[PATCH RFC 12/12] 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       | 12 ++++++++++--
 mm/slub.c       |  3 +++
 3 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 6303a2b1a9d0..09659722ec85 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_objcgp(obj_ext);
@@ -3581,6 +3584,9 @@ 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 948d075cdbef..072cc2506756 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -622,7 +622,15 @@ static inline size_t static_obj_ext_size(void)
 
 static inline size_t slab_obj_ext_size(struct slab *slab)
 {
-	return static_obj_ext_size();
+	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
@@ -741,7 +749,7 @@ static inline struct obj_cgroup **slab_obj_ext_objcgp(struct slabobj_ext *obj_ex
 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 771d73abacb6..09c4931e5435 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2503,6 +2503,9 @@ 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