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Linux Device Drivers | Allocating Memory

ResearchC/C++LinuxLinux Device DriversLinux KernelAbout 2 minAbout 724 words

void *kmalloc(size_t size, int flags);
void kfree(void *obj);

The most frequently used interface to memory allocation.


Flags that control how memory allocations are performed, from the least restrictive to the most. The GFP_USER and GFP_KERNEL priorities allow the current process to be put to sleep to satisfy the request. GFP_NOFS and GFP_NOIO disable filesystem operations and all I/O operations, respectively, while GFP_ATOMIC allocations cannot sleep at all.


These flags modify the kernel’s behavior when allocating memory.

kmem_cache_t *kmem_cache_create(char *name, size_t size, size_t offset, unsigned long flags, constructor(), destructor());
int kmem_cache_destroy(kmem_cache_t *cache);

Create and destroy a slab cache. The cache can be used to allocate several objects of the same size.


Flags that can be specified while creating a cache.


Flags that the allocator can pass to the constructor and the destructor functions.

void *kmem_cache_alloc(kmem_cache_t *cache, int flags);
void kmem_cache_free(kmem_cache_t *cache, const void *obj);

Allocate and release a single object from the cache.


A virtual file containing statistics on slab cache usage.

mempool_t *mempool_create(int min_nr, mempool_alloc_t *alloc_fn, mempool_free_t *free_fn, void *data);
void mempool_destroy(mempool_t *pool);

Functions for the creation of memory pools, which try to avoid memory allocation failures by keeping an “emergency list” of allocated items.

void *mempool_alloc(mempool_t *pool, int gfp_mask);
void mempool_free(void *element, mempool_t *pool);

Functions for allocating items from (and returning them to) memory pools.

unsigned long get_zeroed_page(int flags);
unsigned long __get_free_page(int flags);
unsigned long __get_free_pages(int flags, unsigned long order);

The page-oriented allocation functions. get_zeroed_page returns a single, zero-filled page. All the other versions of the call do not initialize the contents of the returned page(s).

int get_order(unsigned long size);

Returns the allocation order associated to size in the current platform, according to PAGE_SIZE. The argument must be a power of two, and the return value is at least 0.

void free_page(unsigned long addr);
void free_pages(unsigned long addr, unsigned long order);

Functions that release page-oriented allocations.

struct page *alloc_pages_node(int nid, unsigned int flags, unsigned int order);
struct page *alloc_pages(unsigned int flags, unsigned int order);
struct page *alloc_page(unsigned int flags);

All variants of the lowest-level page allocator in the Linux kernel.

void __free_page(struct page *page);
void __free_pages(struct page *page, unsigned int order);
void free_hot_page(struct page *page);
void free_cold_page(struct page *page);

Various ways of freeing pages allocated with one of the forms of alloc_page.

void * vmalloc(unsigned long size);
void vfree(void * addr);
void * ioremap(unsigned long offset, unsigned long size);
void iounmap(void *addr);

Functions that allocate or free a contiguous virtual address space. ioremap accesses physical memory through virtual addresses, while vmalloc allocates free pages. Regions mapped with ioremap are freed with iounmap, while pages obtained from vmalloc are released with vfree.

DEFINE_PER_CPU(type, name);
DECLARE_PER_CPU(type, name);

Macros that define and declare per-CPU variables.

per_cpu(variable, int cpu_id)

Macros that provide access to statically declared per-CPU variables.

void *alloc_percpu(type);
void *__alloc_percpu(size_t size, size_t align);
void free_percpu(void *variable);

Functions that perform runtime allocation and freeing of per-CPU variables.

int get_cpu();
void put_cpu();
per_cpu_ptr(void *variable, int cpu_id)

get_cpu obtains a reference to the current processor (therefore, preventing preemption and movement to another processor) and returns the ID number of the processor; put_cpu returns that reference. To access a dynamically allocated per-CPU variable, use per_cpu_ptr with the ID of the CPU whose version should be accessed. Manipulations of the current CPU’s version of a dynamic, per-CPU variable should probably be surrounded by calls to get_cpu and put_cpu.

void *alloc_bootmem(unsigned long size);
void *alloc_bootmem_low(unsigned long size);
void *alloc_bootmem_pages(unsigned long size);
void *alloc_bootmem_low_pages(unsigned long size);
void free_bootmem(unsigned long addr, unsigned long size);

Functions (which can be used only by drivers directly linked into the kernel) that perform allocation and freeing of memory at system bootstrap time.