@@ -27,6 +27,7 @@ Core utilities
errseq
printk-formats
circular-buffers
+ memory-allocation
mm-api
gfp_mask-from-fs-io
timekeeping
new file mode 100644
@@ -0,0 +1,124 @@
+=======================
+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 or the hardware 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 or
+ the hardware. 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 or the hardware. It is
+ typically used by hardware for buffers that are mapped to
+ userspace (e.g. graphics) that hardware still must DMA to.
+
+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 HW 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`.