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    Subject[PATCH v2 3/5] slab: restrict the number of objects in a slab
    Date
    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_MIN_SIZE ensures that the number of objects in a slab is less or
    equal to 256 for a slab with 1 page.

    Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>

    diff --git a/mm/slab.c b/mm/slab.c
    index ec197b9..3cee122 100644
    --- a/mm/slab.c
    +++ b/mm/slab.c
    @@ -157,6 +157,10 @@
    #define ARCH_KMALLOC_FLAGS SLAB_HWCACHE_ALIGN
    #endif

    +/* We use byte sized index to manage the freelist of a slab */
    +#define NR_PER_BYTE (1 << BITS_PER_BYTE)
    +#define SLAB_MIN_SIZE (PAGE_SIZE >> BITS_PER_BYTE)
    +
    /*
    * true if a page was allocated from pfmemalloc reserves for network-based
    * swap
    @@ -2016,6 +2020,10 @@ static size_t calculate_slab_order(struct kmem_cache *cachep,
    if (!num)
    continue;

    + /* We can't handler number of objects more than NR_PER_BYTE */
    + if (num > NR_PER_BYTE)
    + break;
    +
    if (flags & CFLGS_OFF_SLAB) {
    /*
    * Max number of objs-per-slab for caches which
    @@ -2258,6 +2266,12 @@ __kmem_cache_create (struct kmem_cache *cachep, unsigned long flags)
    flags |= CFLGS_OFF_SLAB;

    size = ALIGN(size, cachep->align);
    + /*
    + * We want to restrict the number of objects in a slab to be equal or
    + * less than 256 in order to manage freelist via byte sized indexes.
    + */
    + if (size < SLAB_MIN_SIZE)
    + size = ALIGN(SLAB_MIN_SIZE, cachep->align);

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

    --
    1.7.9.5


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