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
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 Documentation/core-api/mm-api.rst provides reference documentation for the GFP flags and their combinations and here we briefly outline their recommended usage:
Most of the time
GFP_KERNELis 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_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_NOWAITallocation is likely to fail. Allocations which have a reasonable fallback should be using
If you think that accessing memory reserves is justified and the kernel will be stressed unless allocation succeeds, you may use
Untrusted allocations triggered from userspace should be a subject of kmem accounting and must have
__GFP_ACCOUNTbit set. There is the handy
GFP_KERNELallocations that should be accounted.
Userspace allocations should use either of the
GFP_HIGHUSER_MOVABLEflags. The longer the flag name the less restrictive it is.
GFP_HIGHUSER_MOVABLEdoes not require that allocated memory will be directly accessible by the kernel and implies that the data is movable.
GFP_HIGHUSERmeans 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_USERmeans 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
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
Other legacy GFP flags are
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
Selecting memory allocator¶
The most straightforward way to allocate memory is to use a function
kmalloc() family. And, to be on the safe size it’s
best to use routines that set memory to zero, like
kzalloc(). If you need to allocate memory for an array, there
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.
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
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() before it can be used. Afterwards
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
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