[MAY SLEEP: SEE BELOW]
These routines are used to dynamically request pointer-aligned
chunks of memory, like malloc and free do in userspace, but
kmalloc() takes an extra flag word.
Important values:
GFP_KERNEL
May sleep and swap to free memory. Only allowed in user context, but is the most reliable way to allocate memory.
GFP_ATOMIC
Don't sleep. Less reliable than GFP_KERNEL,
but may be called from interrupt context. You should
really have a good out-of-memory
error-handling strategy.
GFP_DMA
Allocate ISA DMA lower than 16MB. If you don't know what that is you don't need it. Very unreliable.
If you see a sleeping function called from invalid
context warning message, then maybe you called a
sleeping allocation function from interrupt context without
GFP_ATOMIC. You should really fix that.
Run, don't walk.
If you are allocating at least PAGE_SIZE
(include/asm/page.h) bytes,
consider using __get_free_pages()
(include/linux/mm.h). It
takes an order argument (0 for page sized, 1 for double page, 2
for four pages etc.) and the same memory priority flag word as
above.
If you are allocating more than a page worth of bytes you can use
vmalloc(). It'll allocate virtual memory in
the kernel map. This block is not contiguous in physical memory,
but the MMU makes it look like it is for you
(so it'll only look contiguous to the CPUs, not to external device
drivers). If you really need large physically contiguous memory
for some weird device, you have a problem: it is poorly supported
in Linux because after some time memory fragmentation in a running
kernel makes it hard. The best way is to allocate the block early
in the boot process via the alloc_bootmem()
routine.
Before inventing your own cache of often-used objects consider
using a slab cache in
include/linux/slab.h