[PATCH RFC 3/4] mm, slab: introduce kmem_cache_charge()

[Vlastimil Babka]

As suggested by Linus, introduce a slab API function to memcg-charge a
an object that was previously allocated without __GFP_ACCOUNT and from a
cache that's not SLAB_ACCOUNT. This may be useful when it's likely the
object is to be freed soon, and thus the charging/uncharging overhead
can be avoided.

In case kmem_cache_charge() is called on an already-charged object, it's
a no-op.

Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Link: https://lore.kernel.org/all/CAHk-=whYOOdM7jWy5jdrAm8LxcgCMFyk2bt8fYYvZzM4U-zAQA@xxxxxxxxxxxxxx/
Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
---
 include/linux/slab.h | 10 ++++++++++
 mm/slub.c            | 29 +++++++++++++++++++++++++++++
 2 files changed, 39 insertions(+)

diff --git a/include/linux/slab.h b/include/linux/slab.h
index b5f5ee8308d0..0c3acb2fa3e6 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -491,6 +491,16 @@ void *kmem_cache_alloc(struct kmem_cache *cachep, gfp_t flags) __assume_slab_ali
 void *kmem_cache_alloc_lru(struct kmem_cache *s, struct list_lru *lru,
 			   gfp_t gfpflags) __assume_slab_alignment __malloc;
 void kmem_cache_free(struct kmem_cache *s, void *objp);
+#ifdef CONFIG_MEMCG_KMEM
+int kmem_cache_charge(struct kmem_cache *s, gfp_t flags, void *objp);
+#else
+static inline int
+kmem_cache_charge(struct kmem_cache *s, gfp_t flags, void *objp)
+{
+	return 0;
+}
+#endif
+
 
 /*
  * Bulk allocation and freeing operations. These are accelerated in an
diff --git a/mm/slub.c b/mm/slub.c
index 64da169d672a..72b61b379ba1 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -4241,6 +4241,35 @@ void kmem_cache_free(struct kmem_cache *s, void *x)
 }
 EXPORT_SYMBOL(kmem_cache_free);
 
+#ifdef CONFIG_MEMCG_KMEM
+int kmem_cache_charge(struct kmem_cache *s, gfp_t flags, void *x)
+{
+	struct obj_cgroup ** objcg;
+	struct slab *slab;
+
+	s = cache_from_obj(s, x);
+	if (!s)
+		return -EINVAL;
+
+	if (likely(!memcg_kmem_online()))
+		return 0;
+
+	/* was it already accounted? */
+	slab = virt_to_slab(x);
+	if ((objcg = slab_objcgs(slab))) {
+		unsigned int off = obj_to_index(s, slab, x);
+
+		if (objcg[off])
+			return 0;
+	}
+
+	if (!memcg_slab_post_alloc_hook(s, NULL, flags, 1, &x))
+		return -ENOMEM;
+
+	return 0;
+}
+#endif
+
 static void free_large_kmalloc(struct folio *folio, void *object)
 {
 	unsigned int order = folio_order(folio);

-- 
2.44.0