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Commit f315e3fa authored by Joonsoo Kim's avatar Joonsoo Kim Committed by Pekka Enberg
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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: default avatarJoonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: default avatarPekka Enberg <penberg@kernel.org>
parent e5c58dfd

include/linux/slab.h

+11 −0
Original line number Original line Diff line number Diff line
@@ -201,6 +201,17 @@ struct kmem_cache { 
#ifndef KMALLOC_SHIFT_LOW

#ifndef KMALLOC_SHIFT_LOW

#define KMALLOC_SHIFT_LOW	5

#define KMALLOC_SHIFT_LOW	5

#endif

#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

#endif





#ifdef CONFIG_SLUB

#ifdef CONFIG_SLUB

mm/slab.c

+21 −0
Original line number Original line Diff line number Diff line
@@ -157,6 +157,17 @@ 
#define ARCH_KMALLOC_FLAGS SLAB_HWCACHE_ALIGN

#define ARCH_KMALLOC_FLAGS SLAB_HWCACHE_ALIGN

#endif

#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

 * true if a page was allocated from pfmemalloc reserves for network-based

 * swap

 * swap
@@ -2016,6 +2027,10 @@ static size_t calculate_slab_order(struct kmem_cache *cachep, 
		if (!num)

		if (!num)

			continue;

			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) {

		if (flags & CFLGS_OFF_SLAB) {

			/*

			/*

			 * Max number of objs-per-slab for caches which

			 * 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;

		flags |= CFLGS_OFF_SLAB;





	size = ALIGN(size, cachep->align);

	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);

	left_over = calculate_slab_order(cachep, size, cachep->align, flags);