[PATCH RFC 06/14] mm: slub: implement SLUB version of obj_to_index()

[Roman Gushchin <guro@fb.com>]

This commit implements SLUB version of the obj_to_index() function,
which will be required to calculate the offset of memcg_ptr in the
mem_cgroup_vec to store/obtain the memcg ownership data.

To make it faster, let's repeat the SLAB's trick introduced by
commit 6a2d7a955d8d ("[PATCH] SLAB: use a multiply instead of a
divide in obj_to_index()") and avoid an expensive division.

Signed-off-by: Roman Gushchin <guro@fb.com>
---
 include/linux/slub_def.h | 9 +++++++++
 mm/slub.c                | 1 +
 2 files changed, 10 insertions(+)

diff --git a/include/linux/slub_def.h b/include/linux/slub_def.h
index d2153789bd9f..200ea292f250 100644
--- a/include/linux/slub_def.h
+++ b/include/linux/slub_def.h
@@ -8,6 +8,7 @@
  * (C) 2007 SGI, Christoph Lameter
  */
 #include <linux/kobject.h>
+#include <linux/reciprocal_div.h>
 
 enum stat_item {
 	ALLOC_FASTPATH,		/* Allocation from cpu slab */
@@ -86,6 +87,7 @@ struct kmem_cache {
 	unsigned long min_partial;
 	unsigned int size;	/* The size of an object including metadata */
 	unsigned int object_size;/* The size of an object without metadata */
+	struct reciprocal_value reciprocal_size;
 	unsigned int offset;	/* Free pointer offset */
 #ifdef CONFIG_SLUB_CPU_PARTIAL
 	/* Number of per cpu partial objects to keep around */
@@ -182,4 +184,11 @@ static inline void *nearest_obj(struct kmem_cache *cache, struct page *page,
 	return result;
 }
 
+static inline unsigned int obj_to_index(const struct kmem_cache *cache,
+					const struct page *page, void *obj)
+{
+	return reciprocal_divide(kasan_reset_tag(obj) - page_address(page),
+				 cache->reciprocal_size);
+}
+
 #endif /* _LINUX_SLUB_DEF_H */
diff --git a/mm/slub.c b/mm/slub.c
index 3014158c100d..b043cfb673c9 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -3587,6 +3587,7 @@ static int calculate_sizes(struct kmem_cache *s, int forced_order)
 	 */
 	size = ALIGN(size, s->align);
 	s->size = size;
+	s->reciprocal_size = reciprocal_value(size);
 	if (forced_order >= 0)
 		order = forced_order;
 	else
-- 
2.21.0