[PATCH] mm, slab/slub: improve error reporting and overhead of cache_from_obj()

[Vlastimil Babka]

The function cache_from_obj() was added by commit b9ce5ef49f00 ("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 598a0717a816 ("mm/slab: validate
cache membership under freelist hardening").

These checks and warnings can be useful especially for the debugging,
which can be improved.  Commit 598a0717a816 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. Thus, export the SLUB print_tracking() function
and provide an empty one for SLAB.

For SLUB we can also benefit from the static key check in
kmem_cache_debug_flags(), but we need to move this function to slab.h and
declare the static key there.

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

Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
---
 mm/slab.c |  8 --------
 mm/slab.h | 45 +++++++++++++++++++++++++++++++++++++++++++++
 mm/slub.c | 38 +-------------------------------------
 3 files changed, 46 insertions(+), 45 deletions(-)

diff --git a/mm/slab.c b/mm/slab.c
index 6134c4c36d4c..9350062ffc1a 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -3672,14 +3672,6 @@ 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.
diff --git a/mm/slab.h b/mm/slab.h
index a2696d306b62..a9f5ba9ce9a7 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -275,6 +275,34 @@ static inline int cache_vmstat_idx(struct kmem_cache *s)
 		NR_SLAB_RECLAIMABLE : NR_SLAB_UNRECLAIMABLE;
 }
 
+#ifdef CONFIG_SLUB_DEBUG
+#ifdef CONFIG_SLUB_DEBUG_ON
+DECLARE_STATIC_KEY_TRUE(slub_debug_enabled);
+#else
+DECLARE_STATIC_KEY_FALSE(slub_debug_enabled);
+#endif
+extern void print_tracking(struct kmem_cache *s, void *object);
+#else
+static inline void print_tracking(struct kmem_cache *s, void *object)
+{
+}
+#endif
+
+/*
+ * Returns true if any of the specified slub_debug flags is enabled for the
+ * cache. Use only for flags parsed by setup_slub_debug() as it also enables
+ * the static key.
+ */
+static inline bool kmem_cache_debug_flags(struct kmem_cache *s, slab_flags_t flags)
+{
+	VM_WARN_ON_ONCE(!(flags & SLAB_DEBUG_FLAGS));
+#ifdef CONFIG_SLUB_DEBUG
+	if (static_branch_unlikely(&slub_debug_enabled))
+		return s->flags & flags;
+#endif
+	return false;
+}
+
 #ifdef CONFIG_MEMCG_KMEM
 
 /* List of all root caches. */
@@ -503,6 +531,23 @@ 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;
+
+	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;
+}
+
 static inline size_t slab_ksize(const struct kmem_cache *s)
 {
 #ifndef CONFIG_SLUB
diff --git a/mm/slub.c b/mm/slub.c
index 202fb423d195..0e635a8aa340 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -122,21 +122,6 @@ DEFINE_STATIC_KEY_FALSE(slub_debug_enabled);
 #endif
 #endif
 
-/*
- * Returns true if any of the specified slub_debug flags is enabled for the
- * cache. Use only for flags parsed by setup_slub_debug() as it also enables
- * the static key.
- */
-static inline bool kmem_cache_debug_flags(struct kmem_cache *s, slab_flags_t flags)
-{
-	VM_WARN_ON_ONCE(!(flags & SLAB_DEBUG_FLAGS));
-#ifdef CONFIG_SLUB_DEBUG
-	if (static_branch_unlikely(&slub_debug_enabled))
-		return s->flags & flags;
-#endif
-	return false;
-}
-
 static inline bool kmem_cache_debug(struct kmem_cache *s)
 {
 	return kmem_cache_debug_flags(s, SLAB_DEBUG_FLAGS);
@@ -653,7 +638,7 @@ static void print_track(const char *s, struct track *t, unsigned long pr_time)
 #endif
 }
 
-static void print_tracking(struct kmem_cache *s, void *object)
+void print_tracking(struct kmem_cache *s, void *object)
 {
 	unsigned long pr_time = jiffies;
 	if (!(s->flags & SLAB_STORE_USER))
@@ -1525,10 +1510,6 @@ 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 */
 
 /*
@@ -3180,23 +3161,6 @@ 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);
-- 
2.27.0