[RFC V2 PATCH 2/5] mm/slab: abstract slabobj_ext access via new slab_obj_ext() helper

[Harry Yoo <harry.yoo@oracle.com>]

Currently, the slab allocator assumes that slab->obj_exts is a pointer
to an array of struct slabobj_ext objects. However, to support storage
methods where struct slabobj_ext is embedded within objects, the slab
allocator should not make this assumption. Instead of directly
dereferencing the slabobj_exts array, abstract access to
struct slabobj_ext via helper functions.

Introduce a new API slabobj_ext metadata access:

  slab_obj_ext(slab, obj_exts, index) - returns the pointer to
  struct slabobj_ext element at the given index.

Directly dereferencing the return value of slab_obj_exts() is no longer
allowed. Instead, slab_obj_ext() must always be used to access
individual struct slabobj_ext objects.

Convert all users to use these APIs.
No functional changes intended.

Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
---
 mm/memcontrol.c | 23 ++++++++++++++++-------
 mm/slab.h       | 43 +++++++++++++++++++++++++++++++++++++------
 mm/slub.c       | 46 +++++++++++++++++++++++++++-------------------
 3 files changed, 80 insertions(+), 32 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 8dd7fbed5a94..2a9dc246e802 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2566,7 +2566,8 @@ struct mem_cgroup *mem_cgroup_from_obj_folio(struct folio *folio, void *p)
 	 * slab->obj_exts.
 	 */
 	if (folio_test_slab(folio)) {
-		struct slabobj_ext *obj_exts;
+		unsigned long obj_exts;
+		struct slabobj_ext *obj_ext;
 		struct slab *slab;
 		unsigned int off;
 
@@ -2576,8 +2577,9 @@ struct mem_cgroup *mem_cgroup_from_obj_folio(struct folio *folio, void *p)
 			return NULL;
 
 		off = obj_to_index(slab->slab_cache, slab, p);
-		if (obj_exts[off].objcg)
-			return obj_cgroup_memcg(obj_exts[off].objcg);
+		obj_ext = slab_obj_ext(slab, obj_exts, off);
+		if (obj_ext->objcg)
+			return obj_cgroup_memcg(obj_ext->objcg);
 
 		return NULL;
 	}
@@ -3168,6 +3170,9 @@ bool __memcg_slab_post_alloc_hook(struct kmem_cache *s, struct list_lru *lru,
 	}
 
 	for (i = 0; i < size; i++) {
+		unsigned long obj_exts;
+		struct slabobj_ext *obj_ext;
+
 		slab = virt_to_slab(p[i]);
 
 		if (!slab_obj_exts(slab) &&
@@ -3190,29 +3195,33 @@ bool __memcg_slab_post_alloc_hook(struct kmem_cache *s, struct list_lru *lru,
 					slab_pgdat(slab), cache_vmstat_idx(s)))
 			return false;
 
+		obj_exts = slab_obj_exts(slab);
 		off = obj_to_index(s, slab, p[i]);
+		obj_ext = slab_obj_ext(slab, obj_exts, off);
 		obj_cgroup_get(objcg);
-		slab_obj_exts(slab)[off].objcg = objcg;
+		obj_ext->objcg = objcg;
 	}
 
 	return true;
 }
 
 void __memcg_slab_free_hook(struct kmem_cache *s, struct slab *slab,
-			    void **p, int objects, struct slabobj_ext *obj_exts)
+			    void **p, int objects, unsigned long obj_exts)
 {
 	size_t obj_size = obj_full_size(s);
 
 	for (int i = 0; i < objects; i++) {
 		struct obj_cgroup *objcg;
+		struct slabobj_ext *obj_ext;
 		unsigned int off;
 
 		off = obj_to_index(s, slab, p[i]);
-		objcg = obj_exts[off].objcg;
+		obj_ext = slab_obj_ext(slab, obj_exts, off);
+		objcg = obj_ext->objcg;
 		if (!objcg)
 			continue;
 
-		obj_exts[off].objcg = NULL;
+		obj_ext->objcg = NULL;
 		refill_obj_stock(objcg, obj_size, true, -obj_size,
 				 slab_pgdat(slab), cache_vmstat_idx(s));
 		obj_cgroup_put(objcg);
diff --git a/mm/slab.h b/mm/slab.h
index f1866f2d9b21..4eb63360fdb5 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -522,10 +522,12 @@ static inline bool slab_in_kunit_test(void) { return false; }
  * associated with a slab.
  * @slab: a pointer to the slab struct
  *
- * Returns a pointer to the object extension vector associated with the slab,
- * or NULL if no such vector has been associated yet.
+ * Returns the address of the object extension vector associated with the slab,
+ * or zero if no such vector has been associated yet.
+ * Do not dereference the return value directly; use slab_obj_ext() to access
+ * its elements.
  */
-static inline struct slabobj_ext *slab_obj_exts(struct slab *slab)
+static inline unsigned long slab_obj_exts(struct slab *slab)
 {
 	unsigned long obj_exts = READ_ONCE(slab->obj_exts);
 
@@ -534,7 +536,30 @@ static inline struct slabobj_ext *slab_obj_exts(struct slab *slab)
 							slab_page(slab));
 	VM_BUG_ON_PAGE(obj_exts & MEMCG_DATA_KMEM, slab_page(slab));
 #endif
-	return (struct slabobj_ext *)(obj_exts & ~OBJEXTS_FLAGS_MASK);
+
+	return obj_exts & ~OBJEXTS_FLAGS_MASK;
+}
+
+/*
+ * slab_obj_ext - get the pointer to the slab object extension metadata
+ * associated with an object in a slab.
+ * @slab: a pointer to the slab struct
+ * @obj_exts: a pointer to the object extension vector
+ * @index: an index of the object
+ *
+ * Returns a pointer to the object extension associated with the object.
+ */
+static inline struct slabobj_ext *slab_obj_ext(struct slab *slab,
+					       unsigned long obj_exts,
+					       unsigned int index)
+{
+	struct slabobj_ext *obj_ext;
+
+	VM_WARN_ON_ONCE(!slab_obj_exts(slab));
+	VM_WARN_ON_ONCE(obj_exts != slab_obj_exts(slab));
+
+	obj_ext = (struct slabobj_ext *)obj_exts;
+	return &obj_ext[index];
 }
 
 int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
@@ -542,7 +567,13 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
 
 #else /* CONFIG_SLAB_OBJ_EXT */
 
-static inline struct slabobj_ext *slab_obj_exts(struct slab *slab)
+static inline unsigned long slab_obj_exts(struct slab *slab)
+{
+	return false;
+}
+
+static inline struct slabobj_ext *slab_obj_ext(struct slab *slab,
+					       unsigned int index)
 {
 	return NULL;
 }
@@ -559,7 +590,7 @@ static inline enum node_stat_item cache_vmstat_idx(struct kmem_cache *s)
 bool __memcg_slab_post_alloc_hook(struct kmem_cache *s, struct list_lru *lru,
 				  gfp_t flags, size_t size, void **p);
 void __memcg_slab_free_hook(struct kmem_cache *s, struct slab *slab,
-			    void **p, int objects, struct slabobj_ext *obj_exts);
+			    void **p, int objects, unsigned long obj_exts);
 #endif
 
 void kvfree_rcu_cb(struct rcu_head *head);
diff --git a/mm/slub.c b/mm/slub.c
index 9e91f2016697..33e2692ca618 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2028,9 +2028,12 @@ static inline void mark_objexts_empty(struct slabobj_ext *obj_exts)
 	if (slab_exts) {
 		unsigned int offs = obj_to_index(obj_exts_slab->slab_cache,
 						 obj_exts_slab, obj_exts);
+		struct slab slabobj_ext *ext = slab_obj_ext(obj_exts_slab,
+							    slab_exts, offs);
+
 		/* codetag should be NULL */
-		WARN_ON(slab_exts[offs].ref.ct);
-		set_codetag_empty(&slab_exts[offs].ref);
+		WARN_ON(ext->ref.ct);
+		set_codetag_empty(&ext->ref);
 	}
 }
 
@@ -2129,7 +2132,7 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
 
 static inline void free_slab_obj_exts(struct slab *slab)
 {
-	struct slabobj_ext *obj_exts;
+	unsigned long obj_exts;
 
 	obj_exts = slab_obj_exts(slab);
 	if (!obj_exts)
@@ -2142,8 +2145,8 @@ static inline void free_slab_obj_exts(struct slab *slab)
 	 * 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);
+	mark_objexts_empty((struct slabobj_ext *)obj_exts);
+	kfree((void *)obj_exts);
 	slab->obj_exts = 0;
 }
 
@@ -2168,9 +2171,10 @@ static inline void free_slab_obj_exts(struct slab *slab)
 #ifdef CONFIG_MEM_ALLOC_PROFILING
 
 static inline struct slabobj_ext *
-prepare_slab_obj_exts_hook(struct kmem_cache *s, gfp_t flags, void *p)
+prepare_slab_obj_ext_hook(struct kmem_cache *s, gfp_t flags, void *p)
 {
 	struct slab *slab;
+	unsigned long obj_exts;
 
 	if (!p)
 		return NULL;
@@ -2182,30 +2186,32 @@ prepare_slab_obj_exts_hook(struct kmem_cache *s, gfp_t flags, void *p)
 		return NULL;
 
 	slab = virt_to_slab(p);
-	if (!slab_obj_exts(slab) &&
+	obj_exts = slab_obj_exts(slab);
+	if (!obj_exts &&
 	    alloc_slab_obj_exts(slab, s, flags, false)) {
 		pr_warn_once("%s, %s: Failed to create slab extension vector!\n",
 			     __func__, s->name);
 		return NULL;
 	}
 
-	return slab_obj_exts(slab) + obj_to_index(s, slab, p);
+	obj_exts = slab_obj_exts(slab);
+	return slab_obj_ext(slab, obj_exts, obj_to_index(s, slab, p));
 }
 
 /* Should be called only if mem_alloc_profiling_enabled() */
 static noinline void
 __alloc_tagging_slab_alloc_hook(struct kmem_cache *s, void *object, gfp_t flags)
 {
-	struct slabobj_ext *obj_exts;
+	struct slabobj_ext *obj_ext;
 
-	obj_exts = prepare_slab_obj_exts_hook(s, flags, object);
+	obj_ext = prepare_slab_obj_ext_hook(s, flags, object);
 	/*
 	 * Currently obj_exts is used only for allocation profiling.
 	 * If other users appear then mem_alloc_profiling_enabled()
 	 * check should be added before alloc_tag_add().
 	 */
-	if (likely(obj_exts))
-		alloc_tag_add(&obj_exts->ref, current->alloc_tag, s->size);
+	if (likely(obj_ext))
+		alloc_tag_add(&obj_ext->ref, current->alloc_tag, s->size);
 }
 
 static inline void
@@ -2220,8 +2226,8 @@ static noinline void
 __alloc_tagging_slab_free_hook(struct kmem_cache *s, struct slab *slab, void **p,
 			       int objects)
 {
-	struct slabobj_ext *obj_exts;
 	int i;
+	unsigned long obj_exts;
 
 	/* slab->obj_exts might not be NULL if it was created for MEMCG accounting. */
 	if (s->flags & (SLAB_NO_OBJ_EXT | SLAB_NOLEAKTRACE))
@@ -2234,7 +2240,7 @@ __alloc_tagging_slab_free_hook(struct kmem_cache *s, struct slab *slab, void **p
 	for (i = 0; i < objects; i++) {
 		unsigned int off = obj_to_index(s, slab, p[i]);
 
-		alloc_tag_sub(&obj_exts[off].ref, s->size);
+		alloc_tag_sub(&slab_obj_ext(slab, obj_exts, off)->ref, s->size);
 	}
 }
 
@@ -2293,7 +2299,7 @@ static __fastpath_inline
 void memcg_slab_free_hook(struct kmem_cache *s, struct slab *slab, void **p,
 			  int objects)
 {
-	struct slabobj_ext *obj_exts;
+	unsigned long obj_exts;
 
 	if (!memcg_kmem_online())
 		return;
@@ -2308,7 +2314,8 @@ void memcg_slab_free_hook(struct kmem_cache *s, struct slab *slab, void **p,
 static __fastpath_inline
 bool memcg_slab_post_charge(void *p, gfp_t flags)
 {
-	struct slabobj_ext *slab_exts;
+	unsigned long obj_exts;
+	struct slabobj_ext *obj_ext;
 	struct kmem_cache *s;
 	struct folio *folio;
 	struct slab *slab;
@@ -2348,10 +2355,11 @@ bool memcg_slab_post_charge(void *p, gfp_t flags)
 		return true;
 
 	/* Ignore already charged objects. */
-	slab_exts = slab_obj_exts(slab);
-	if (slab_exts) {
+	obj_exts = slab_obj_exts(slab);
+	if (obj_exts) {
 		off = obj_to_index(s, slab, p);
-		if (unlikely(slab_exts[off].objcg))
+		obj_ext = slab_obj_ext(slab, obj_exts, off);
+		if (unlikely(obj_ext->objcg))
 			return true;
 	}
 
-- 
2.43.0