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><A
NAME="ROUTINES-KMALLOC"
></A
><TT
CLASS="FUNCTION"
>kmalloc()</TT
>/<TT
CLASS="FUNCTION"
>kfree()</TT
>
<TT
CLASS="FILENAME"
>include/linux/slab.h</TT
></H1
><P
> <I
CLASS="EMPHASIS"
>[MAY SLEEP: SEE BELOW]</I
>
</P
><P
> These routines are used to dynamically request pointer-aligned
chunks of memory, like malloc and free do in userspace, but
<TT
CLASS="FUNCTION"
>kmalloc()</TT
> takes an extra flag word.
Important values:
</P
><P
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><DIV
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><DL
><DT
><TT
CLASS="CONSTANT"
> GFP_KERNEL
</TT
></DT
><DD
><P
> May sleep and swap to free memory. Only allowed in user
context, but is the most reliable way to allocate memory.
</P
></DD
><DT
><TT
CLASS="CONSTANT"
> GFP_ATOMIC
</TT
></DT
><DD
><P
> Don't sleep. Less reliable than <TT
CLASS="CONSTANT"
>GFP_KERNEL</TT
>,
but may be called from interrupt context. You should
<I
CLASS="EMPHASIS"
>really</I
> have a good out-of-memory
error-handling strategy.
</P
></DD
><DT
><TT
CLASS="CONSTANT"
> GFP_DMA
</TT
></DT
><DD
><P
> Allocate ISA DMA lower than 16MB. If you don't know what that
is you don't need it. Very unreliable.
</P
></DD
></DL
></DIV
><P
> If you see a <SPAN
CLASS="ERRORNAME"
>kmem_grow: Called nonatomically from int
</SPAN
> warning message you called a memory allocation function
from interrupt context without <TT
CLASS="CONSTANT"
>GFP_ATOMIC</TT
>.
You should really fix that. Run, don't walk.
</P
><P
> If you are allocating at least <TT
CLASS="CONSTANT"
>PAGE_SIZE</TT
>
(<TT
CLASS="FILENAME"
>include/asm/page.h</TT
>) bytes,
consider using <TT
CLASS="FUNCTION"
>__get_free_pages()</TT
>
(<TT
CLASS="FILENAME"
>include/linux/mm.h</TT
>). 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.
</P
><P
> If you are allocating more than a page worth of bytes you can use
<TT
CLASS="FUNCTION"
>vmalloc()</TT
>. It'll allocate virtual memory in
the kernel map. This block is not contiguous in physical memory,
but the <SPAN
CLASS="ACRONYM"
>MMU</SPAN
> 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 <TT
CLASS="FUNCTION"
>alloc_bootmem()</TT
>
routine.
</P
><P
> Before inventing your own cache of often-used objects consider
using a slab cache in
<TT
CLASS="FILENAME"
>include/linux/slab.h</TT
>
</P
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