.. _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.

Selecting memory allocator
==========================

The most straightforward way to allocate memory is to use a function
from the :c:func:`kmalloc` family. And, to be on the safe size it's
best to use routines that set memory to zero, like
:c:func:`kzalloc`. If you need to allocate memory for an array, there
are :c:func:`kmalloc_array` and :c:func:`kcalloc` helpers.

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.

For large allocations you can use :c:func:`vmalloc` and
:c:func:`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 :c:func:`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 :c:func:`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
:c:func:`kmem_cache_create` before it can be used. Afterwards
:c:func:`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 :c:func:`kvfree` for the memory allocated with `kmalloc`,
`vmalloc` and `kvmalloc`. The slab caches should be freed with
:c:func:`kmem_cache_free`. And don't forget to destroy the cache with
:c:func:`kmem_cache_destroy`.
