.. _memory_allocation: =======================Memory Allocation Guide======================= Linux provides a variety of APIs for memory allocation. You canallocate small chunks using `kmalloc` or `kmem_cache_alloc` families,large virtually contiguous areas using `vmalloc` and its derivatives,or you can directly request pages from the page allocator with`alloc_pages`. It is also possible to use more specialized allocators,for instance `cma_alloc` or `zs_malloc`. Most of the memory allocation APIs use GFP flags to express how thatmemory should be allocated. The GFP acronym stands for "get freepages", the underlying memory allocation function. Diversity of the allocation APIs combined with the numerous GFP flagsmakes the question "How should I allocate memory?" not that easy toanswer, although very likely you should use ::   kzalloc(<size>, GFP_KERNEL); Of course there are cases when other allocation APIs and different GFPflags must be used. Get Free Page flags=================== The GFP flags control the allocators behavior. They tell what memoryzones can be used, how hard the allocator should try to find freememory, whether the memory can be accessed by the userspace etc. The:ref:`Documentation/core-api/mm-api.rst <mm-api-gfp-flags>` providesreference documentation for the GFP flags and their combinations andhere we briefly outline their recommended usage:   * Most of the time ``GFP_KERNEL`` is what you need. Memory for the    kernel data structures, DMAable memory, inode cache, all these and    many other allocations types can use ``GFP_KERNEL``. Note, that    using ``GFP_KERNEL`` implies ``GFP_RECLAIM``, which means that    direct reclaim may be triggered under memory pressure; the calling    context must be allowed to sleep.  * If the allocation is performed from an atomic context, e.g interrupt    handler, use ``GFP_NOWAIT``. This flag prevents direct reclaim and    IO or filesystem operations. Consequently, under memory pressure    ``GFP_NOWAIT`` allocation is likely to fail. Allocations which    have a reasonable fallback should be using ``GFP_NOWARN``.  * If you think that accessing memory reserves is justified and the kernel    will be stressed unless allocation succeeds, you may use ``GFP_ATOMIC``.  * Untrusted allocations triggered from userspace should be a subject    of kmem accounting and must have ``__GFP_ACCOUNT`` bit set. There    is the handy ``GFP_KERNEL_ACCOUNT`` shortcut for ``GFP_KERNEL``    allocations that should be accounted.  * Userspace allocations should use either of the ``GFP_USER``,    ``GFP_HIGHUSER`` or ``GFP_HIGHUSER_MOVABLE`` flags. The longer    the flag name the less restrictive it is.     ``GFP_HIGHUSER_MOVABLE`` does not require that allocated memory    will be directly accessible by the kernel and implies that the    data is movable.     ``GFP_HIGHUSER`` means that the allocated memory is not movable,    but it is not required to be directly accessible by the kernel. An    example may be a hardware allocation that maps data directly into    userspace but has no addressing limitations.     ``GFP_USER`` means that the allocated memory is not movable and it    must be directly accessible by the kernel. You may notice that quite a few allocations in the existing codespecify ``GFP_NOIO`` or ``GFP_NOFS``. Historically, they were used toprevent recursion deadlocks caused by direct memory reclaim callingback into the FS or IO paths and blocking on already heldresources. Since 4.12 the preferred way to address this issue is touse new scope APIs described in:ref:`Documentation/core-api/gfp_mask-from-fs-io.rst <gfp_mask_from_fs_io>`. Other legacy GFP flags are ``GFP_DMA`` and ``GFP_DMA32``. They areused to ensure that the allocated memory is accessible by hardwarewith limited addressing capabilities. So unless you are writing adriver for a device with such restrictions, avoid using these flags.And even with hardware with restrictions it is preferable to use`dma_alloc*` APIs. GFP flags and reclaim behavior------------------------------Memory allocations may trigger direct or background reclaim and it isuseful to understand how hard the page allocator will try to satisfy thator another request.   * ``GFP_KERNEL & ~__GFP_RECLAIM`` - optimistic allocation without _any_    attempt to free memory at all. The most light weight mode which even    doesn't kick the background reclaim. Should be used carefully because it    might deplete the memory and the next user might hit the more aggressive    reclaim.   * ``GFP_KERNEL & ~__GFP_DIRECT_RECLAIM`` (or ``GFP_NOWAIT``)- optimistic    allocation without any attempt to free memory from the current    context but can wake kswapd to reclaim memory if the zone is below    the low watermark. Can be used from either atomic contexts or when    the request is a performance optimization and there is another    fallback for a slow path.   * ``(GFP_KERNEL|__GFP_HIGH) & ~__GFP_DIRECT_RECLAIM`` (aka ``GFP_ATOMIC``) -    non sleeping allocation with an expensive fallback so it can access    some portion of memory reserves. Usually used from interrupt/bottom-half    context with an expensive slow path fallback.   * ``GFP_KERNEL`` - both background and direct reclaim are allowed and the    **default** page allocator behavior is used. That means that not costly    allocation requests are basically no-fail but there is no guarantee of    that behavior so failures have to be checked properly by callers    (e.g. OOM killer victim is allowed to fail currently).   * ``GFP_KERNEL | __GFP_NORETRY`` - overrides the default allocator behavior    and all allocation requests fail early rather than cause disruptive    reclaim (one round of reclaim in this implementation). The OOM killer    is not invoked.   * ``GFP_KERNEL | __GFP_RETRY_MAYFAIL`` - overrides the default allocator    behavior and all allocation requests try really hard. The request    will fail if the reclaim cannot make any progress. The OOM killer    won't be triggered.   * ``GFP_KERNEL | __GFP_NOFAIL`` - overrides the default allocator behavior    and all allocation requests will loop endlessly until they succeed.    This might be really dangerous especially for larger orders. Selecting memory allocator========================== The most straightforward way to allocate memory is to use a functionfrom the kmalloc() family. And, to be on the safe side it's best to useroutines that set memory to zero, like kzalloc(). If you need toallocate memory for an array, there are kmalloc_array() and kcalloc()helpers. The helpers struct_size(), array_size() and array3_size() canbe used to safely calculate object sizes without overflowing. The maximal size of a chunk that can be allocated with `kmalloc` islimited. The actual limit depends on the hardware and the kernelconfiguration, but it is a good practice to use `kmalloc` for objectssmaller than page size. The address of a chunk allocated with `kmalloc` is aligned to at leastARCH_KMALLOC_MINALIGN bytes.  For sizes which are a power of two, thealignment is also guaranteed to be at least the respective size. Chunks allocated with kmalloc() can be resized with krealloc(). Similarlyto kmalloc_array(): a helper for resizing arrays is provided in the form ofkrealloc_array(). For large allocations you can use vmalloc() and vzalloc(), or directlyrequest pages from the page allocator. The memory allocated by `vmalloc`and related functions is not physically contiguous. If you are not sure whether the allocation size is too large for`kmalloc`, it is possible to use kvmalloc() and its derivatives. It willtry to allocate memory with `kmalloc` and if the allocation fails itwill be retried with `vmalloc`. There are restrictions on which GFPflags can be used with `kvmalloc`; please see kvmalloc_node() referencedocumentation. Note that `kvmalloc` may return memory that is notphysically contiguous. If you need to allocate many identical objects you can use the slabcache allocator. The cache should be set up with kmem_cache_create() orkmem_cache_create_usercopy() before it can be used. The second functionshould be used if a part of the cache might be copied to the userspace.After the cache is created kmem_cache_alloc() and its conveniencewrappers can allocate memory from that cache. When the allocated memory is no longer needed it must be freed. You canuse kvfree() for the memory allocated with `kmalloc`, `vmalloc` and`kvmalloc`. The slab caches should be freed with kmem_cache_free(). Anddon't forget to destroy the cache with kmem_cache_destroy().