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 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_KERNEL
is what you need. Memory for the kernel data structures, DMAable memory, inode cache, all these and many other allocations types can useGFP_KERNEL
. Note, that usingGFP_KERNEL
impliesGFP_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 pressureGFP_NOWAIT
allocation is likely to fail. Allocations which have a reasonable fallback should be usingGFP_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 handyGFP_KERNEL_ACCOUNT
shortcut forGFP_KERNEL
allocations that should be accounted.Userspace allocations should use either of the
GFP_USER
,GFP_HIGHUSER
orGFP_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
Documentation/core-api/gfp_mask-from-fs-io.rst.
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.
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().