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分类: LINUX

2010-10-20 19:12:22

The base of kmalloc is an array that groups slab caches for memory areas of graded sizes. The array entries are instances of the cache_sizes data structure that is defined as follows:
 

/* Size description struct for general caches. */
struct cache_sizes {
    size_t             cs_size;
    struct kmem_cache    *cs_cachep;
#ifdef CONFIG_ZONE_DMA
    struct kmem_cache    *cs_dmacachep;
#endif
};


cs_size
specifies the size of the memory area for which the entry is responsible. There are two slab caches for each size, one of which supplies DMA-suitable memory.

The statically defined malloc_sizes array groups the available sizes essentially using powers of 2 between 25 = 32 and 225 = 131, 072, depending on the setting of KMALLOC_MAX_SIZE.

/*
 * These are the default caches for kmalloc(). Custom caches can have other sizes.
 */
struct cache_sizes malloc_sizes[] = {
#define CACHE(x) { .cs_size = (x) },
#if (PAGE_SIZE == 4096)
    CACHE(32)
#endif
    CACHE(64)
#if L1_CACHE_BYTES < 64
    CACHE(96)
#endif
    CACHE(128)
#if L1_CACHE_BYTES < 128
    CACHE(192)
#endif
    CACHE(256)
    CACHE(512)
    CACHE(1024)
    CACHE(2048)
    CACHE(4096)
    CACHE(8192)
    CACHE(16384)
    CACHE(32768)
    CACHE(65536)
    CACHE(131072)
#if KMALLOC_MAX_SIZE >= 262144
    CACHE(262144)
#endif
#if KMALLOC_MAX_SIZE >= 524288
    CACHE(524288)
#endif
#if KMALLOC_MAX_SIZE >= 1048576
    CACHE(1048576)
#endif
#if KMALLOC_MAX_SIZE >= 2097152
    CACHE(2097152)
#endif
#if KMALLOC_MAX_SIZE >= 4194304
    CACHE(4194304)
#endif
#if KMALLOC_MAX_SIZE >= 8388608
    CACHE(8388608)
#endif
#if KMALLOC_MAX_SIZE >= 16777216
    CACHE(16777216)
#endif
#if KMALLOC_MAX_SIZE >= 33554432
    CACHE(33554432)
#endif

    CACHE(ULONG_MAX)
#undef CACHE
};


kfree() hands over the actual work to the __cache_free().


/**
 * kfree - free previously allocated memory
 * @objp: pointer returned by kmalloc.
 *
 * If @objp is NULL, no operation is performed.
 *
 * Don't free memory not originally allocated by kmalloc()
 * or you will run into trouble.
 */

void kfree(const void *objp)
{
    struct kmem_cache *c;
    unsigned long flags;

    trace_kfree(_RET_IP_, objp);

    if (unlikely(ZERO_OR_NULL_PTR(objp)))
        return;
    local_irq_save(flags);
    kfree_debugcheck(objp);
    c = virt_to_cache(objp);
    debug_check_no_locks_freed(objp, obj_size(c));
    debug_check_no_obj_freed(objp, obj_size(c));
    __cache_free(c, (void *)objp);
    local_irq_restore(flags);
}
EXPORT_SYMBOL(kfree);


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