[PATCH RFC 11/12] mm/slab: add slab_needs_objcg() helper

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

Slabs of some caches never need the objcg part of struct slabobj_ext.
Introduce a helper to query this for a slab.

Currently only is_kmalloc_normal() caches are considered as not needing
objcg's. We could also consider all kmem caches without SLAB_ACCOUNT,
however some might be used with and without __GFP_ACCOUNT concurrently
and we currently don't restrict that. This can be improved later.

To make the evaluation of slab_needs_objcg() faster in the allocation
and free fast paths, add a obj_exts_needs_objcg flag into slab itself.
This optimization is only available on 64bit architectures where free
bits are available for the flag.

Signed-off-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
---
 mm/kfence/core.c |  3 +++
 mm/slab.h        | 31 +++++++++++++++++++++++++++++--
 mm/slub.c        |  4 ++++
 3 files changed, 36 insertions(+), 2 deletions(-)

diff --git a/mm/kfence/core.c b/mm/kfence/core.c
index 717e8baf7e5d..afeaf80484ad 100644
--- a/mm/kfence/core.c
+++ b/mm/kfence/core.c
@@ -640,6 +640,9 @@ static unsigned long kfence_init_pool(void)
 		struct slab *slab = page_slab(page);
 		slab->obj_exts = (unsigned long)&kfence_metadata_init[i / 2 - 1].obj_exts |
 				 MEMCG_DATA_OBJEXTS;
+#ifdef CONFIG_64BIT
+		slab->obj_exts_needs_objcg = 1;
+#endif
 #endif
 	}
 
diff --git a/mm/slab.h b/mm/slab.h
index a50347c9dbe3..948d075cdbef 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -81,10 +81,11 @@ struct freelist_counters {
 #ifdef CONFIG_64BIT
 					/*
 					 * Some optimizations use free bits in 'counters' field
-					 * to save memory. If these free bits are not available,
-					 * such optimizations are disabled.
+					 * to save memory or CPU. If these free bits are not
+					 * available, such optimizations are disabled.
 					 */
 					unsigned obj_exts_in_object:1;
+					unsigned obj_exts_needs_objcg:1;
 #endif
 				};
 			};
@@ -580,6 +581,32 @@ static inline bool slab_obj_ext_has_codetag(void)
 }
 #endif
 
+#ifdef CONFIG_MEMCG
+static inline bool cache_needs_objcg(struct kmem_cache *cache)
+{
+	return !is_kmalloc_normal(cache);
+}
+
+static inline bool slab_needs_objcg(struct slab *slab)
+{
+#ifdef CONFIG_64BIT
+	return slab->obj_exts_needs_objcg;
+#else
+	return cache_needs_objcg(slab->slab_cache);
+#endif
+}
+#else
+static inline bool cache_needs_objcg(struct kmem_cache *cache)
+{
+	return false;
+}
+
+static inline bool slab_needs_objcg(struct slab *slab)
+{
+	return false;
+}
+#endif
+
 static inline size_t static_obj_ext_size(void)
 {
 	size_t sz = 0;
diff --git a/mm/slub.c b/mm/slub.c
index 4200e7105b30..771d73abacb6 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -3426,6 +3426,10 @@ static struct slab *allocate_slab(struct kmem_cache *s, gfp_t flags,
 
 	slab->objects = oo_objects(oo);
 
+#ifdef CONFIG_64BIT
+	if (cache_needs_objcg(s))
+		slab->obj_exts_needs_objcg = 1;
+#endif
 	slab->slab_cache = s;
 
 	kasan_poison_slab(slab);

-- 
2.55.0