Linux Kernel Architecture

Chapter3:MemoryManagement

Destroying Caches

Thekmem_cache_destroyfunction must be invoked to destroy a complete cache in which there are

only unused objects. This function is needed primarily when removing modules that want to return all

memory space allocated to them.^36

Since the implementation itself reveals nothing new, we will confine ourselves to outlining the main steps

needed to remove a cache:

❑ The slabs on theslabs_freelist are scanned successively. The destructor is first invoked for

each object on each slab, and then the slab memory space is returned to the buddy system.

❑ The memory space for the per-CPU caches is freed.

❑ The data are removed from thecache_cachelist.

3.6.5 General Caches

Thekmallocandkfreefunctions must be used to allocate and free memory in the classic sense rather

than objects. These are the kernel equivalents to themallocandfreefunctions from the C standard

library in userspace.^37

I have already noted several times thatkmallocandkfreeare implemented as slab allocator front-ends

and mimic the semantics ofmalloc/freeas best they can. We can therefore deal with their implementa-

tion succinctly.

Implementationofkmalloc

Thebaseofkmallocis an array that groups slab caches for memory areas of graded sizes. The array

entries are instances of thecache_sizesdata structure that is defined as follows:

<slab_def.h>

struct cache_sizes {

size_t cs_size;

kmem_cache_t *cs_cachep;

kmem_cache_t *cs_dmacachep;

#ifdef CONFIG_ZONE_DMA

struct kmem_cache *cs_dmacachep;

#endif

}

sizespecifies 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 definedmalloc_sizesarray groups the available sizes essentially using powers of 2

between 2^5 =32 and 2^25 =131, 072, depending on the setting ofKMALLOC_MAX_SIZEas discussed above.

(^36) This is not mandatory. If a module wants to obtain persistent memory that is preserved between unloading a module and reload-
ing the next time (assuming, of course, that the system is not rebooted in the meantime), it can retain a cache so that the data it
contains are available for reuse.
(^37) Use ofprintk,kmallocandkfreein userspace programs is an unmistakable sign of too much contact with kernel
programming.