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+.. _memory_allocation:
+
+=======================
+Memory Allocation Guide
+=======================
+
+Linux provides a variety of APIs for memory allocation. You can
+allocate 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 that
+memory should be allocated. The GFP acronym stands for "get free
+pages", the underlying memory allocation function.
+
+Diversity of the allocation APIs combined with the numerous GFP flags
+makes the question "How should I allocate memory?" not that easy to
+answer, although very likely you should use
+
+::
+
+  kzalloc(<size>, GFP_KERNEL);
+
+Of course there are cases when other allocation APIs and different GFP
+flags must be used.
+
+Get Free Page flags
+===================
+
+The GFP flags control the allocators behavior. They tell what memory
+zones can be used, how hard the allocator should try to find free
+memory, whether the memory can be accessed by the userspace etc. The
+:ref:`Documentation/core-api/mm-api.rst <mm-api-gfp-flags>` provides
+reference documentation for the GFP flags and their combinations and
+here 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 code
+specify ``GFP_NOIO`` or ``GFP_NOFS``. Historically, they were used to
+prevent recursion deadlocks caused by direct memory reclaim calling
+back into the FS or IO paths and blocking on already held
+resources. Since 4.12 the preferred way to address this issue is to
+use 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 are
+used to ensure that the allocated memory is accessible by hardware
+with limited addressing capabilities. So unless you are writing a
+driver 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 is
+useful to understand how hard the page allocator will try to satisfy that
+or 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 function
+from the kmalloc() family. And, to be on the safe side it's best to use
+routines that set memory to zero, like kzalloc(). If you need to
+allocate memory for an array, there are kmalloc_array() and kcalloc()
+helpers. The helpers struct_size(), array_size() and array3_size() can
+be used to safely calculate object sizes without overflowing.
+
+The maximal size of a chunk that can be allocated with `kmalloc` is
+limited. The actual limit depends on the hardware and the kernel
+configuration, but it is a good practice to use `kmalloc` for objects
+smaller than page size.
+
+The address of a chunk allocated with `kmalloc` is aligned to at least
+ARCH_KMALLOC_MINALIGN bytes.  For sizes which are a power of two, the
+alignment is also guaranteed to be at least the respective size.
+
+Chunks allocated with kmalloc() can be resized with krealloc(). Similarly
+to kmalloc_array(): a helper for resizing arrays is provided in the form of
+krealloc_array().
+
+For large allocations you can use vmalloc() and vzalloc(), or directly
+request 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 will
+try to allocate memory with `kmalloc` and if the allocation fails it
+will be retried with `vmalloc`. There are restrictions on which GFP
+flags can be used with `kvmalloc`; please see kvmalloc_node() reference
+documentation. Note that `kvmalloc` may return memory that is not
+physically contiguous.
+
+If you need to allocate many identical objects you can use the slab
+cache allocator. The cache should be set up with kmem_cache_create() or
+kmem_cache_create_usercopy() before it can be used. The second function
+should be used if a part of the cache might be copied to the userspace.
+After the cache is created kmem_cache_alloc() and its convenience
+wrappers can allocate memory from that cache.
+
+When the allocated memory is no longer needed it must be freed. You can
+use kvfree() for the memory allocated with `kmalloc`, `vmalloc` and
+`kvmalloc`. The slab caches should be freed with kmem_cache_free(). And
+don't forget to destroy the cache with kmem_cache_destroy().