diff options
author | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-27 10:05:51 +0000 |
---|---|---|
committer | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-27 10:05:51 +0000 |
commit | 5d1646d90e1f2cceb9f0828f4b28318cd0ec7744 (patch) | |
tree | a94efe259b9009378be6d90eb30d2b019d95c194 /Documentation/core-api/memory-allocation.rst | |
parent | Initial commit. (diff) | |
download | linux-5d1646d90e1f2cceb9f0828f4b28318cd0ec7744.tar.xz linux-5d1646d90e1f2cceb9f0828f4b28318cd0ec7744.zip |
Adding upstream version 5.10.209.upstream/5.10.209upstream
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
Diffstat (limited to '')
-rw-r--r-- | Documentation/core-api/memory-allocation.rst | 172 |
1 files changed, 172 insertions, 0 deletions
diff --git a/Documentation/core-api/memory-allocation.rst b/Documentation/core-api/memory-allocation.rst new file mode 100644 index 000000000..4446a1ac3 --- /dev/null +++ b/Documentation/core-api/memory-allocation.rst @@ -0,0 +1,172 @@ +.. _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. + +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(). |