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