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