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slab: restrict the number of objects in a slab 

To prepare to implement byte sized index for managing the freelist
of a slab, we should restrict the number of objects in a slab to be less
or equal to 256, since byte only represent 256 different values.
Setting the size of object to value equal or more than newly introduced
SLAB_OBJ_MIN_SIZE ensures that the number of objects in a slab is less or
equal to 256 for a slab with 1 page.

If page size is rather larger than 4096, above assumption would be wrong.
In this case, we would fall back on 2 bytes sized index.

If minimum size of kmalloc is less than 16, we use it as minimum object
size and give up this optimization.

Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>
This commit is contained in:
Joonsoo Kim 2013-12-02 17:49:41 +09:00 committed by Pekka Enberg
parent e5c58dfdcb
commit f315e3fa1c
2 changed files with 32 additions and 0 deletions
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11
include/linux/slab.h
View File

@ -201,6 +201,17 @@ struct kmem_cache {
#ifndef KMALLOC_SHIFT_LOW
#define KMALLOC_SHIFT_LOW 5
#endif
/*
* This restriction comes from byte sized index implementation.
* Page size is normally 2^12 bytes and, in this case, if we want to use
* byte sized index which can represent 2^8 entries, the size of the object
* should be equal or greater to 2^12 / 2^8 = 2^4 = 16.
* If minimum size of kmalloc is less than 16, we use it as minimum object
* size and give up to use byte sized index.
*/
#define SLAB_OBJ_MIN_SIZE (KMALLOC_SHIFT_LOW < 4 ? \
(1 << KMALLOC_SHIFT_LOW) : 16)
#endif
#ifdef CONFIG_SLUB

21
mm/slab.c
View File

@ -157,6 +157,17 @@
#define ARCH_KMALLOC_FLAGS SLAB_HWCACHE_ALIGN
#endif
#define FREELIST_BYTE_INDEX (((PAGE_SIZE >> BITS_PER_BYTE) \
<= SLAB_OBJ_MIN_SIZE) ? 1 : 0)
#if FREELIST_BYTE_INDEX
typedef unsigned char freelist_idx_t;
#else
typedef unsigned short freelist_idx_t;
#endif
#define SLAB_OBJ_MAX_NUM (1 << sizeof(freelist_idx_t) * BITS_PER_BYTE)
/*
* true if a page was allocated from pfmemalloc reserves for network-based
* swap
@ -2016,6 +2027,10 @@ static size_t calculate_slab_order(struct kmem_cache *cachep,
if (!num)
continue;
/* Can't handle number of objects more than SLAB_OBJ_MAX_NUM */
if (num > SLAB_OBJ_MAX_NUM)
break;
if (flags & CFLGS_OFF_SLAB) {
/*
* Max number of objs-per-slab for caches which
@ -2258,6 +2273,12 @@ __kmem_cache_create (struct kmem_cache *cachep, unsigned long flags)
flags |= CFLGS_OFF_SLAB;
size = ALIGN(size, cachep->align);
/*
* We should restrict the number of objects in a slab to implement
* byte sized index. Refer comment on SLAB_OBJ_MIN_SIZE definition.
*/
if (FREELIST_BYTE_INDEX && size < SLAB_OBJ_MIN_SIZE)
size = ALIGN(SLAB_OBJ_MIN_SIZE, cachep->align);
left_over = calculate_slab_order(cachep, size, cachep->align, flags);