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.

.. _posted: https://lwn.net/Articles/125842/

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

.. kernel-doc:: lib/genalloc.c
   :functions: gen_pool_create          

.. kernel-doc:: lib/genalloc.c
   :functions: devm_gen_pool_create

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:

.. kernel-doc:: lib/genalloc.c
   :functions: gen_pool_destroy

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:

.. kernel-doc:: include/linux/genalloc.h
   :functions: gen_pool_add

.. kernel-doc:: lib/genalloc.c
   :functions: gen_pool_add_owner

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:

.. kernel-doc:: include/linux/genalloc.h
   :functions: gen_pool_alloc

.. kernel-doc:: lib/genalloc.c
   :functions: gen_pool_dma_alloc

.. kernel-doc:: lib/genalloc.c
   :functions: gen_pool_free_owner

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:

.. kernel-doc:: lib/genalloc.c
   :functions: gen_pool_alloc_algo_owner

.. kernel-doc:: lib/genalloc.c
   :functions: gen_pool_set_algo

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.

.. kernel-doc:: lib/genalloc.c
   :functions: gen_pool_virt_to_phys

.. kernel-doc:: lib/genalloc.c
   :functions: gen_pool_for_each_chunk

.. kernel-doc:: lib/genalloc.c
   :functions: gen_pool_has_addr

.. kernel-doc:: lib/genalloc.c
   :functions: gen_pool_avail

.. kernel-doc:: lib/genalloc.c
   :functions: gen_pool_size

.. kernel-doc:: lib/genalloc.c
   :functions: gen_pool_get

.. kernel-doc:: lib/genalloc.c
   :functions: of_gen_pool_get