177 lines
8.5 KiB
ReStructuredText
177 lines
8.5 KiB
ReStructuredText
|
.. _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().
|