Slab allocator maintains a set of generic slab caches to cache memory blocks of unit sizes in multiples of 8. It maintains two sets of slab caches for each unit size, one to maintain a pool of memory blocks allocated from ZONE_NORMAL pages and another from ZONE_DMA pages. These caches are global and shared by all kernel code. Users can track the status of these caches through a special file /proc/slabinfo. Kernel services can allocate and release memory blocks from these caches through the kmalloc family of routines. They are referred to as kmalloc caches:
#cat /proc/slabinfo
slabinfo - version: 2.1
# name <active_objs> <num_objs> <objsize> <objperslab> <pagesperslab> : tunables <limit> <batchcount> <sharedfactor> : slabdata <active_slabs> <num_slabs> <sharedavail>
dma-kmalloc-8192 0 0 8192 4 8 : tunables 0 0 0 : slabdata 0 0 0
dma-kmalloc-4096 0 0 4096 8 8 : tunables 0 0 0 : slabdata 0 0 0
dma-kmalloc-2048 0 0 2048 16 8 : tunables 0 0 0 : slabdata 0 0 0
dma-kmalloc-1024 0 0 1024 16 4 : tunables 0 0 0 : slabdata 0 0 0
dma-kmalloc-512 0 0 512 16 2 : tunables 0 0 0 : slabdata 0 0 0
dma-kmalloc-256 0 0 256 16 1 : tunables 0 0 0 : slabdata 0 0 0
dma-kmalloc-128 0 0 128 32 1 : tunables 0 0 0 : slabdata 0 0 0
dma-kmalloc-64 0 0 64 64 1 : tunables 0 0 0 : slabdata 0 0 0
dma-kmalloc-32 0 0 32 128 1 : tunables 0 0 0 : slabdata 0 0 0
dma-kmalloc-16 0 0 16 256 1 : tunables 0 0 0 : slabdata 0 0 0
dma-kmalloc-8 0 0 8 512 1 : tunables 0 0 0 : slabdata 0 0 0
dma-kmalloc-192 0 0 192 21 1 : tunables 0 0 0 : slabdata 0 0 0
dma-kmalloc-96 0 0 96 42 1 : tunables 0 0 0 : slabdata 0 0 0
kmalloc-8192 156 156 8192 4 8 : tunables 0 0 0 : slabdata 39 39 0
kmalloc-4096 325 352 4096 8 8 : tunables 0 0 0 : slabdata 44 44 0
kmalloc-2048 1105 1184 2048 16 8 : tunables 0 0 0 : slabdata 74 74 0
kmalloc-1024 2374 2448 1024 16 4 : tunables 0 0 0 : slabdata 153 153 0
kmalloc-512 1445 1520 512 16 2 : tunables 0 0 0 : slabdata 95 95 0
kmalloc-256 9988 10400 256 16 1 : tunables 0 0 0 : slabdata 650 650 0
kmalloc-192 3561 4053 192 21 1 : tunables 0 0 0 : slabdata 193 193 0
kmalloc-128 3588 5728 128 32 1 : tunables 0 0 0 : slabdata 179 179 0
kmalloc-96 3402 3402 96 42 1 : tunables 0 0 0 : slabdata 81 81 0
kmalloc-64 42672 45184 64 64 1 : tunables 0 0 0 : slabdata 706 706 0
kmalloc-32 15095 16000 32 128 1 : tunables 0 0 0 : slabdata 125 125 0
kmalloc-16 6400 6400 16 256 1 : tunables 0 0 0 : slabdata 25 25 0
kmalloc-8 6144 6144 8 512 1 : tunables 0 0 0 : slabdata 12 12 0
kmalloc-96 and kmalloc-192 are caches used to maintain memory blocks aligned with the level 1 hardware cache. For allocations above 8k (large blocks), the slab allocator falls back on buddy system.
Following are the kmalloc family of allocator routines; all of these need appropriate GFP flags:
/**
* kmalloc - allocate memory.
* @size: bytes of memory required.
* @flags: the type of memory to allocate.
*/
void *kmalloc(size_t size, gfp_t flags)
/**
* kzalloc - allocate memory. The memory is set to zero.
* @size: bytes of memory required.
* @flags: the type of memory to allocate.
*/
inline void *kzalloc(size_t size, gfp_t flags)
/**
* kmalloc_array - allocate memory for an array.
* @n: number of elements.
* @size: element size.
* @flags: the type of memory to allocate (see kmalloc).
*/
inline void *kmalloc_array(size_t n, size_t size, gfp_t flags)
/**
* kcalloc - allocate memory for an array. The memory is set to zero.
* @n: number of elements.
* @size: element size.
* @flags: the type of memory to allocate (see kmalloc).
*/
inline void *kcalloc(size_t n, size_t size, gfp_t flags)
/**
* krealloc - reallocate memory. The contents will remain unchanged.
* @p: object to reallocate memory for.
* @new_size: bytes of memory are required.
* @flags: the type of memory to allocate.
*
* The contents of the object pointed to are preserved up to the
* lesser of the new and old sizes. If @p is %NULL, krealloc()
* behaves exactly like kmalloc(). If @new_size is 0 and @p is not a
* %NULL pointer, the object pointed to is freed
*/
void *krealloc(const void *p, size_t new_size, gfp_t flags)
/**
* kmalloc_node - allocate memory from a particular memory node.
* @size: bytes of memory are required.
* @flags: the type of memory to allocate.
* @node: memory node from which to allocate
*/
void *kmalloc_node(size_t size, gfp_t flags, int node)
/**
* kzalloc_node - allocate zeroed memory from a particular memory node.
* @size: how many bytes of memory are required.
* @flags: the type of memory to allocate (see kmalloc).
* @node: memory node from which to allocate
*/
void *kzalloc_node(size_t size, gfp_t flags, int node)
Following routines return the allocated block to the free pool. Callers need to ensure that address passed as argument is of a valid allocated block:
/**
* 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)
/**
* kzfree - like kfree but zero memory
* @p: object to free memory of
*
* The memory of the object @p points to is zeroed before freed.
* If @p is %NULL, kzfree() does nothing.
*
* Note: this function zeroes the whole allocated buffer which can be a good
* deal bigger than the requested buffer size passed to kmalloc(). So be
* careful when using this function in performance sensitive code.
*/
void kzfree(const void *p)