1 .. _memory_allocation: 2 3 ======================= 4 Memory Allocation Guide 5 ======================= 6 7 Linux provides a variety of APIs for memory allocation. You can 8 allocate small chunks using `kmalloc` or `kmem_cache_alloc` families, 9 large virtually contiguous areas using `vmalloc` and its derivatives, 10 or you can directly request pages from the page allocator with 11 `alloc_pages`. It is also possible to use more specialized allocators, 12 for instance `cma_alloc` or `zs_malloc`. 13 14 Most of the memory allocation APIs use GFP flags to express how that 15 memory should be allocated. The GFP acronym stands for "get free 16 pages", the underlying memory allocation function. 17 18 Diversity of the allocation APIs combined with the numerous GFP flags 19 makes the question "How should I allocate memory?" not that easy to 20 answer, although very likely you should use 21 22 :: 23 24 kzalloc(<size>, GFP_KERNEL); 25 26 Of course there are cases when other allocation APIs and different GFP 27 flags must be used. 28 29 Get Free Page flags 30 =================== 31 32 The GFP flags control the allocators behavior. They tell what memory 33 zones can be used, how hard the allocator should try to find free 34 memory, whether the memory can be accessed by the userspace etc. The 35 :ref:`Documentation/core-api/mm-api.rst <mm-api-gfp-flags>` provides 36 reference documentation for the GFP flags and their combinations and 37 here we briefly outline their recommended usage: 38 39 * Most of the time ``GFP_KERNEL`` is what you need. Memory for the 40 kernel data structures, DMAable memory, inode cache, all these and 41 many other allocations types can use ``GFP_KERNEL``. Note, that 42 using ``GFP_KERNEL`` implies ``GFP_RECLAIM``, which means that 43 direct reclaim may be triggered under memory pressure; the calling 44 context must be allowed to sleep. 45 * If the allocation is performed from an atomic context, e.g interrupt 46 handler, use ``GFP_NOWAIT``. This flag prevents direct reclaim and 47 IO or filesystem operations. Consequently, under memory pressure 48 ``GFP_NOWAIT`` allocation is likely to fail. Users of this flag need 49 to provide a suitable fallback to cope with such failures where 50 appropriate. 51 * If you think that accessing memory reserves is justified and the kernel 52 will be stressed unless allocation succeeds, you may use ``GFP_ATOMIC``. 53 * Untrusted allocations triggered from userspace should be a subject 54 of kmem accounting and must have ``__GFP_ACCOUNT`` bit set. There 55 is the handy ``GFP_KERNEL_ACCOUNT`` shortcut for ``GFP_KERNEL`` 56 allocations that should be accounted. 57 * Userspace allocations should use either of the ``GFP_USER``, 58 ``GFP_HIGHUSER`` or ``GFP_HIGHUSER_MOVABLE`` flags. The longer 59 the flag name the less restrictive it is. 60 61 ``GFP_HIGHUSER_MOVABLE`` does not require that allocated memory 62 will be directly accessible by the kernel and implies that the 63 data is movable. 64 65 ``GFP_HIGHUSER`` means that the allocated memory is not movable, 66 but it is not required to be directly accessible by the kernel. An 67 example may be a hardware allocation that maps data directly into 68 userspace but has no addressing limitations. 69 70 ``GFP_USER`` means that the allocated memory is not movable and it 71 must be directly accessible by the kernel. 72 73 You may notice that quite a few allocations in the existing code 74 specify ``GFP_NOIO`` or ``GFP_NOFS``. Historically, they were used to 75 prevent recursion deadlocks caused by direct memory reclaim calling 76 back into the FS or IO paths and blocking on already held 77 resources. Since 4.12 the preferred way to address this issue is to 78 use new scope APIs described in 79 :ref:`Documentation/core-api/gfp_mask-from-fs-io.rst <gfp_mask_from_fs_io>`. 80 81 Other legacy GFP flags are ``GFP_DMA`` and ``GFP_DMA32``. They are 82 used to ensure that the allocated memory is accessible by hardware 83 with limited addressing capabilities. So unless you are writing a 84 driver for a device with such restrictions, avoid using these flags. 85 And even with hardware with restrictions it is preferable to use 86 `dma_alloc*` APIs. 87 88 GFP flags and reclaim behavior 89 ------------------------------ 90 Memory allocations may trigger direct or background reclaim and it is 91 useful to understand how hard the page allocator will try to satisfy that 92 or another request. 93 94 * ``GFP_KERNEL & ~__GFP_RECLAIM`` - optimistic allocation without _any_ 95 attempt to free memory at all. The most light weight mode which even 96 doesn't kick the background reclaim. Should be used carefully because it 97 might deplete the memory and the next user might hit the more aggressive 98 reclaim. 99 100 * ``GFP_KERNEL & ~__GFP_DIRECT_RECLAIM`` (or ``GFP_NOWAIT``)- optimistic 101 allocation without any attempt to free memory from the current 102 context but can wake kswapd to reclaim memory if the zone is below 103 the low watermark. Can be used from either atomic contexts or when 104 the request is a performance optimization and there is another 105 fallback for a slow path. 106 107 * ``(GFP_KERNEL|__GFP_HIGH) & ~__GFP_DIRECT_RECLAIM`` (aka ``GFP_ATOMIC``) - 108 non sleeping allocation with an expensive fallback so it can access 109 some portion of memory reserves. Usually used from interrupt/bottom-half 110 context with an expensive slow path fallback. 111 112 * ``GFP_KERNEL`` - both background and direct reclaim are allowed and the 113 **default** page allocator behavior is used. That means that not costly 114 allocation requests are basically no-fail but there is no guarantee of 115 that behavior so failures have to be checked properly by callers 116 (e.g. OOM killer victim is allowed to fail currently). 117 118 * ``GFP_KERNEL | __GFP_NORETRY`` - overrides the default allocator behavior 119 and all allocation requests fail early rather than cause disruptive 120 reclaim (one round of reclaim in this implementation). The OOM killer 121 is not invoked. 122 123 * ``GFP_KERNEL | __GFP_RETRY_MAYFAIL`` - overrides the default allocator 124 behavior and all allocation requests try really hard. The request 125 will fail if the reclaim cannot make any progress. The OOM killer 126 won't be triggered. 127 128 * ``GFP_KERNEL | __GFP_NOFAIL`` - overrides the default allocator behavior 129 and all allocation requests will loop endlessly until they succeed. 130 This might be really dangerous especially for larger orders. 131 132 Selecting memory allocator 133 ========================== 134 135 The most straightforward way to allocate memory is to use a function 136 from the kmalloc() family. And, to be on the safe side it's best to use 137 routines that set memory to zero, like kzalloc(). If you need to 138 allocate memory for an array, there are kmalloc_array() and kcalloc() 139 helpers. The helpers struct_size(), array_size() and array3_size() can 140 be used to safely calculate object sizes without overflowing. 141 142 The maximal size of a chunk that can be allocated with `kmalloc` is 143 limited. The actual limit depends on the hardware and the kernel 144 configuration, but it is a good practice to use `kmalloc` for objects 145 smaller than page size. 146 147 The address of a chunk allocated with `kmalloc` is aligned to at least 148 ARCH_KMALLOC_MINALIGN bytes. For sizes which are a power of two, the 149 alignment is also guaranteed to be at least the respective size. For other 150 sizes, the alignment is guaranteed to be at least the largest power-of-two 151 divisor of the size. 152 153 Chunks allocated with kmalloc() can be resized with krealloc(). Similarly 154 to kmalloc_array(): a helper for resizing arrays is provided in the form of 155 krealloc_array(). 156 157 For large allocations you can use vmalloc() and vzalloc(), or directly 158 request pages from the page allocator. The memory allocated by `vmalloc` 159 and related functions is not physically contiguous. 160 161 If you are not sure whether the allocation size is too large for 162 `kmalloc`, it is possible to use kvmalloc() and its derivatives. It will 163 try to allocate memory with `kmalloc` and if the allocation fails it 164 will be retried with `vmalloc`. There are restrictions on which GFP 165 flags can be used with `kvmalloc`; please see kvmalloc_node() reference 166 documentation. Note that `kvmalloc` may return memory that is not 167 physically contiguous. 168 169 If you need to allocate many identical objects you can use the slab 170 cache allocator. The cache should be set up with kmem_cache_create() or 171 kmem_cache_create_usercopy() before it can be used. The second function 172 should be used if a part of the cache might be copied to the userspace. 173 After the cache is created kmem_cache_alloc() and its convenience 174 wrappers can allocate memory from that cache. 175 176 When the allocated memory is no longer needed it must be freed. 177 178 Objects allocated by `kmalloc` can be freed by `kfree` or `kvfree`. Objects 179 allocated by `kmem_cache_alloc` can be freed with `kmem_cache_free`, `kfree` 180 or `kvfree`, where the latter two might be more convenient thanks to not 181 needing the kmem_cache pointer. 182 183 The same rules apply to _bulk and _rcu flavors of freeing functions. 184 185 Memory allocated by `vmalloc` can be freed with `vfree` or `kvfree`. 186 Memory allocated by `kvmalloc` can be freed with `kvfree`. 187 Caches created by `kmem_cache_create` should be freed with 188 `kmem_cache_destroy` only after freeing all the allocated objects first.
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