The genalloc/genpool subsystem

There are a number of memory-allocation subsystems in the kernel, each aimed at a specific need. Sometimes, however, a kernel developer needs to implement a new allocator for a specific range of special-purpose memory; often that memory is located on a device somewhere. The author of the driver for that device can certainly write a little allocator to get the job done, but that is the way to fill the kernel with dozens of poorly tested allocators. Back in 2005, Jes Sorensen lifted one of those allocators from the sym53c8xx_2 driver and posted it as a generic module for the creation of ad hoc memory allocators. This code was merged for the 2.6.13 release; it has been modified considerably since then.

Code using this allocator should include <linux/genalloc.h>. The action begins with the creation of a pool using one of:

struct gen_pool * gen_pool_create(int min_alloc_order, int nid)

create a new special memory pool

Parameters

int min_alloc_order

log base 2 of number of bytes each bitmap bit represents

int nid

node id of the node the pool structure should be allocated on, or -1

Description

Create a new special memory pool that can be used to manage special purpose memory not managed by the regular kmalloc/kfree interface.

struct gen_pool * devm_gen_pool_create(struct device * dev, int min_alloc_order, int nid, const char * name)

managed gen_pool_create

Parameters

struct device * dev

device that provides the gen_pool

int min_alloc_order

log base 2 of number of bytes each bitmap bit represents

int nid

node selector for allocated gen_pool, NUMA_NO_NODE for all nodes

const char * name

name of a gen_pool or NULL, identifies a particular gen_pool on device

Description

Create a new special memory pool that can be used to manage special purpose memory not managed by the regular kmalloc/kfree interface. The pool will be automatically destroyed by the device management code.

A call to gen_pool_create() will create a pool. The granularity of allocations is set with min_alloc_order; it is a log-base-2 number like those used by the page allocator, but it refers to bytes rather than pages. So, if min_alloc_order is passed as 3, then all allocations will be a multiple of eight bytes. Increasing min_alloc_order decreases the memory required to track the memory in the pool. The nid parameter specifies which NUMA node should be used for the allocation of the housekeeping structures; it can be -1 if the caller doesn't care.

The "managed" interface devm_gen_pool_create() ties the pool to a specific device. Among other things, it will automatically clean up the pool when the given device is destroyed.

A pool is shut down with:

void gen_pool_destroy(struct gen_pool * pool)

destroy a special memory pool

Parameters

struct gen_pool * pool

pool to destroy

Description

Destroy the specified special memory pool. Verifies that there are no outstanding allocations.

It's worth noting that, if there are still allocations outstanding from the given pool, this function will take the rather extreme step of invoking BUG(), crashing the entire system. You have been warned.

A freshly created pool has no memory to allocate. It is fairly useless in that state, so one of the first orders of business is usually to add memory to the pool. That can be done with one of:

int gen_pool_add(struct gen_pool * pool, unsigned long addr, size_t size, int nid)

add a new chunk of special memory to the pool

Parameters

struct gen_pool * pool

pool to add new memory chunk to

unsigned long addr

starting address of memory chunk to add to pool

size_t size

size in bytes of the memory chunk to add to pool

int nid

node id of the node the chunk structure and bitmap should be allocated on, or -1

Description

Add a new chunk of special memory to the specified pool.

Returns 0 on success or a -ve errno on failure.

A call to gen_pool_add() will place the size bytes of memory starting at addr (in the kernel's virtual address space) into the given pool, once again using nid as the node ID for ancillary memory allocations. The gen_pool_add_virt() variant associates an explicit physical address with the memory; this is only necessary if the pool will be used for DMA allocations.

The functions for allocating memory from the pool (and putting it back) are:

void * gen_pool_dma_alloc(struct gen_pool * pool, size_t size, dma_addr_t * dma)

allocate special memory from the pool for DMA usage

Parameters

struct gen_pool * pool

pool to allocate from

size_t size

number of bytes to allocate from the pool

dma_addr_t * dma

dma-view physical address return value. Use NULL if unneeded.

Description

Allocate the requested number of bytes from the specified pool. Uses the pool allocation function (with first-fit algorithm by default). Can not be used in NMI handler on architectures without NMI-safe cmpxchg implementation.

As one would expect, gen_pool_alloc() will allocate size< bytes from the given pool. The gen_pool_dma_alloc() variant allocates memory for use with DMA operations, returning the associated physical address in the space pointed to by dma. This will only work if the memory was added with gen_pool_add_virt(). Note that this function departs from the usual genpool pattern of using unsigned long values to represent kernel addresses; it returns a void * instead.

That all seems relatively simple; indeed, some developers clearly found it to be too simple. After all, the interface above provides no control over how the allocation functions choose which specific piece of memory to return. If that sort of control is needed, the following functions will be of interest:

void gen_pool_set_algo(struct gen_pool * pool, genpool_algo_t algo, void * data)

set the allocation algorithm

Parameters

struct gen_pool * pool

pool to change allocation algorithm

genpool_algo_t algo

custom algorithm function

void * data

additional data used by algo

Description

Call algo for each memory allocation in the pool. If algo is NULL use gen_pool_first_fit as default memory allocation function.

Allocations with gen_pool_alloc_algo() specify an algorithm to be used to choose the memory to be allocated; the default algorithm can be set with gen_pool_set_algo(). The data value is passed to the algorithm; most ignore it, but it is occasionally needed. One can, naturally, write a special-purpose algorithm, but there is a fair set already available:

  • gen_pool_first_fit is a simple first-fit allocator; this is the default algorithm if none other has been specified.

  • gen_pool_first_fit_align forces the allocation to have a specific alignment (passed via data in a genpool_data_align structure).

  • gen_pool_first_fit_order_align aligns the allocation to the order of the size. A 60-byte allocation will thus be 64-byte aligned, for example.

  • gen_pool_best_fit, as one would expect, is a simple best-fit allocator.

  • gen_pool_fixed_alloc allocates at a specific offset (passed in a genpool_data_fixed structure via the data parameter) within the pool. If the indicated memory is not available the allocation fails.

There is a handful of other functions, mostly for purposes like querying the space available in the pool or iterating through chunks of memory. Most users, however, should not need much beyond what has been described above. With luck, wider awareness of this module will help to prevent the writing of special-purpose memory allocators in the future.

phys_addr_t gen_pool_virt_to_phys(struct gen_pool * pool, unsigned long addr)

return the physical address of memory

Parameters

struct gen_pool * pool

pool to allocate from

unsigned long addr

starting address of memory

Description

Returns the physical address on success, or -1 on error.

void gen_pool_for_each_chunk(struct gen_pool * pool, void (*func) (struct gen_pool *pool, struct gen_pool_chunk *chunk, void *data, void * data)

call func for every chunk of generic memory pool

Parameters

struct gen_pool * pool

the generic memory pool

void (*)(struct gen_pool *pool, struct gen_pool_chunk *chunk, void *data) func

func to call

void * data

additional data used by func

Description

Call func for every chunk of generic memory pool. The func is called with rcu_read_lock held.

bool addr_in_gen_pool(struct gen_pool * pool, unsigned long start, size_t size)

checks if an address falls within the range of a pool

Parameters

struct gen_pool * pool

the generic memory pool

unsigned long start

start address

size_t size

size of the region

Description

Check if the range of addresses falls within the specified pool. Returns true if the entire range is contained in the pool and false otherwise.

size_t gen_pool_avail(struct gen_pool * pool)

get available free space of the pool

Parameters

struct gen_pool * pool

pool to get available free space

Description

Return available free space of the specified pool.

size_t gen_pool_size(struct gen_pool * pool)

get size in bytes of memory managed by the pool

Parameters

struct gen_pool * pool

pool to get size

Description

Return size in bytes of memory managed by the pool.

struct gen_pool * gen_pool_get(struct device * dev, const char * name)

Obtain the gen_pool (if any) for a device

Parameters

struct device * dev

device to retrieve the gen_pool from

const char * name

name of a gen_pool or NULL, identifies a particular gen_pool on device

Description

Returns the gen_pool for the device if one is present, or NULL.

struct gen_pool * of_gen_pool_get(struct device_node * np, const char * propname, int index)

find a pool by phandle property

Parameters

struct device_node * np

device node

const char * propname

property name containing phandle(s)

int index

index into the phandle array

Description

Returns the pool that contains the chunk starting at the physical address of the device tree node pointed at by the phandle property, or NULL if not found.