mm, slab/slub: move and improve cache_from_obj()

The function cache_from_obj() was added by commit b9ce5ef49f ("sl[au]b:
always get the cache from its page in kmem_cache_free()") to support
kmemcg, where per-memcg cache can be different from the root one, so we
can't use the kmem_cache pointer given to kmem_cache_free().

Prior to that commit, SLUB already had debugging check+warning that could
be enabled to compare the given kmem_cache pointer to one referenced by
the slab page where the object-to-be-freed resides.  This check was moved
to cache_from_obj().  Later the check was also enabled for
SLAB_FREELIST_HARDENED configs by commit 598a0717a8 ("mm/slab: validate
cache membership under freelist hardening").

These checks and warnings can be useful especially for the debugging,
which can be improved.  Commit 598a0717a8 changed the pr_err() with
WARN_ON_ONCE() to WARN_ONCE() so only the first hit is now reported,
others are silent.  This patch changes it to WARN() so that all errors are
reported.

It's also useful to print SLUB allocation/free tracking info for the
offending object, if tracking is enabled.  We could export the SLUB
print_tracking() function and provide an empty one for SLAB, or realize
that both the debugging and hardening cases in cache_from_obj() are only
supported by SLUB anyway.  So this patch moves cache_from_obj() from
slab.h to separate instances in slab.c and slub.c, where the SLAB version
only does the kmemcg lookup and even could be completely removed once the
kmemcg rework [1] is merged.  The SLUB version can thus easily use the
print_tracking() function.  It can also use the kmem_cache_debug_flags()
static key check for improved performance in kernels without the hardening
and with debugging not enabled on boot.

[1] https://lore.kernel.org/r/20200608230654.828134-18-guro@fb.com

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Christoph Lameter <cl@linux.com>
Cc: Jann Horn <jannh@google.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Vijayanand Jitta <vjitta@codeaurora.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Pekka Enberg <penberg@kernel.org>
Link: http://lkml.kernel.org/r/20200610163135.17364-10-vbabka@suse.cz
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/slab.c

@@ -3678,6 +3678,14 @@ void *__kmalloc_track_caller(size_t size, gfp_t flags, unsigned long caller)
 }
 EXPORT_SYMBOL(__kmalloc_track_caller);
 
+static inline struct kmem_cache *cache_from_obj(struct kmem_cache *s, void *x)
+{
+	if (memcg_kmem_enabled())
+		return virt_to_cache(x);
+	else
+		return s;
+}
+
 /**
  * kmem_cache_free - Deallocate an object
  * @cachep: The cache the allocation was from.

mm/slab.h

@@ -504,29 +504,6 @@ static __always_inline void uncharge_slab_page(struct page *page, int order,
 	memcg_uncharge_slab(page, order, s);
 }
 
-static inline struct kmem_cache *cache_from_obj(struct kmem_cache *s, void *x)
-{
-	struct kmem_cache *cachep;
-
-	/*
-	 * When kmemcg is not being used, both assignments should return the
-	 * same value. but we don't want to pay the assignment price in that
-	 * case. If it is not compiled in, the compiler should be smart enough
-	 * to not do even the assignment. In that case, slab_equal_or_root
-	 * will also be a constant.
-	 */
-	if (!memcg_kmem_enabled() &&
-	    !IS_ENABLED(CONFIG_SLAB_FREELIST_HARDENED) &&
-	    !unlikely(s->flags & SLAB_CONSISTENCY_CHECKS))
-		return s;
-
-	cachep = virt_to_cache(x);
-	WARN_ONCE(cachep && !slab_equal_or_root(cachep, s),
-		  "%s: Wrong slab cache. %s but object is from %s\n",
-		  __func__, s->name, cachep->name);
-	return cachep;
-}
-
 static inline size_t slab_ksize(const struct kmem_cache *s)
 {
 #ifndef CONFIG_SLUB

mm/slub.c

@@ -1525,6 +1525,10 @@ static bool freelist_corrupted(struct kmem_cache *s, struct page *page,
 {
 	return false;
 }
+
+static void print_tracking(struct kmem_cache *s, void *object)
+{
+}
 #endif /* CONFIG_SLUB_DEBUG */
 
 /*
@@ -3171,6 +3175,23 @@ void ___cache_free(struct kmem_cache *cache, void *x, unsigned long addr)
 }
 #endif
 
+static inline struct kmem_cache *cache_from_obj(struct kmem_cache *s, void *x)
+{
+	struct kmem_cache *cachep;
+
+	if (!IS_ENABLED(CONFIG_SLAB_FREELIST_HARDENED) &&
+	    !memcg_kmem_enabled() &&
+	    !kmem_cache_debug_flags(s, SLAB_CONSISTENCY_CHECKS))
+		return s;
+
+	cachep = virt_to_cache(x);
+	if (WARN(cachep && !slab_equal_or_root(cachep, s),
+		  "%s: Wrong slab cache. %s but object is from %s\n",
+		  __func__, s->name, cachep->name))
+		print_tracking(cachep, x);
+	return cachep;
+}
+
 void kmem_cache_free(struct kmem_cache *s, void *x)
 {
 	s = cache_from_obj(s, x);