145 lines
5.7 KiB
ReStructuredText
145 lines
5.7 KiB
ReStructuredText
The genalloc/genpool subsystem
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==============================
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There are a number of memory-allocation subsystems in the kernel, each
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aimed at a specific need. Sometimes, however, a kernel developer needs to
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implement a new allocator for a specific range of special-purpose memory;
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often that memory is located on a device somewhere. The author of the
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driver for that device can certainly write a little allocator to get the
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job done, but that is the way to fill the kernel with dozens of poorly
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tested allocators. Back in 2005, Jes Sorensen lifted one of those
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allocators from the sym53c8xx_2 driver and posted_ it as a generic module
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for the creation of ad hoc memory allocators. This code was merged
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for the 2.6.13 release; it has been modified considerably since then.
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.. _posted: https://lwn.net/Articles/125842/
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Code using this allocator should include <linux/genalloc.h>. The action
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begins with the creation of a pool using one of:
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.. kernel-doc:: lib/genalloc.c
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:functions: gen_pool_create
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.. kernel-doc:: lib/genalloc.c
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:functions: devm_gen_pool_create
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A call to gen_pool_create() will create a pool. The granularity of
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allocations is set with min_alloc_order; it is a log-base-2 number like
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those used by the page allocator, but it refers to bytes rather than pages.
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So, if min_alloc_order is passed as 3, then all allocations will be a
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multiple of eight bytes. Increasing min_alloc_order decreases the memory
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required to track the memory in the pool. The nid parameter specifies
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which NUMA node should be used for the allocation of the housekeeping
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structures; it can be -1 if the caller doesn't care.
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The "managed" interface devm_gen_pool_create() ties the pool to a
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specific device. Among other things, it will automatically clean up the
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pool when the given device is destroyed.
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A pool is shut down with:
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.. kernel-doc:: lib/genalloc.c
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:functions: gen_pool_destroy
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It's worth noting that, if there are still allocations outstanding from the
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given pool, this function will take the rather extreme step of invoking
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BUG(), crashing the entire system. You have been warned.
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A freshly created pool has no memory to allocate. It is fairly useless in
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that state, so one of the first orders of business is usually to add memory
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to the pool. That can be done with one of:
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.. kernel-doc:: include/linux/genalloc.h
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:functions: gen_pool_add
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.. kernel-doc:: lib/genalloc.c
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:functions: gen_pool_add_owner
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A call to gen_pool_add() will place the size bytes of memory
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starting at addr (in the kernel's virtual address space) into the given
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pool, once again using nid as the node ID for ancillary memory allocations.
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The gen_pool_add_virt() variant associates an explicit physical
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address with the memory; this is only necessary if the pool will be used
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for DMA allocations.
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The functions for allocating memory from the pool (and putting it back)
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are:
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.. kernel-doc:: include/linux/genalloc.h
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:functions: gen_pool_alloc
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.. kernel-doc:: lib/genalloc.c
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:functions: gen_pool_dma_alloc
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.. kernel-doc:: lib/genalloc.c
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:functions: gen_pool_free_owner
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As one would expect, gen_pool_alloc() will allocate size< bytes
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from the given pool. The gen_pool_dma_alloc() variant allocates
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memory for use with DMA operations, returning the associated physical
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address in the space pointed to by dma. This will only work if the memory
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was added with gen_pool_add_virt(). Note that this function
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departs from the usual genpool pattern of using unsigned long values to
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represent kernel addresses; it returns a void * instead.
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That all seems relatively simple; indeed, some developers clearly found it
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to be too simple. After all, the interface above provides no control over
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how the allocation functions choose which specific piece of memory to
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return. If that sort of control is needed, the following functions will be
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of interest:
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.. kernel-doc:: lib/genalloc.c
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:functions: gen_pool_alloc_algo_owner
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.. kernel-doc:: lib/genalloc.c
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:functions: gen_pool_set_algo
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Allocations with gen_pool_alloc_algo() specify an algorithm to be
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used to choose the memory to be allocated; the default algorithm can be set
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with gen_pool_set_algo(). The data value is passed to the
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algorithm; most ignore it, but it is occasionally needed. One can,
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naturally, write a special-purpose algorithm, but there is a fair set
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already available:
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- gen_pool_first_fit is a simple first-fit allocator; this is the default
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algorithm if none other has been specified.
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- gen_pool_first_fit_align forces the allocation to have a specific
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alignment (passed via data in a genpool_data_align structure).
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- gen_pool_first_fit_order_align aligns the allocation to the order of the
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size. A 60-byte allocation will thus be 64-byte aligned, for example.
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- gen_pool_best_fit, as one would expect, is a simple best-fit allocator.
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- gen_pool_fixed_alloc allocates at a specific offset (passed in a
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genpool_data_fixed structure via the data parameter) within the pool.
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If the indicated memory is not available the allocation fails.
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There is a handful of other functions, mostly for purposes like querying
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the space available in the pool or iterating through chunks of memory.
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Most users, however, should not need much beyond what has been described
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above. With luck, wider awareness of this module will help to prevent the
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writing of special-purpose memory allocators in the future.
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.. kernel-doc:: lib/genalloc.c
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:functions: gen_pool_virt_to_phys
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.. kernel-doc:: lib/genalloc.c
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:functions: gen_pool_for_each_chunk
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.. kernel-doc:: lib/genalloc.c
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:functions: gen_pool_has_addr
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.. kernel-doc:: lib/genalloc.c
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:functions: gen_pool_avail
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.. kernel-doc:: lib/genalloc.c
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:functions: gen_pool_size
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.. kernel-doc:: lib/genalloc.c
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:functions: gen_pool_get
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.. kernel-doc:: lib/genalloc.c
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:functions: of_gen_pool_get
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