB_CONSISTENCY_CHECKS))
		return s;
		cachep = slab->slab_cache;

		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)
		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 */