The genalloc/genpool subsystem¶

There are a number of memory-allocation subsystems in the kernel, eachaimed at a specific need. Sometimes, however, a kernel developer needs toimplement a new allocator for a specific range of special-purpose memory;often that memory is located on a device somewhere. The author of thedriver for that device can certainly write a little allocator to get thejob done, but that is the way to fill the kernel with dozens of poorlytested allocators. Back in 2005, Jes Sorensen lifted one of thoseallocators from the sym53c8xx_2 driver andposted it as a generic modulefor the creation of ad hoc memory allocators. This code was mergedfor the 2.6.13 release; it has been modified considerably since then.

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

structgen_pool*gen_pool_create(intmin_alloc_order,intnid)¶

create a new special memory pool

structgen_pool*devm_gen_pool_create(structdevice*dev,intmin_alloc_order,intnid,constchar*name)¶

managed gen_pool_create

A call togen_pool_create() will create a pool. The granularity ofallocations is set with min_alloc_order; it is a log-base-2 number likethose 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 amultiple of eight bytes. Increasing min_alloc_order decreases the memoryrequired to track the memory in the pool. The nid parameter specifieswhich NUMA node should be used for the allocation of the housekeepingstructures; it can be -1 if the caller doesn’t care.

The “managed” interfacedevm_gen_pool_create() ties the pool to aspecific device. Among other things, it will automatically clean up thepool when the given device is destroyed.

A pool is shut down with:

voidgen_pool_destroy(structgen_pool*pool)¶

destroy a special memory pool

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

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

intgen_pool_add(structgen_pool*pool,unsignedlongaddr,size_tsize,intnid)¶

add a new chunk of special memory to the pool

intgen_pool_add_owner(structgen_pool*pool,unsignedlongvirt,phys_addr_tphys,size_tsize,intnid,void*owner)¶

add a new chunk of special memory to the pool

A call togen_pool_add() will place the size bytes of memorystarting at addr (in the kernel’s virtual address space) into the givenpool, once again using nid as the node ID for ancillary memory allocations.Thegen_pool_add_virt() variant associates an explicit physicaladdress with the memory; this is only necessary if the pool will be usedfor DMA allocations.

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

unsignedlonggen_pool_alloc(structgen_pool*pool,size_tsize)¶

allocate special memory from the pool

void*gen_pool_dma_alloc(structgen_pool*pool,size_tsize,dma_addr_t*dma)¶

allocate special memory from the pool for DMA usage

voidgen_pool_free_owner(structgen_pool*pool,unsignedlongaddr,size_tsize,void**owner)¶

free allocated special memory back to the pool

As one would expect,gen_pool_alloc() will allocate size< bytesfrom the given pool. Thegen_pool_dma_alloc() variant allocatesmemory for use with DMA operations, returning the associated physicaladdress in the space pointed to by dma. This will only work if the memorywas added withgen_pool_add_virt(). Note that this functiondeparts from the usual genpool pattern of using unsigned long values torepresent kernel addresses; it returns a void * instead.

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

unsignedlonggen_pool_alloc_algo_owner(structgen_pool*pool,size_tsize,genpool_algo_talgo,void*data,void**owner)¶

allocate special memory from the pool

voidgen_pool_set_algo(structgen_pool*pool,genpool_algo_talgo,void*data)¶

set the allocation algorithm

Allocations withgen_pool_alloc_algo() specify an algorithm to beused to choose the memory to be allocated; the default algorithm can be setwithgen_pool_set_algo(). The data value is passed to thealgorithm; most ignore it, but it is occasionally needed. One can,naturally, write a special-purpose algorithm, but there is a fair setalready available:

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

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

• gen_pool_first_fit_order_align aligns the allocation to the order of thesize. 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 agenpool_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 queryingthe space available in the pool or iterating through chunks of memory.Most users, however, should not need much beyond what has been describedabove. With luck, wider awareness of this module will help to prevent thewriting of special-purpose memory allocators in the future.

phys_addr_tgen_pool_virt_to_phys(structgen_pool*pool,unsignedlongaddr)¶

return the physical address of memory

voidgen_pool_for_each_chunk(structgen_pool*pool,void(*func)(structgen_pool*pool,structgen_pool_chunk*chunk,void*data),void*data)¶

call func for every chunk of generic memory pool

boolgen_pool_has_addr(structgen_pool*pool,unsignedlongstart,size_tsize)¶

checks if an address falls within the range of a pool

size_tgen_pool_avail(structgen_pool*pool)¶

get available free space of the pool

size_tgen_pool_size(structgen_pool*pool)¶

get size in bytes of memory managed by the pool

structgen_pool*gen_pool_get(structdevice*dev,constchar*name)¶

Obtain the gen_pool (if any) for a device

structgen_pool*of_gen_pool_get(structdevice_node*np,constchar*propname,intindex)¶

find a pool by phandle property