[PATCH v2 13/13] mm/slab: stop allocating objcg pointers when unnecessary

["Vlastimil Babka (SUSE)" <vbabka@kernel.org>]

Start using the slab_needs_objcg() helper to calculate slabobj_ext size.
Caches that we know to never need objcg pointers (currently
KMALLOC_NORMAL caches) will thus stop wasting memory on them when memory
allocation profiling is enabled.

For things to work properly, we need to also add slab_needs_objcg()
checks to mem_cgroup_from_obj_slab() and memcg_slab_free_hook(), because
when obj_exts array exists for a slab only due to mem_alloc profiling,
we would otherwise attempt to access a non-existing objcg pointer in
that slab.

The function __memcg_slab_post_alloc_hook() should not be possible to
call for a slab where slab_needs_objcg() is false, but add a DEBUG_VM
check there to prevent breaking this assumption accidentally.

Signed-off-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
---
 mm/memcontrol.c |  6 ++++++
 mm/slab.h       | 14 +++++++++++---
 mm/slub.c       |  3 +++
 3 files changed, 20 insertions(+), 3 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index cb1e97b4edc1..aace85fb99f9 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2871,6 +2871,9 @@ struct mem_cgroup *mem_cgroup_from_obj_slab(struct slab *slab, void *p)
 	if (!obj_exts)
 		return NULL;
 
+	if (!slab_needs_objcg(slab))
+		return NULL;
+
 	get_slab_obj_exts(obj_exts);
 	obj_ext = slab_obj_ext(slab->slab_cache, slab, obj_exts, p);
 	objcg = slab_obj_ext_objcg(obj_ext);
@@ -3580,6 +3583,9 @@ bool __memcg_slab_post_alloc_hook(struct kmem_cache *s, struct list_lru *lru,
 
 		slab = virt_to_slab(p[i]);
 
+		if (IS_ENABLED(CONFIG_DEBUG_VM) && WARN_ON_ONCE(!slab_needs_objcg(slab)))
+			continue;
+
 		if (!slab_obj_exts(slab) &&
 		    alloc_slab_obj_exts(slab, s, flags, slab_alloc_flags)) {
 			continue;
diff --git a/mm/slab.h b/mm/slab.h
index 65b44902c06e..18c2a807f023 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -615,7 +615,7 @@ static inline size_t cache_obj_ext_size(struct kmem_cache *s)
 {
 	size_t sz = 0;
 
-	if (IS_ENABLED(CONFIG_MEMCG))
+	if (cache_needs_objcg(s))
 		sz += 1;
 
 	if (slab_obj_ext_has_codetag())
@@ -626,7 +626,15 @@ static inline size_t cache_obj_ext_size(struct kmem_cache *s)
 
 static inline size_t slab_obj_ext_size(struct slab *slab)
 {
-	return cache_obj_ext_size(slab->slab_cache);
+	size_t sz = 0;
+
+	if (slab_needs_objcg(slab))
+		sz += 1;
+
+	if (slab_obj_ext_has_codetag())
+		sz += 1;
+
+	return sizeof(struct slabobj_ext) * sz;
 }
 
 #ifdef CONFIG_SLAB_OBJ_EXT
@@ -753,7 +761,7 @@ static inline void slab_obj_ext_set_objcg(struct slabobj_ext *obj_ext,
 static inline union codetag_ref *
 slab_obj_ext_codetag_ref(struct slab *slab, struct slabobj_ext *obj_ext)
 {
-	if (IS_ENABLED(CONFIG_MEMCG))
+	if (slab_needs_objcg(slab))
 		obj_ext += 1;
 
 	return &obj_ext->_ctref;
diff --git a/mm/slub.c b/mm/slub.c
index 30435e2509ea..e6148b73b92f 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2512,6 +2512,9 @@ void memcg_slab_free_hook(struct kmem_cache *s, struct slab *slab, void **p,
 	if (likely(!obj_exts))
 		return;
 
+	if (!slab_needs_objcg(slab))
+		return;
+
 	get_slab_obj_exts(obj_exts);
 	__memcg_slab_free_hook(s, slab, p, objects, obj_exts);
 	put_slab_obj_exts(obj_exts);

-- 
2.55.0