1.. _memory_allocation:
2
3=======================
4Memory Allocation Guide
5=======================
6
7Linux provides a variety of APIs for memory allocation. You can
8allocate small chunks using `kmalloc` or `kmem_cache_alloc` families,
9large virtually contiguous areas using `vmalloc` and its derivatives,
10or you can directly request pages from the page allocator with
11`alloc_pages`. It is also possible to use more specialized allocators,
12for instance `cma_alloc` or `zs_malloc`.
13
14Most of the memory allocation APIs use GFP flags to express how that
15memory should be allocated. The GFP acronym stands for "get free
16pages", the underlying memory allocation function.
17
18Diversity of the allocation APIs combined with the numerous GFP flags
19makes the question "How should I allocate memory?" not that easy to
20answer, although very likely you should use
21
22::
23
24  kzalloc(<size>, GFP_KERNEL);
25
26Of course there are cases when other allocation APIs and different GFP
27flags must be used.
28
29Get Free Page flags
30===================
31
32The GFP flags control the allocators behavior. They tell what memory
33zones can be used, how hard the allocator should try to find free
34memory, whether the memory can be accessed by the userspace etc. The
35:ref:`Documentation/core-api/mm-api.rst <mm-api-gfp-flags>` provides
36reference documentation for the GFP flags and their combinations and
37here 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. Allocations which
49    have a reasonable fallback should be using ``GFP_NOWARN``.
50  * If you think that accessing memory reserves is justified and the kernel
51    will be stressed unless allocation succeeds, you may use ``GFP_ATOMIC``.
52  * Untrusted allocations triggered from userspace should be a subject
53    of kmem accounting and must have ``__GFP_ACCOUNT`` bit set. There
54    is the handy ``GFP_KERNEL_ACCOUNT`` shortcut for ``GFP_KERNEL``
55    allocations that should be accounted.
56  * Userspace allocations should use either of the ``GFP_USER``,
57    ``GFP_HIGHUSER`` or ``GFP_HIGHUSER_MOVABLE`` flags. The longer
58    the flag name the less restrictive it is.
59
60    ``GFP_HIGHUSER_MOVABLE`` does not require that allocated memory
61    will be directly accessible by the kernel and implies that the
62    data is movable.
63
64    ``GFP_HIGHUSER`` means that the allocated memory is not movable,
65    but it is not required to be directly accessible by the kernel. An
66    example may be a hardware allocation that maps data directly into
67    userspace but has no addressing limitations.
68
69    ``GFP_USER`` means that the allocated memory is not movable and it
70    must be directly accessible by the kernel.
71
72You may notice that quite a few allocations in the existing code
73specify ``GFP_NOIO`` or ``GFP_NOFS``. Historically, they were used to
74prevent recursion deadlocks caused by direct memory reclaim calling
75back into the FS or IO paths and blocking on already held
76resources. Since 4.12 the preferred way to address this issue is to
77use new scope APIs described in
78:ref:`Documentation/core-api/gfp_mask-from-fs-io.rst <gfp_mask_from_fs_io>`.
79
80Other legacy GFP flags are ``GFP_DMA`` and ``GFP_DMA32``. They are
81used to ensure that the allocated memory is accessible by hardware
82with limited addressing capabilities. So unless you are writing a
83driver for a device with such restrictions, avoid using these flags.
84And even with hardware with restrictions it is preferable to use
85`dma_alloc*` APIs.
86
87Selecting memory allocator
88==========================
89
90The most straightforward way to allocate memory is to use a function
91from the :c:func:`kmalloc` family. And, to be on the safe size it's
92best to use routines that set memory to zero, like
93:c:func:`kzalloc`. If you need to allocate memory for an array, there
94are :c:func:`kmalloc_array` and :c:func:`kcalloc` helpers.
95
96The maximal size of a chunk that can be allocated with `kmalloc` is
97limited. The actual limit depends on the hardware and the kernel
98configuration, but it is a good practice to use `kmalloc` for objects
99smaller than page size.
100
101For large allocations you can use :c:func:`vmalloc` and
102:c:func:`vzalloc`, or directly request pages from the page
103allocator. The memory allocated by `vmalloc` and related functions is
104not physically contiguous.
105
106If you are not sure whether the allocation size is too large for
107`kmalloc`, it is possible to use :c:func:`kvmalloc` and its
108derivatives. It will try to allocate memory with `kmalloc` and if the
109allocation fails it will be retried with `vmalloc`. There are
110restrictions on which GFP flags can be used with `kvmalloc`; please
111see :c:func:`kvmalloc_node` reference documentation. Note that
112`kvmalloc` may return memory that is not physically contiguous.
113
114If you need to allocate many identical objects you can use the slab
115cache allocator. The cache should be set up with
116:c:func:`kmem_cache_create` or :c:func:`kmem_cache_create_usercopy`
117before it can be used. The second function should be used if a part of
118the cache might be copied to the userspace.  After the cache is
119created :c:func:`kmem_cache_alloc` and its convenience wrappers can
120allocate memory from that cache.
121
122When the allocated memory is no longer needed it must be freed. You
123can use :c:func:`kvfree` for the memory allocated with `kmalloc`,
124`vmalloc` and `kvmalloc`. The slab caches should be freed with
125:c:func:`kmem_cache_free`. And don't forget to destroy the cache with
126:c:func:`kmem_cache_destroy`.
127