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144 lines
5.7 KiB
144 lines
5.7 KiB
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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