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outdated comment in build_detached_freelist() reminds us.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>

diff --git a/mm/slub.c b/mm/slub.c
index f77b7407c51bca0712df0fea402936b98bfb3d0b..28af56acc3ab02e3cd16f3c2c6fc1f849b416a6e 100644 (file)
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -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)
-               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 */