[PATCH v10 11/22] mm: move obj_to_index to include/linux/slab_def.h

[Andrey Konovalov <andreyknvl@google.com>]

While with SLUB we can actually preassign tags for caches with contructors
and store them in pointers in the freelist, SLAB doesn't allow that since
the freelist is stored as an array of indexes, so there are no pointers to
store the tags.

Instead we compute the tag twice, once when a slab is created before
calling the constructor and then again each time when an object is
allocated with kmalloc. Tag is computed simply by taking the lowest byte of
the index that corresponds to the object. However in kasan_kmalloc we only
have access to the objects pointer, so we need a way to find out which
index this object corresponds to.

This patch moves obj_to_index from slab.c to include/linux/slab_def.h to
be reused by KASAN.

Acked-by: Christoph Lameter <cl@linux.com>
Reviewed-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
---
 include/linux/slab_def.h | 13 +++++++++++++
 mm/slab.c                | 13 -------------
 2 files changed, 13 insertions(+), 13 deletions(-)

diff --git a/include/linux/slab_def.h b/include/linux/slab_def.h
index 3485c58cfd1c..9a5eafb7145b 100644
--- a/include/linux/slab_def.h
+++ b/include/linux/slab_def.h
@@ -104,4 +104,17 @@ static inline void *nearest_obj(struct kmem_cache *cache, struct page *page,
 		return object;
 }
 
+/*
+ * We want to avoid an expensive divide : (offset / cache->size)
+ *   Using the fact that size is a constant for a particular cache,
+ *   we can replace (offset / cache->size) by
+ *   reciprocal_divide(offset, cache->reciprocal_buffer_size)
+ */
+static inline unsigned int obj_to_index(const struct kmem_cache *cache,
+					const struct page *page, void *obj)
+{
+	u32 offset = (obj - page->s_mem);
+	return reciprocal_divide(offset, cache->reciprocal_buffer_size);
+}
+
 #endif	/* _LINUX_SLAB_DEF_H */
diff --git a/mm/slab.c b/mm/slab.c
index 27859fb39889..d2f827316dfc 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -406,19 +406,6 @@ static inline void *index_to_obj(struct kmem_cache *cache, struct page *page,
 	return page->s_mem + cache->size * idx;
 }
 
-/*
- * We want to avoid an expensive divide : (offset / cache->size)
- *   Using the fact that size is a constant for a particular cache,
- *   we can replace (offset / cache->size) by
- *   reciprocal_divide(offset, cache->reciprocal_buffer_size)
- */
-static inline unsigned int obj_to_index(const struct kmem_cache *cache,
-					const struct page *page, void *obj)
-{
-	u32 offset = (obj - page->s_mem);
-	return reciprocal_divide(offset, cache->reciprocal_buffer_size);
-}
-
 #define BOOT_CPUCACHE_ENTRIES	1
 /* internal cache of cache description objs */
 static struct kmem_cache kmem_cache_boot = {
-- 
2.19.1.930.g4563a0d9d0-goog