codetag: debug: introduce OBJEXTS_ALLOC_FAIL to mark failed slab_ext allocations

If slabobj_ext vector allocation for a slab object fails and later on it
succeeds for another object in the same slab, the slabobj_ext for the
original object will be NULL and will be flagged in case when
CONFIG_MEM_ALLOC_PROFILING_DEBUG is enabled.
Mark failed slabobj_ext vector allocations using a new objext_flags flag
stored in the lower bits of slab->obj_exts. When new allocation succeeds
it marks all tag references in the same slabobj_ext vector as empty to
avoid warnings implemented by CONFIG_MEM_ALLOC_PROFILING_DEBUG checks.

Signed-off-by: Suren Baghdasaryan <surenb@google.com>

include/linux/memcontrol.h

@@ -356,8 +356,10 @@ enum page_memcg_data_flags {
 #endif /* CONFIG_MEMCG */
 
 enum objext_flags {
+	/* slabobj_ext vector failed to allocate */
+	OBJEXTS_ALLOC_FAIL = __FIRST_OBJEXT_FLAG,
 	/* the next bit after the last actual flag */
-	__NR_OBJEXTS_FLAGS  = __FIRST_OBJEXT_FLAG,
+	__NR_OBJEXTS_FLAGS  = (__FIRST_OBJEXT_FLAG << 1),
 };
 
 #define OBJEXTS_FLAGS_MASK (__NR_OBJEXTS_FLAGS - 1)

mm/slab.h

@@ -474,9 +474,34 @@ static inline void mark_objexts_empty(struct slabobj_ext *obj_exts)
 	}
 }
 
+static inline void mark_failed_objexts_alloc(struct slab *slab)
+{
+	slab->obj_exts = OBJEXTS_ALLOC_FAIL;
+}
+
+static inline void handle_failed_objexts_alloc(unsigned long obj_exts,
+			struct slabobj_ext *vec, unsigned int objects)
+{
+	/*
+	 * If vector previously failed to allocate then we have live
+	 * objects with no tag reference. Mark all references in this
+	 * vector as empty to avoid warnings later on.
+	 */
+	if (obj_exts & OBJEXTS_ALLOC_FAIL) {
+		unsigned int i;
+
+		for (i = 0; i < objects; i++)
+			set_codetag_empty(&vec[i].ref);
+	}
+}
+
+
 #else /* CONFIG_MEM_ALLOC_PROFILING_DEBUG */
 
 static inline void mark_objexts_empty(struct slabobj_ext *obj_exts) {}
+static inline void mark_failed_objexts_alloc(struct slab *slab) {}
+static inline void handle_failed_objexts_alloc(unsigned long obj_exts,
+			struct slabobj_ext *vec, unsigned int objects) {}
 
 #endif /* CONFIG_MEM_ALLOC_PROFILING_DEBUG */
 

mm/slab_common.c

@@ -218,29 +218,37 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
 			gfp_t gfp, bool new_slab)
 {
 	unsigned int objects = objs_per_slab(s, slab);
-	unsigned long obj_exts;
-	void *vec;
+	unsigned long new_exts;
+	unsigned long old_exts;
+	struct slabobj_ext *vec;
 
 	gfp &= ~OBJCGS_CLEAR_MASK;
 	/* Prevent recursive extension vector allocation */
 	gfp |= __GFP_NO_OBJ_EXT;
 	vec = kcalloc_node(objects, sizeof(struct slabobj_ext), gfp,
 			   slab_nid(slab));
-	if (!vec)
+	if (!vec) {
+		/* Mark vectors which failed to allocate */
+		if (new_slab)
+			mark_failed_objexts_alloc(slab);
+
 		return -ENOMEM;
+	}
 
-	obj_exts = (unsigned long)vec;
+	new_exts = (unsigned long)vec;
 #ifdef CONFIG_MEMCG
-	obj_exts |= MEMCG_DATA_OBJEXTS;
+	new_exts |= MEMCG_DATA_OBJEXTS;
 #endif
+	old_exts = slab->obj_exts;
+	handle_failed_objexts_alloc(old_exts, vec, objects);
 	if (new_slab) {
 		/*
 		 * If the slab is brand new and nobody can yet access its
 		 * obj_exts, no synchronization is required and obj_exts can
 		 * be simply assigned.
 		 */
-		slab->obj_exts = obj_exts;
-	} else if (cmpxchg(&slab->obj_exts, 0, obj_exts)) {
+		slab->obj_exts = new_exts;
+	} else if (cmpxchg(&slab->obj_exts, old_exts, new_exts) != old_exts) {
 		/*
 		 * If the slab is already in use, somebody can allocate and
 		 * assign slabobj_exts in parallel. In this case the existing