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back and consider that cache_from_obj() is a relict from when memcgs created their separate kmem_cache copies, as the outdated comment in build_detached_freelist() reminds us.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 Closes: https://lore.kernel.org/all/20260115130642.3419324-1-edumazet@google.com/ Reviewed-by: Harry Yoo Reviewed-by: Hao Li Signed-off-by: Vlastimil Babka --- I'll put this to slab/for-next. Thanks for the reviews! Changes in v2: - Add a comment and mention in the commit log that we are leaking the freed object also in cases where s != slab->slab_cache (Hao Li) - Link to v1: https://patch.msgid.link/20260120-b4-remove_cache_from_obj-v1-1-ace30c41eecf@suse.cz --- mm/slub.c | 56 +++++++++++++++++++++++++++++++++----------------------- 1 file changed, 33 insertions(+), 23 deletions(-) diff --git a/mm/slub.c b/mm/slub.c index 861592ac5425..fd915ea95121 100644 --- a/mm/slub.c +++ b/mm/slub.c @@ -6738,30 +6738,26 @@ 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; + } } /** @@ -6774,11 +6770,25 @@ 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); @@ -7305,7 +7315,7 @@ 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 */ --- base-commit: 0f61b1860cc3f52aef9036d7235ed1f017632193 change-id: 20260120-b4-remove_cache_from_obj-190fcaf16789 Best regards, -- Vlastimil Babka