objext objects are created with __GFP_NO_OBJ_EXT flag and therefore have
no corresponding objext themselves (otherwise we would get an infinite
recursion). When freeing these objects their codetag will be empty and
when CONFIG_MEM_ALLOC_PROFILING_DEBUG is enabled this will lead to false
warnings. Introduce CODETAG_EMPTY special codetag value to mark
allocations which intentionally lack codetag to avoid these warnings.
Set objext codetags to CODETAG_EMPTY before freeing to indicate that
the codetag is expected to be empty.

Signed-off-by: Suren Baghdasaryan <surenb@google.com>
---
 include/linux/alloc_tag.h | 26 ++++++++++++++++++++++++++
 mm/slab.h                 | 25 +++++++++++++++++++++++++
 mm/slab_common.c          |  1 +
 mm/slub.c                 |  8 ++++++++
 4 files changed, 60 insertions(+)

diff --git a/include/linux/alloc_tag.h b/include/linux/alloc_tag.h
index 0a5973c4ad77..1f3207097b03 100644

index c4bd0d5348cb..cf332a839bf4 100644

--- a/mm/slab.h
+++ b/mm/slab.h

@@ -567,6 +567,31 @@ static inline struct slabobj_ext *slab_obj_exts(struct slab *slab)
 int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
 			gfp_t gfp, bool new_slab);
 
+
+#ifdef CONFIG_MEM_ALLOC_PROFILING_DEBUG
+
+static inline void mark_objexts_empty(struct slabobj_ext *obj_exts)
+{
+	struct slabobj_ext *slab_exts;
+	struct slab *obj_exts_slab;
+
+	obj_exts_slab = virt_to_slab(obj_exts);
+	slab_exts = slab_obj_exts(obj_exts_slab);
+	if (slab_exts) {
+		unsigned int offs = obj_to_index(obj_exts_slab->slab_cache,
+						 obj_exts_slab, obj_exts);
+		/* codetag should be NULL */
+		WARN_ON(slab_exts[offs].ref.ct);
+		set_codetag_empty(&slab_exts[offs].ref);
+	}
+}
+
+#else /* CONFIG_MEM_ALLOC_PROFILING_DEBUG */
+
+static inline void mark_objexts_empty(struct slabobj_ext *obj_exts) {}
+
+#endif /* CONFIG_MEM_ALLOC_PROFILING_DEBUG */
+

 static inline bool need_slab_obj_ext(void)
 {
 #ifdef CONFIG_MEM_ALLOC_PROFILING

diff --git a/mm/slab_common.c b/mm/slab_common.c
index 21b0b9e9cd9e..d5f75d04ced2 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c

@@ -242,6 +242,7 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
 		 * assign slabobj_exts in parallel. In this case the existing
 		 * objcg vector should be reused.
 		 */
+		mark_objexts_empty(vec);
 		kfree(vec);
 		return 0;
 	}

diff --git a/mm/slub.c b/mm/slub.c
index 4d480784942e..1136ff18b4fe 100644
--- a/mm/slub.c
+++ b/mm/slub.c

@@ -1890,6 +1890,14 @@ static inline void free_slab_obj_exts(struct slab *slab)
 	if (!obj_exts)
 		return;
 
+	/*
+	 * obj_exts was created with __GFP_NO_OBJ_EXT flag, therefore its
+	 * corresponding extension will be NULL. alloc_tag_sub() will throw a
+	 * warning if slab has extensions but the extension of an object is
+	 * NULL, therefore replace NULL with CODETAG_EMPTY to indicate that
+	 * the extension for obj_exts is expected to be NULL.
+	 */
+	mark_objexts_empty(obj_exts);
 	kfree(obj_exts);
 	slab->obj_exts = 0;
 }