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<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=ANSI_X3.4-1968" /><title>kmalloc()/kfree() include/linux/slab.h</title><meta name="generator" content="DocBook XSL Stylesheets V1.75.2" /><link rel="home" href="index.html" title="Unreliable Guide To Hacking The Linux Kernel" /><link rel="up" href="ch06.html" title="Chapter 6. Common Routines" /><link rel="prev" href="ch06s02.html" title="copy_[to/from]_user() / get_user() / put_user() include/asm/uaccess.h" /><link rel="next" href="ch06s04.html" title="current include/asm/current.h" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center"><code class="function">kmalloc()</code>/<code class="function">kfree()</code>
<code class="filename">include/linux/slab.h</code></th></tr><tr><td width="20%" align="left"><a accesskey="p" href="ch06s02.html">Prev</a> </td><th width="60%" align="center">Chapter 6. Common Routines</th><td width="20%" align="right"> <a accesskey="n" href="ch06s04.html">Next</a></td></tr></table><hr /></div><div class="sect1" title="kmalloc()/kfree() include/linux/slab.h"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="routines-kmalloc"></a><code class="function">kmalloc()</code>/<code class="function">kfree()</code>
<code class="filename">include/linux/slab.h</code></h2></div></div></div><p>
<span class="emphasis"><em>[MAY SLEEP: SEE BELOW]</em></span>
</p><p>
These routines are used to dynamically request pointer-aligned
chunks of memory, like malloc and free do in userspace, but
<code class="function">kmalloc()</code> takes an extra flag word.
Important values:
</p><div class="variablelist"><dl><dt><span class="term">
<code class="constant">
GFP_KERNEL
</code>
</span></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><span class="term">
<code class="constant">
GFP_ATOMIC
</code>
</span></dt><dd><p>
Don't sleep. Less reliable than <code class="constant">GFP_KERNEL</code>,
but may be called from interrupt context. You should
<span class="emphasis"><em>really</em></span> have a good out-of-memory
error-handling strategy.
</p></dd><dt><span class="term">
<code class="constant">
GFP_DMA
</code>
</span></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">sleeping function called from invalid
context</span> warning message, then maybe you called a
sleeping allocation function from interrupt context without
<code class="constant">GFP_ATOMIC</code>. You should really fix that.
Run, don't walk.
</p><p>
If you are allocating at least <code class="constant">PAGE_SIZE</code>
(<code class="filename">include/asm/page.h</code>) bytes,
consider using <code class="function">__get_free_pages()</code>
(<code class="filename">include/linux/mm.h</code>). 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
<code class="function">vmalloc()</code>. It'll allocate virtual memory in
the kernel map. This block is not contiguous in physical memory,
but the <acronym class="acronym">MMU</acronym> 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 <code class="function">alloc_bootmem()</code>
routine.
</p><p>
Before inventing your own cache of often-used objects consider
using a slab cache in
<code class="filename">include/linux/slab.h</code>
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<code class="function">copy_[to/from]_user()</code>
/
<code class="function">get_user()</code>
/
<code class="function">put_user()</code>
<code class="filename">include/asm/uaccess.h</code>
 </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> <code class="function">current</code>
<code class="filename">include/asm/current.h</code></td></tr></table></div></body></html>
