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<section id="high-memory-handling">
<h1>High Memory Handling<a class="headerlink" href="#high-memory-handling" title="Link to this heading">¶</a></h1>
<p>By: Peter Zijlstra <<a class="reference external" href="mailto:a.p.zijlstra%40chello.nl">a<span>.</span>p<span>.</span>zijlstra<span>@</span>chello<span>.</span>nl</a>></p>
<nav class="contents local" id="contents">
<ul class="simple">
<li><p><a class="reference internal" href="#what-is-high-memory" id="id1">What Is High Memory?</a></p></li>
<li><p><a class="reference internal" href="#temporary-virtual-mappings" id="id2">Temporary Virtual Mappings</a></p></li>
<li><p><a class="reference internal" href="#cost-of-temporary-mappings" id="id3">Cost of Temporary Mappings</a></p></li>
<li><p><a class="reference internal" href="#i386-pae" id="id4">i386 PAE</a></p></li>
<li><p><a class="reference internal" href="#functions" id="id5">Functions</a></p></li>
</ul>
</nav>
<section id="what-is-high-memory">
<h2><a class="toc-backref" href="#id1" role="doc-backlink">What Is High Memory?</a><a class="headerlink" href="#what-is-high-memory" title="Link to this heading">¶</a></h2>
<p>High memory (highmem) is used when the size of physical memory approaches or
exceeds the maximum size of virtual memory. At that point it becomes
impossible for the kernel to keep all of the available physical memory mapped
at all times. This means the kernel needs to start using temporary mappings of
the pieces of physical memory that it wants to access.</p>
<p>The part of (physical) memory not covered by a permanent mapping is what we
refer to as ‘highmem’. There are various architecture dependent constraints on
where exactly that border lies.</p>
<p>In the i386 arch, for example, we choose to map the kernel into every process’s
VM space so that we don’t have to pay the full TLB invalidation costs for
kernel entry/exit. This means the available virtual memory space (4GiB on
i386) has to be divided between user and kernel space.</p>
<p>The traditional split for architectures using this approach is 3:1, 3GiB for
userspace and the top 1GiB for kernel space:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>+--------+ 0xffffffff
| Kernel |
+--------+ 0xc0000000
| |
| User |
| |
+--------+ 0x00000000
</pre></div>
</div>
<p>This means that the kernel can at most map 1GiB of physical memory at any one
time, but because we need virtual address space for other things - including
temporary maps to access the rest of the physical memory - the actual direct
map will typically be less (usually around ~896MiB).</p>
<p>Other architectures that have mm context tagged TLBs can have separate kernel
and user maps. Some hardware (like some ARMs), however, have limited virtual
space when they use mm context tags.</p>
</section>
<section id="temporary-virtual-mappings">
<h2><a class="toc-backref" href="#id2" role="doc-backlink">Temporary Virtual Mappings</a><a class="headerlink" href="#temporary-virtual-mappings" title="Link to this heading">¶</a></h2>
<p>The kernel contains several ways of creating temporary mappings. The following
list shows them in order of preference of use.</p>
<ul>
<li><p><a class="reference internal" href="#c.kmap_local_page" title="kmap_local_page"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap_local_page()</span></code></a>, <a class="reference internal" href="#c.kmap_local_folio" title="kmap_local_folio"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap_local_folio()</span></code></a> - These functions are used to create
short term mappings. They can be invoked from any context (including
interrupts) but the mappings can only be used in the context which acquired
them. The only differences between them consist in the first taking a pointer
to a <code class="xref c c-struct broken_xref docutils literal notranslate"><span class="pre">struct</span> <span class="pre">page</span></code> and the second taking a pointer to <a class="reference internal" href="../core-api/mm-api.html#c.folio" title="folio"><code class="xref c c-struct docutils literal notranslate"><span class="pre">struct</span> <span class="pre">folio</span></code></a> and the byte
offset within the folio which identifies the page.</p>
<p>These functions should always be used, whereas <a class="reference internal" href="#c.kmap_atomic" title="kmap_atomic"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap_atomic()</span></code></a> and <a class="reference internal" href="#c.kmap" title="kmap"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap()</span></code></a> have
been deprecated.</p>
<p>These mappings are thread-local and CPU-local, meaning that the mapping
can only be accessed from within this thread and the thread is bound to the
CPU while the mapping is active. Although preemption is never disabled by
this function, the CPU can not be unplugged from the system via
CPU-hotplug until the mapping is disposed.</p>
<p>It’s valid to take pagefaults in a local kmap region, unless the context
in which the local mapping is acquired does not allow it for other reasons.</p>
<p>As said, pagefaults and preemption are never disabled. There is no need to
disable preemption because, when context switches to a different task, the
maps of the outgoing task are saved and those of the incoming one are
restored.</p>
<p><a class="reference internal" href="#c.kmap_local_page" title="kmap_local_page"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap_local_page()</span></code></a>, as well as <a class="reference internal" href="#c.kmap_local_folio" title="kmap_local_folio"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap_local_folio()</span></code></a> always returns valid virtual
kernel addresses and it is assumed that <a class="reference internal" href="#c.kunmap_local" title="kunmap_local"><code class="xref c c-func docutils literal notranslate"><span class="pre">kunmap_local()</span></code></a> will never fail.</p>
<p>On CONFIG_HIGHMEM=n kernels and for low memory pages they return the
virtual address of the direct mapping. Only real highmem pages are
temporarily mapped. Therefore, users may call a plain <a class="reference internal" href="#c.page_address" title="page_address"><code class="xref c c-func docutils literal notranslate"><span class="pre">page_address()</span></code></a>
for pages which are known to not come from ZONE_HIGHMEM. However, it is
always safe to use kmap_local_{page,folio}() / <a class="reference internal" href="#c.kunmap_local" title="kunmap_local"><code class="xref c c-func docutils literal notranslate"><span class="pre">kunmap_local()</span></code></a>.</p>
<p>While they are significantly faster than <a class="reference internal" href="#c.kmap" title="kmap"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap()</span></code></a>, for the highmem case they
come with restrictions about the pointers validity. Contrary to <a class="reference internal" href="#c.kmap" title="kmap"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap()</span></code></a>
mappings, the local mappings are only valid in the context of the caller
and cannot be handed to other contexts. This implies that users must
be absolutely sure to keep the use of the return address local to the
thread which mapped it.</p>
<p>Most code can be designed to use thread local mappings. User should
therefore try to design their code to avoid the use of <a class="reference internal" href="#c.kmap" title="kmap"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap()</span></code></a> by mapping
pages in the same thread the address will be used and prefer
<a class="reference internal" href="#c.kmap_local_page" title="kmap_local_page"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap_local_page()</span></code></a> or <a class="reference internal" href="#c.kmap_local_folio" title="kmap_local_folio"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap_local_folio()</span></code></a>.</p>
<p>Nesting <a class="reference internal" href="#c.kmap_local_page" title="kmap_local_page"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap_local_page()</span></code></a> and <a class="reference internal" href="#c.kmap_atomic" title="kmap_atomic"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap_atomic()</span></code></a> mappings is allowed to a certain
extent (up to KMAP_TYPE_NR) but their invocations have to be strictly ordered
because the map implementation is stack based. See <a class="reference internal" href="#c.kmap_local_page" title="kmap_local_page"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap_local_page()</span></code></a> kdocs
(included in the “Functions” section) for details on how to manage nested
mappings.</p>
</li>
<li><p><a class="reference internal" href="#c.kmap_atomic" title="kmap_atomic"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap_atomic()</span></code></a>. This function has been deprecated; use <a class="reference internal" href="#c.kmap_local_page" title="kmap_local_page"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap_local_page()</span></code></a>.</p>
<p>NOTE: Conversions to <a class="reference internal" href="#c.kmap_local_page" title="kmap_local_page"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap_local_page()</span></code></a> must take care to follow the mapping
restrictions imposed on <a class="reference internal" href="#c.kmap_local_page" title="kmap_local_page"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap_local_page()</span></code></a>. Furthermore, the code between
calls to <a class="reference internal" href="#c.kmap_atomic" title="kmap_atomic"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap_atomic()</span></code></a> and <a class="reference internal" href="#c.kunmap_atomic" title="kunmap_atomic"><code class="xref c c-func docutils literal notranslate"><span class="pre">kunmap_atomic()</span></code></a> may implicitly depend on the side
effects of atomic mappings, i.e. disabling page faults or preemption, or both.
In that case, explicit calls to <code class="xref c c-func broken_xref docutils literal notranslate"><span class="pre">pagefault_disable()</span></code> or <code class="xref c c-func broken_xref docutils literal notranslate"><span class="pre">preempt_disable()</span></code> or
both must be made in conjunction with the use of <a class="reference internal" href="#c.kmap_local_page" title="kmap_local_page"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap_local_page()</span></code></a>.</p>
<p>[Legacy documentation]</p>
<p>This permits a very short duration mapping of a single page. Since the
mapping is restricted to the CPU that issued it, it performs well, but
the issuing task is therefore required to stay on that CPU until it has
finished, lest some other task displace its mappings.</p>
<p><a class="reference internal" href="#c.kmap_atomic" title="kmap_atomic"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap_atomic()</span></code></a> may also be used by interrupt contexts, since it does not
sleep and the callers too may not sleep until after <a class="reference internal" href="#c.kunmap_atomic" title="kunmap_atomic"><code class="xref c c-func docutils literal notranslate"><span class="pre">kunmap_atomic()</span></code></a> is
called.</p>
<p>Each call of <a class="reference internal" href="#c.kmap_atomic" title="kmap_atomic"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap_atomic()</span></code></a> in the kernel creates a non-preemptible section
and disable pagefaults. This could be a source of unwanted latency. Therefore
users should prefer <a class="reference internal" href="#c.kmap_local_page" title="kmap_local_page"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap_local_page()</span></code></a> instead of <a class="reference internal" href="#c.kmap_atomic" title="kmap_atomic"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap_atomic()</span></code></a>.</p>
<p>It is assumed that k[un]<code class="xref c c-func broken_xref docutils literal notranslate"><span class="pre">map_atomic()</span></code> won’t fail.</p>
</li>
<li><p><a class="reference internal" href="#c.kmap" title="kmap"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap()</span></code></a>. This function has been deprecated; use <a class="reference internal" href="#c.kmap_local_page" title="kmap_local_page"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap_local_page()</span></code></a>.</p>
<p>NOTE: Conversions to <a class="reference internal" href="#c.kmap_local_page" title="kmap_local_page"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap_local_page()</span></code></a> must take care to follow the mapping
restrictions imposed on <a class="reference internal" href="#c.kmap_local_page" title="kmap_local_page"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap_local_page()</span></code></a>. In particular, it is necessary to
make sure that the kernel virtual memory pointer is only valid in the thread
that obtained it.</p>
<p>[Legacy documentation]</p>
<p>This should be used to make short duration mapping of a single page with no
restrictions on preemption or migration. It comes with an overhead as mapping
space is restricted and protected by a global lock for synchronization. When
mapping is no longer needed, the address that the page was mapped to must be
released with <a class="reference internal" href="#c.kunmap" title="kunmap"><code class="xref c c-func docutils literal notranslate"><span class="pre">kunmap()</span></code></a>.</p>
<p>Mapping changes must be propagated across all the CPUs. <a class="reference internal" href="#c.kmap" title="kmap"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap()</span></code></a> also
requires global TLB invalidation when the kmap’s pool wraps and it might
block when the mapping space is fully utilized until a slot becomes
available. Therefore, <a class="reference internal" href="#c.kmap" title="kmap"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap()</span></code></a> is only callable from preemptible context.</p>
<p>All the above work is necessary if a mapping must last for a relatively
long time but the bulk of high-memory mappings in the kernel are
short-lived and only used in one place. This means that the cost of
<a class="reference internal" href="#c.kmap" title="kmap"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap()</span></code></a> is mostly wasted in such cases. <a class="reference internal" href="#c.kmap" title="kmap"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap()</span></code></a> was not intended for long
term mappings but it has morphed in that direction and its use is
strongly discouraged in newer code and the set of the preceding functions
should be preferred.</p>
<p>On 64-bit systems, calls to <a class="reference internal" href="#c.kmap_local_page" title="kmap_local_page"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap_local_page()</span></code></a>, <a class="reference internal" href="#c.kmap_atomic" title="kmap_atomic"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap_atomic()</span></code></a> and <a class="reference internal" href="#c.kmap" title="kmap"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap()</span></code></a> have
no real work to do because a 64-bit address space is more than sufficient to
address all the physical memory whose pages are permanently mapped.</p>
</li>
<li><p><a class="reference internal" href="../core-api/mm-api.html#c.vmap" title="vmap"><code class="xref c c-func docutils literal notranslate"><span class="pre">vmap()</span></code></a>. This can be used to make a long duration mapping of multiple
physical pages into a contiguous virtual space. It needs global
synchronization to unmap.</p></li>
</ul>
</section>
<section id="cost-of-temporary-mappings">
<h2><a class="toc-backref" href="#id3" role="doc-backlink">Cost of Temporary Mappings</a><a class="headerlink" href="#cost-of-temporary-mappings" title="Link to this heading">¶</a></h2>
<p>The cost of creating temporary mappings can be quite high. The arch has to
manipulate the kernel’s page tables, the data TLB and/or the MMU’s registers.</p>
<p>If CONFIG_HIGHMEM is not set, then the kernel will try and create a mapping
simply with a bit of arithmetic that will convert the page <code class="xref c c-struct broken_xref docutils literal notranslate"><span class="pre">struct</span> <span class="pre">address</span></code> into
a pointer to the page contents rather than juggling mappings about. In such a
case, the unmap operation may be a null operation.</p>
<p>If CONFIG_MMU is not set, then there can be no temporary mappings and no
highmem. In such a case, the arithmetic approach will also be used.</p>
</section>
<section id="i386-pae">
<h2><a class="toc-backref" href="#id4" role="doc-backlink">i386 PAE</a><a class="headerlink" href="#i386-pae" title="Link to this heading">¶</a></h2>
<p>The i386 arch, under some circumstances, will permit you to stick up to 64GiB
of RAM into your 32-bit machine. This has a number of consequences:</p>
<ul class="simple">
<li><p>Linux needs a page-frame structure for each page in the system and the
pageframes need to live in the permanent mapping, which means:</p></li>
<li><p>you can have 896M/sizeof(<code class="xref c c-struct broken_xref docutils literal notranslate"><span class="pre">struct</span> <span class="pre">page</span></code>) page-frames at most; with <code class="xref c c-struct broken_xref docutils literal notranslate"><span class="pre">struct</span>
<span class="pre">page</span></code> being 32-bytes that would end up being something in the order of 112G
worth of pages; the kernel, however, needs to store more than just
page-frames in that memory...</p></li>
<li><p>PAE makes your page tables larger - which slows the system down as more
data has to be accessed to traverse in TLB fills and the like. One
advantage is that PAE has more PTE bits and can provide advanced features
like NX and PAT.</p></li>
</ul>
<p>The general recommendation is that you don’t use more than 8GiB on a 32-bit
machine - although more might work for you and your workload, you’re pretty
much on your own - don’t expect kernel developers to really care much if things
come apart.</p>
</section>
<section id="functions">
<h2><a class="toc-backref" href="#id5" role="doc-backlink">Functions</a><a class="headerlink" href="#functions" title="Link to this heading">¶</a></h2>
<dl class="c function">
<dt class="sig sig-object c" id="c.kmap">
<span class="kt"><span class="pre">void</span></span><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="sig-name descname"><span class="n"><span class="pre">kmap</span></span></span><span class="sig-paren">(</span><span class="k"><span class="pre">struct</span></span><span class="w"> </span><a class="reference internal" href="#c.kmap" title="page"><span class="n"><span class="pre">page</span></span></a><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">page</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.kmap" title="Link to this definition">¶</a><br /></dt>
<dd><p>Map a page for long term usage</p>
</dd></dl>
<div class="kernelindent docutils container">
<p><strong>Parameters</strong></p>
<dl class="simple">
<dt><code class="docutils literal notranslate"><span class="pre">struct</span> <span class="pre">page</span> <span class="pre">*page</span></code></dt><dd><p>Pointer to the page to be mapped</p>
</dd>
</dl>
<p><strong>Return</strong></p>
<p>The virtual address of the mapping</p>
<p><strong>Description</strong></p>
<p>Can only be invoked from preemptible task context because on 32bit
systems with CONFIG_HIGHMEM enabled this function might sleep.</p>
<p>For systems with CONFIG_HIGHMEM=n and for pages in the low memory area
this returns the virtual address of the direct kernel mapping.</p>
<p>The returned virtual address is globally visible and valid up to the
point where it is unmapped via <a class="reference internal" href="#c.kunmap" title="kunmap"><code class="xref c c-func docutils literal notranslate"><span class="pre">kunmap()</span></code></a>. The pointer can be handed to
other contexts.</p>
<p>For highmem pages on 32bit systems this can be slow as the mapping space
is limited and protected by a global lock. In case that there is no
mapping slot available the function blocks until a slot is released via
<a class="reference internal" href="#c.kunmap" title="kunmap"><code class="xref c c-func docutils literal notranslate"><span class="pre">kunmap()</span></code></a>.</p>
</div>
<dl class="c function">
<dt class="sig sig-object c" id="c.kunmap">
<span class="kt"><span class="pre">void</span></span><span class="w"> </span><span class="sig-name descname"><span class="n"><span class="pre">kunmap</span></span></span><span class="sig-paren">(</span><span class="k"><span class="pre">const</span></span><span class="w"> </span><span class="k"><span class="pre">struct</span></span><span class="w"> </span><a class="reference internal" href="#c.kunmap" title="page"><span class="n"><span class="pre">page</span></span></a><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">page</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.kunmap" title="Link to this definition">¶</a><br /></dt>
<dd><p>Unmap the virtual address mapped by <a class="reference internal" href="#c.kmap" title="kmap"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap()</span></code></a></p>
</dd></dl>
<div class="kernelindent docutils container">
<p><strong>Parameters</strong></p>
<dl class="simple">
<dt><code class="docutils literal notranslate"><span class="pre">const</span> <span class="pre">struct</span> <span class="pre">page</span> <span class="pre">*page</span></code></dt><dd><p>Pointer to the page which was mapped by <a class="reference internal" href="#c.kmap" title="kmap"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap()</span></code></a></p>
</dd>
</dl>
<p><strong>Description</strong></p>
<p>Counterpart to <a class="reference internal" href="#c.kmap" title="kmap"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap()</span></code></a>. A NOOP for CONFIG_HIGHMEM=n and for mappings of
pages in the low memory area.</p>
</div>
<dl class="c function">
<dt class="sig sig-object c" id="c.kmap_to_page">
<span class="k"><span class="pre">struct</span></span><span class="w"> </span><span class="n"><span class="pre">page</span></span><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="sig-name descname"><span class="n"><span class="pre">kmap_to_page</span></span></span><span class="sig-paren">(</span><span class="kt"><span class="pre">void</span></span><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">addr</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.kmap_to_page" title="Link to this definition">¶</a><br /></dt>
<dd><p>Get the page for a kmap’ed address</p>
</dd></dl>
<div class="kernelindent docutils container">
<p><strong>Parameters</strong></p>
<dl class="simple">
<dt><code class="docutils literal notranslate"><span class="pre">void</span> <span class="pre">*addr</span></code></dt><dd><p>The address to look up</p>
</dd>
</dl>
<p><strong>Return</strong></p>
<p>The page which is mapped to <strong>addr</strong>.</p>
</div>
<dl class="c function">
<dt class="sig sig-object c" id="c.kmap_flush_unused">
<span class="kt"><span class="pre">void</span></span><span class="w"> </span><span class="sig-name descname"><span class="n"><span class="pre">kmap_flush_unused</span></span></span><span class="sig-paren">(</span><span class="kt"><span class="pre">void</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.kmap_flush_unused" title="Link to this definition">¶</a><br /></dt>
<dd><p>Flush all unused kmap mappings in order to remove stray mappings</p>
</dd></dl>
<div class="kernelindent docutils container">
<p><strong>Parameters</strong></p>
<dl class="simple">
<dt><code class="docutils literal notranslate"><span class="pre">void</span></code></dt><dd><p>no arguments</p>
</dd>
</dl>
</div>
<dl class="c function">
<dt class="sig sig-object c" id="c.kmap_local_page">
<span class="kt"><span class="pre">void</span></span><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="sig-name descname"><span class="n"><span class="pre">kmap_local_page</span></span></span><span class="sig-paren">(</span><span class="k"><span class="pre">const</span></span><span class="w"> </span><span class="k"><span class="pre">struct</span></span><span class="w"> </span><a class="reference internal" href="#c.kmap_local_page" title="page"><span class="n"><span class="pre">page</span></span></a><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">page</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.kmap_local_page" title="Link to this definition">¶</a><br /></dt>
<dd><p>Map a page for temporary usage</p>
</dd></dl>
<div class="kernelindent docutils container">
<p><strong>Parameters</strong></p>
<dl class="simple">
<dt><code class="docutils literal notranslate"><span class="pre">const</span> <span class="pre">struct</span> <span class="pre">page</span> <span class="pre">*page</span></code></dt><dd><p>Pointer to the page to be mapped</p>
</dd>
</dl>
<p><strong>Return</strong></p>
<p>The virtual address of the mapping</p>
<p><strong>Description</strong></p>
<p>Can be invoked from any context, including interrupts.</p>
<p>Requires careful handling when nesting multiple mappings because the map
management is stack based. The unmap has to be in the reverse order of
the map operation:</p>
<p>addr1 = kmap_local_page(page1);
addr2 = kmap_local_page(page2);
...
kunmap_local(addr2);
kunmap_local(addr1);</p>
<p>Unmapping addr1 before addr2 is invalid and causes malfunction.</p>
<p>Contrary to <a class="reference internal" href="#c.kmap" title="kmap"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap()</span></code></a> mappings the mapping is only valid in the context of
the caller and cannot be handed to other contexts.</p>
<p>On CONFIG_HIGHMEM=n kernels and for low memory pages this returns the
virtual address of the direct mapping. Only real highmem pages are
temporarily mapped.</p>
<p>While <a class="reference internal" href="#c.kmap_local_page" title="kmap_local_page"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap_local_page()</span></code></a> is significantly faster than <a class="reference internal" href="#c.kmap" title="kmap"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap()</span></code></a> for the highmem
case it comes with restrictions about the pointer validity.</p>
<p>On HIGHMEM enabled systems mapping a highmem page has the side effect of
disabling migration in order to keep the virtual address stable across
preemption. No caller of <a class="reference internal" href="#c.kmap_local_page" title="kmap_local_page"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap_local_page()</span></code></a> can rely on this side effect.</p>
</div>
<dl class="c function">
<dt class="sig sig-object c" id="c.kmap_local_folio">
<span class="kt"><span class="pre">void</span></span><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="sig-name descname"><span class="n"><span class="pre">kmap_local_folio</span></span></span><span class="sig-paren">(</span><span class="k"><span class="pre">const</span></span><span class="w"> </span><span class="k"><span class="pre">struct</span></span><span class="w"> </span><a class="reference internal" href="#c.kmap_local_folio" title="folio"><span class="n"><span class="pre">folio</span></span></a><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">folio</span></span>, <span class="n"><span class="pre">size_t</span></span><span class="w"> </span><span class="n"><span class="pre">offset</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.kmap_local_folio" title="Link to this definition">¶</a><br /></dt>
<dd><p>Map a page in this folio for temporary usage</p>
</dd></dl>
<div class="kernelindent docutils container">
<p><strong>Parameters</strong></p>
<dl class="simple">
<dt><code class="docutils literal notranslate"><span class="pre">const</span> <span class="pre">struct</span> <span class="pre">folio</span> <span class="pre">*folio</span></code></dt><dd><p>The folio containing the page.</p>
</dd>
<dt><code class="docutils literal notranslate"><span class="pre">size_t</span> <span class="pre">offset</span></code></dt><dd><p>The byte offset within the folio which identifies the page.</p>
</dd>
</dl>
<p><strong>Description</strong></p>
<p>Requires careful handling when nesting multiple mappings because the map
management is stack based. The unmap has to be in the reverse order of
the map operation:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>addr1 = kmap_local_folio(folio1, offset1);
addr2 = kmap_local_folio(folio2, offset2);
...
kunmap_local(addr2);
kunmap_local(addr1);
</pre></div>
</div>
<p>Unmapping addr1 before addr2 is invalid and causes malfunction.</p>
<p>Contrary to <a class="reference internal" href="#c.kmap" title="kmap"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap()</span></code></a> mappings the mapping is only valid in the context of
the caller and cannot be handed to other contexts.</p>
<p>On CONFIG_HIGHMEM=n kernels and for low memory pages this returns the
virtual address of the direct mapping. Only real highmem pages are
temporarily mapped.</p>
<p>While it is significantly faster than <a class="reference internal" href="#c.kmap" title="kmap"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap()</span></code></a> for the highmem case it
comes with restrictions about the pointer validity.</p>
<p>On HIGHMEM enabled systems mapping a highmem page has the side effect of
disabling migration in order to keep the virtual address stable across
preemption. No caller of <a class="reference internal" href="#c.kmap_local_folio" title="kmap_local_folio"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap_local_folio()</span></code></a> can rely on this side effect.</p>
<p><strong>Context</strong></p>
<p>Can be invoked from any context.</p>
<p><strong>Return</strong></p>
<p>The virtual address of <strong>offset</strong>.</p>
</div>
<dl class="c function">
<dt class="sig sig-object c" id="c.kmap_atomic">
<span class="kt"><span class="pre">void</span></span><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="sig-name descname"><span class="n"><span class="pre">kmap_atomic</span></span></span><span class="sig-paren">(</span><span class="k"><span class="pre">const</span></span><span class="w"> </span><span class="k"><span class="pre">struct</span></span><span class="w"> </span><a class="reference internal" href="#c.kmap_atomic" title="page"><span class="n"><span class="pre">page</span></span></a><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">page</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.kmap_atomic" title="Link to this definition">¶</a><br /></dt>
<dd><p>Atomically map a page for temporary usage - Deprecated!</p>
</dd></dl>
<div class="kernelindent docutils container">
<p><strong>Parameters</strong></p>
<dl class="simple">
<dt><code class="docutils literal notranslate"><span class="pre">const</span> <span class="pre">struct</span> <span class="pre">page</span> <span class="pre">*page</span></code></dt><dd><p>Pointer to the page to be mapped</p>
</dd>
</dl>
<p><strong>Return</strong></p>
<p>The virtual address of the mapping</p>
<p><strong>Description</strong></p>
<p>In fact a wrapper around <a class="reference internal" href="#c.kmap_local_page" title="kmap_local_page"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap_local_page()</span></code></a> which also disables pagefaults
and, depending on PREEMPT_RT configuration, also CPU migration and
preemption. Therefore users should not count on the latter two side effects.</p>
<p>Mappings should always be released by <a class="reference internal" href="#c.kunmap_atomic" title="kunmap_atomic"><code class="xref c c-func docutils literal notranslate"><span class="pre">kunmap_atomic()</span></code></a>.</p>
<p>Do not use in new code. Use <a class="reference internal" href="#c.kmap_local_page" title="kmap_local_page"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap_local_page()</span></code></a> instead.</p>
<p>It is used in atomic context when code wants to access the contents of a
page that might be allocated from high memory (see __GFP_HIGHMEM), for
example a page in the pagecache. The API has two functions, and they
can be used in a manner similar to the following:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>// Find the page of interest.
struct page *page = find_get_page(mapping, offset);
// Gain access to the contents of that page.
void *vaddr = kmap_atomic(page);
// Do something to the contents of that page.
memset(vaddr, 0, PAGE_SIZE);
// Unmap that page.
kunmap_atomic(vaddr);
</pre></div>
</div>
<p>Note that the <a class="reference internal" href="#c.kunmap_atomic" title="kunmap_atomic"><code class="xref c c-func docutils literal notranslate"><span class="pre">kunmap_atomic()</span></code></a> call takes the result of the <a class="reference internal" href="#c.kmap_atomic" title="kmap_atomic"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap_atomic()</span></code></a>
call, not the argument.</p>
<p>If you need to map two pages because you want to copy from one page to
another you need to keep the kmap_atomic calls strictly nested, like:</p>
<p>vaddr1 = kmap_atomic(page1);
vaddr2 = kmap_atomic(page2);</p>
<p>memcpy(vaddr1, vaddr2, PAGE_SIZE);</p>
<p>kunmap_atomic(vaddr2);
kunmap_atomic(vaddr1);</p>
</div>
<dl class="c function">
<dt class="sig sig-object c" id="c.vma_alloc_zeroed_movable_folio">
<span class="k"><span class="pre">struct</span></span><span class="w"> </span><a class="reference internal" href="../core-api/mm-api.html#c.folio" title="folio"><span class="n"><span class="pre">folio</span></span></a><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="sig-name descname"><span class="n"><span class="pre">vma_alloc_zeroed_movable_folio</span></span></span><span class="sig-paren">(</span><span class="k"><span class="pre">struct</span></span><span class="w"> </span><span class="n"><span class="pre">vm_area_struct</span></span><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">vma</span></span>, <span class="kt"><span class="pre">unsigned</span></span><span class="w"> </span><span class="kt"><span class="pre">long</span></span><span class="w"> </span><span class="n"><span class="pre">vaddr</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.vma_alloc_zeroed_movable_folio" title="Link to this definition">¶</a><br /></dt>
<dd><p>Allocate a zeroed page for a VMA.</p>
</dd></dl>
<div class="kernelindent docutils container">
<p><strong>Parameters</strong></p>
<dl class="simple">
<dt><code class="docutils literal notranslate"><span class="pre">struct</span> <span class="pre">vm_area_struct</span> <span class="pre">*vma</span></code></dt><dd><p>The VMA the page is to be allocated for.</p>
</dd>
<dt><code class="docutils literal notranslate"><span class="pre">unsigned</span> <span class="pre">long</span> <span class="pre">vaddr</span></code></dt><dd><p>The virtual address the page will be inserted into.</p>
</dd>
</dl>
<p><strong>Description</strong></p>
<p>This function will allocate a page suitable for inserting into this
VMA at this virtual address. It may be allocated from highmem or
the movable zone. An architecture may provide its own implementation.</p>
<p><strong>Return</strong></p>
<p>A folio containing one allocated and zeroed page or NULL if
we are out of memory.</p>
</div>
<dl class="c function">
<dt class="sig sig-object c" id="c.memcpy_from_folio">
<span class="kt"><span class="pre">void</span></span><span class="w"> </span><span class="sig-name descname"><span class="n"><span class="pre">memcpy_from_folio</span></span></span><span class="sig-paren">(</span><span class="kt"><span class="pre">char</span></span><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">to</span></span>, <span class="k"><span class="pre">struct</span></span><span class="w"> </span><a class="reference internal" href="#c.memcpy_from_folio" title="folio"><span class="n"><span class="pre">folio</span></span></a><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">folio</span></span>, <span class="n"><span class="pre">size_t</span></span><span class="w"> </span><span class="n"><span class="pre">offset</span></span>, <span class="n"><span class="pre">size_t</span></span><span class="w"> </span><span class="n"><span class="pre">len</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.memcpy_from_folio" title="Link to this definition">¶</a><br /></dt>
<dd><p>Copy a range of bytes from a folio.</p>
</dd></dl>
<div class="kernelindent docutils container">
<p><strong>Parameters</strong></p>
<dl class="simple">
<dt><code class="docutils literal notranslate"><span class="pre">char</span> <span class="pre">*to</span></code></dt><dd><p>The memory to copy to.</p>
</dd>
<dt><code class="docutils literal notranslate"><span class="pre">struct</span> <span class="pre">folio</span> <span class="pre">*folio</span></code></dt><dd><p>The folio to read from.</p>
</dd>
<dt><code class="docutils literal notranslate"><span class="pre">size_t</span> <span class="pre">offset</span></code></dt><dd><p>The first byte in the folio to read.</p>
</dd>
<dt><code class="docutils literal notranslate"><span class="pre">size_t</span> <span class="pre">len</span></code></dt><dd><p>The number of bytes to copy.</p>
</dd>
</dl>
</div>
<dl class="c function">
<dt class="sig sig-object c" id="c.memcpy_to_folio">
<span class="kt"><span class="pre">void</span></span><span class="w"> </span><span class="sig-name descname"><span class="n"><span class="pre">memcpy_to_folio</span></span></span><span class="sig-paren">(</span><span class="k"><span class="pre">struct</span></span><span class="w"> </span><a class="reference internal" href="#c.memcpy_to_folio" title="folio"><span class="n"><span class="pre">folio</span></span></a><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">folio</span></span>, <span class="n"><span class="pre">size_t</span></span><span class="w"> </span><span class="n"><span class="pre">offset</span></span>, <span class="k"><span class="pre">const</span></span><span class="w"> </span><span class="kt"><span class="pre">char</span></span><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">from</span></span>, <span class="n"><span class="pre">size_t</span></span><span class="w"> </span><span class="n"><span class="pre">len</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.memcpy_to_folio" title="Link to this definition">¶</a><br /></dt>
<dd><p>Copy a range of bytes to a folio.</p>
</dd></dl>
<div class="kernelindent docutils container">
<p><strong>Parameters</strong></p>
<dl class="simple">
<dt><code class="docutils literal notranslate"><span class="pre">struct</span> <span class="pre">folio</span> <span class="pre">*folio</span></code></dt><dd><p>The folio to write to.</p>
</dd>
<dt><code class="docutils literal notranslate"><span class="pre">size_t</span> <span class="pre">offset</span></code></dt><dd><p>The first byte in the folio to store to.</p>
</dd>
<dt><code class="docutils literal notranslate"><span class="pre">const</span> <span class="pre">char</span> <span class="pre">*from</span></code></dt><dd><p>The memory to copy from.</p>
</dd>
<dt><code class="docutils literal notranslate"><span class="pre">size_t</span> <span class="pre">len</span></code></dt><dd><p>The number of bytes to copy.</p>
</dd>
</dl>
</div>
<dl class="c function">
<dt class="sig sig-object c" id="c.folio_zero_tail">
<span class="kt"><span class="pre">void</span></span><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="sig-name descname"><span class="n"><span class="pre">folio_zero_tail</span></span></span><span class="sig-paren">(</span><span class="k"><span class="pre">struct</span></span><span class="w"> </span><a class="reference internal" href="#c.folio_zero_tail" title="folio"><span class="n"><span class="pre">folio</span></span></a><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">folio</span></span>, <span class="n"><span class="pre">size_t</span></span><span class="w"> </span><span class="n"><span class="pre">offset</span></span>, <span class="kt"><span class="pre">void</span></span><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">kaddr</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.folio_zero_tail" title="Link to this definition">¶</a><br /></dt>
<dd><p>Zero the tail of a folio.</p>
</dd></dl>
<div class="kernelindent docutils container">
<p><strong>Parameters</strong></p>
<dl class="simple">
<dt><code class="docutils literal notranslate"><span class="pre">struct</span> <span class="pre">folio</span> <span class="pre">*folio</span></code></dt><dd><p>The folio to zero.</p>
</dd>
<dt><code class="docutils literal notranslate"><span class="pre">size_t</span> <span class="pre">offset</span></code></dt><dd><p>The byte offset in the folio to start zeroing at.</p>
</dd>
<dt><code class="docutils literal notranslate"><span class="pre">void</span> <span class="pre">*kaddr</span></code></dt><dd><p>The address the folio is currently mapped to.</p>
</dd>
</dl>
<p><strong>Description</strong></p>
<p>If you have already used <a class="reference internal" href="#c.kmap_local_folio" title="kmap_local_folio"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap_local_folio()</span></code></a> to map a folio, written
some data to it and now need to zero the end of the folio (and flush
the dcache), you can use this function. If you do not have the
folio kmapped (eg the folio has been partially populated by DMA),
use <a class="reference internal" href="#c.folio_zero_range" title="folio_zero_range"><code class="xref c c-func docutils literal notranslate"><span class="pre">folio_zero_range()</span></code></a> or <a class="reference internal" href="#c.folio_zero_segment" title="folio_zero_segment"><code class="xref c c-func docutils literal notranslate"><span class="pre">folio_zero_segment()</span></code></a> instead.</p>
<p><strong>Return</strong></p>
<p>An address which can be passed to <a class="reference internal" href="#c.kunmap_local" title="kunmap_local"><code class="xref c c-func docutils literal notranslate"><span class="pre">kunmap_local()</span></code></a>.</p>
</div>
<dl class="c function">
<dt class="sig sig-object c" id="c.folio_fill_tail">
<span class="kt"><span class="pre">void</span></span><span class="w"> </span><span class="sig-name descname"><span class="n"><span class="pre">folio_fill_tail</span></span></span><span class="sig-paren">(</span><span class="k"><span class="pre">struct</span></span><span class="w"> </span><a class="reference internal" href="#c.folio_fill_tail" title="folio"><span class="n"><span class="pre">folio</span></span></a><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">folio</span></span>, <span class="n"><span class="pre">size_t</span></span><span class="w"> </span><span class="n"><span class="pre">offset</span></span>, <span class="k"><span class="pre">const</span></span><span class="w"> </span><span class="kt"><span class="pre">char</span></span><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">from</span></span>, <span class="n"><span class="pre">size_t</span></span><span class="w"> </span><span class="n"><span class="pre">len</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.folio_fill_tail" title="Link to this definition">¶</a><br /></dt>
<dd><p>Copy some data to a folio and pad with zeroes.</p>
</dd></dl>
<div class="kernelindent docutils container">
<p><strong>Parameters</strong></p>
<dl class="simple">
<dt><code class="docutils literal notranslate"><span class="pre">struct</span> <span class="pre">folio</span> <span class="pre">*folio</span></code></dt><dd><p>The destination folio.</p>
</dd>
<dt><code class="docutils literal notranslate"><span class="pre">size_t</span> <span class="pre">offset</span></code></dt><dd><p>The offset into <strong>folio</strong> at which to start copying.</p>
</dd>
<dt><code class="docutils literal notranslate"><span class="pre">const</span> <span class="pre">char</span> <span class="pre">*from</span></code></dt><dd><p>The data to copy.</p>
</dd>
<dt><code class="docutils literal notranslate"><span class="pre">size_t</span> <span class="pre">len</span></code></dt><dd><p>How many bytes of data to copy.</p>
</dd>
</dl>
<p><strong>Description</strong></p>
<p>This function is most useful for filesystems which support inline data.
When they want to copy data from the inode into the page cache, this
function does everything for them. It supports large folios even on
HIGHMEM configurations.</p>
</div>
<dl class="c function">
<dt class="sig sig-object c" id="c.memcpy_from_file_folio">
<span class="n"><span class="pre">size_t</span></span><span class="w"> </span><span class="sig-name descname"><span class="n"><span class="pre">memcpy_from_file_folio</span></span></span><span class="sig-paren">(</span><span class="kt"><span class="pre">char</span></span><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">to</span></span>, <span class="k"><span class="pre">struct</span></span><span class="w"> </span><a class="reference internal" href="#c.memcpy_from_file_folio" title="folio"><span class="n"><span class="pre">folio</span></span></a><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">folio</span></span>, <span class="n"><span class="pre">loff_t</span></span><span class="w"> </span><span class="n"><span class="pre">pos</span></span>, <span class="n"><span class="pre">size_t</span></span><span class="w"> </span><span class="n"><span class="pre">len</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.memcpy_from_file_folio" title="Link to this definition">¶</a><br /></dt>
<dd><p>Copy some bytes from a file folio.</p>
</dd></dl>
<div class="kernelindent docutils container">
<p><strong>Parameters</strong></p>
<dl class="simple">
<dt><code class="docutils literal notranslate"><span class="pre">char</span> <span class="pre">*to</span></code></dt><dd><p>The destination buffer.</p>
</dd>
<dt><code class="docutils literal notranslate"><span class="pre">struct</span> <span class="pre">folio</span> <span class="pre">*folio</span></code></dt><dd><p>The folio to copy from.</p>
</dd>
<dt><code class="docutils literal notranslate"><span class="pre">loff_t</span> <span class="pre">pos</span></code></dt><dd><p>The position in the file.</p>
</dd>
<dt><code class="docutils literal notranslate"><span class="pre">size_t</span> <span class="pre">len</span></code></dt><dd><p>The maximum number of bytes to copy.</p>
</dd>
</dl>
<p><strong>Description</strong></p>
<p>Copy up to <strong>len</strong> bytes from this folio. This may be limited by PAGE_SIZE
if the folio comes from HIGHMEM, and by the size of the folio.</p>
<p><strong>Return</strong></p>
<p>The number of bytes copied from the folio.</p>
</div>
<dl class="c function">
<dt class="sig sig-object c" id="c.folio_zero_segments">
<span class="kt"><span class="pre">void</span></span><span class="w"> </span><span class="sig-name descname"><span class="n"><span class="pre">folio_zero_segments</span></span></span><span class="sig-paren">(</span><span class="k"><span class="pre">struct</span></span><span class="w"> </span><a class="reference internal" href="#c.folio_zero_segments" title="folio"><span class="n"><span class="pre">folio</span></span></a><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">folio</span></span>, <span class="n"><span class="pre">size_t</span></span><span class="w"> </span><span class="n"><span class="pre">start1</span></span>, <span class="n"><span class="pre">size_t</span></span><span class="w"> </span><span class="n"><span class="pre">xend1</span></span>, <span class="n"><span class="pre">size_t</span></span><span class="w"> </span><span class="n"><span class="pre">start2</span></span>, <span class="n"><span class="pre">size_t</span></span><span class="w"> </span><span class="n"><span class="pre">xend2</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.folio_zero_segments" title="Link to this definition">¶</a><br /></dt>
<dd><p>Zero two byte ranges in a folio.</p>
</dd></dl>
<div class="kernelindent docutils container">
<p><strong>Parameters</strong></p>
<dl class="simple">
<dt><code class="docutils literal notranslate"><span class="pre">struct</span> <span class="pre">folio</span> <span class="pre">*folio</span></code></dt><dd><p>The folio to write to.</p>
</dd>
<dt><code class="docutils literal notranslate"><span class="pre">size_t</span> <span class="pre">start1</span></code></dt><dd><p>The first byte to zero.</p>
</dd>
<dt><code class="docutils literal notranslate"><span class="pre">size_t</span> <span class="pre">xend1</span></code></dt><dd><p>One more than the last byte in the first range.</p>
</dd>
<dt><code class="docutils literal notranslate"><span class="pre">size_t</span> <span class="pre">start2</span></code></dt><dd><p>The first byte to zero in the second range.</p>
</dd>
<dt><code class="docutils literal notranslate"><span class="pre">size_t</span> <span class="pre">xend2</span></code></dt><dd><p>One more than the last byte in the second range.</p>
</dd>
</dl>
</div>
<dl class="c function">
<dt class="sig sig-object c" id="c.folio_zero_segment">
<span class="kt"><span class="pre">void</span></span><span class="w"> </span><span class="sig-name descname"><span class="n"><span class="pre">folio_zero_segment</span></span></span><span class="sig-paren">(</span><span class="k"><span class="pre">struct</span></span><span class="w"> </span><a class="reference internal" href="#c.folio_zero_segment" title="folio"><span class="n"><span class="pre">folio</span></span></a><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">folio</span></span>, <span class="n"><span class="pre">size_t</span></span><span class="w"> </span><span class="n"><span class="pre">start</span></span>, <span class="n"><span class="pre">size_t</span></span><span class="w"> </span><span class="n"><span class="pre">xend</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.folio_zero_segment" title="Link to this definition">¶</a><br /></dt>
<dd><p>Zero a byte range in a folio.</p>
</dd></dl>
<div class="kernelindent docutils container">
<p><strong>Parameters</strong></p>
<dl class="simple">
<dt><code class="docutils literal notranslate"><span class="pre">struct</span> <span class="pre">folio</span> <span class="pre">*folio</span></code></dt><dd><p>The folio to write to.</p>
</dd>
<dt><code class="docutils literal notranslate"><span class="pre">size_t</span> <span class="pre">start</span></code></dt><dd><p>The first byte to zero.</p>
</dd>
<dt><code class="docutils literal notranslate"><span class="pre">size_t</span> <span class="pre">xend</span></code></dt><dd><p>One more than the last byte to zero.</p>
</dd>
</dl>
</div>
<dl class="c function">
<dt class="sig sig-object c" id="c.folio_zero_range">
<span class="kt"><span class="pre">void</span></span><span class="w"> </span><span class="sig-name descname"><span class="n"><span class="pre">folio_zero_range</span></span></span><span class="sig-paren">(</span><span class="k"><span class="pre">struct</span></span><span class="w"> </span><a class="reference internal" href="#c.folio_zero_range" title="folio"><span class="n"><span class="pre">folio</span></span></a><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">folio</span></span>, <span class="n"><span class="pre">size_t</span></span><span class="w"> </span><span class="n"><span class="pre">start</span></span>, <span class="n"><span class="pre">size_t</span></span><span class="w"> </span><span class="n"><span class="pre">length</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.folio_zero_range" title="Link to this definition">¶</a><br /></dt>
<dd><p>Zero a byte range in a folio.</p>
</dd></dl>
<div class="kernelindent docutils container">
<p><strong>Parameters</strong></p>
<dl class="simple">
<dt><code class="docutils literal notranslate"><span class="pre">struct</span> <span class="pre">folio</span> <span class="pre">*folio</span></code></dt><dd><p>The folio to write to.</p>
</dd>
<dt><code class="docutils literal notranslate"><span class="pre">size_t</span> <span class="pre">start</span></code></dt><dd><p>The first byte to zero.</p>
</dd>
<dt><code class="docutils literal notranslate"><span class="pre">size_t</span> <span class="pre">length</span></code></dt><dd><p>The number of bytes to zero.</p>
</dd>
</dl>
</div>
<dl class="c function">
<dt class="sig sig-object c" id="c.folio_release_kmap">
<span class="kt"><span class="pre">void</span></span><span class="w"> </span><span class="sig-name descname"><span class="n"><span class="pre">folio_release_kmap</span></span></span><span class="sig-paren">(</span><span class="k"><span class="pre">struct</span></span><span class="w"> </span><a class="reference internal" href="#c.folio_release_kmap" title="folio"><span class="n"><span class="pre">folio</span></span></a><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">folio</span></span>, <span class="kt"><span class="pre">void</span></span><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">addr</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.folio_release_kmap" title="Link to this definition">¶</a><br /></dt>
<dd><p>Unmap a folio and drop a refcount.</p>
</dd></dl>
<div class="kernelindent docutils container">
<p><strong>Parameters</strong></p>
<dl class="simple">
<dt><code class="docutils literal notranslate"><span class="pre">struct</span> <span class="pre">folio</span> <span class="pre">*folio</span></code></dt><dd><p>The folio to release.</p>
</dd>
<dt><code class="docutils literal notranslate"><span class="pre">void</span> <span class="pre">*addr</span></code></dt><dd><p>The address previously returned by a call to <a class="reference internal" href="#c.kmap_local_folio" title="kmap_local_folio"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap_local_folio()</span></code></a>.</p>
</dd>
</dl>
<p><strong>Description</strong></p>
<p>It is common, eg in directory handling to kmap a folio. This function
unmaps the folio and drops the refcount that was being held to keep the
folio alive while we accessed it.</p>
</div>
<dl class="c function">
<dt class="sig sig-object c" id="c.kmap_high">
<span class="kt"><span class="pre">void</span></span><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="sig-name descname"><span class="n"><span class="pre">kmap_high</span></span></span><span class="sig-paren">(</span><span class="k"><span class="pre">struct</span></span><span class="w"> </span><a class="reference internal" href="#c.kmap_high" title="page"><span class="n"><span class="pre">page</span></span></a><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">page</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.kmap_high" title="Link to this definition">¶</a><br /></dt>
<dd><p>map a highmem page into memory</p>
</dd></dl>
<div class="kernelindent docutils container">
<p><strong>Parameters</strong></p>
<dl class="simple">
<dt><code class="docutils literal notranslate"><span class="pre">struct</span> <span class="pre">page</span> <span class="pre">*page</span></code></dt><dd><p><code class="xref c c-type docutils literal notranslate"><span class="pre">struct</span> <span class="pre">page</span></code> to map</p>
</dd>
</dl>
<p><strong>Description</strong></p>
<p>Returns the page’s virtual memory address.</p>
<p>We cannot call this from interrupts, as it may block.</p>
</div>
<dl class="c function">
<dt class="sig sig-object c" id="c.kmap_high_get">
<span class="kt"><span class="pre">void</span></span><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="sig-name descname"><span class="n"><span class="pre">kmap_high_get</span></span></span><span class="sig-paren">(</span><span class="k"><span class="pre">const</span></span><span class="w"> </span><span class="k"><span class="pre">struct</span></span><span class="w"> </span><a class="reference internal" href="#c.kmap_high_get" title="page"><span class="n"><span class="pre">page</span></span></a><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">page</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.kmap_high_get" title="Link to this definition">¶</a><br /></dt>
<dd><p>pin a highmem page into memory</p>
</dd></dl>
<div class="kernelindent docutils container">
<p><strong>Parameters</strong></p>
<dl class="simple">
<dt><code class="docutils literal notranslate"><span class="pre">const</span> <span class="pre">struct</span> <span class="pre">page</span> <span class="pre">*page</span></code></dt><dd><p><code class="xref c c-type docutils literal notranslate"><span class="pre">struct</span> <span class="pre">page</span></code> to pin</p>
</dd>
</dl>
<p><strong>Description</strong></p>
<p>Returns the page’s current virtual memory address, or NULL if no mapping
exists. If and only if a non null address is returned then a
matching call to <a class="reference internal" href="#c.kunmap_high" title="kunmap_high"><code class="xref c c-func docutils literal notranslate"><span class="pre">kunmap_high()</span></code></a> is necessary.</p>
<p>This can be called from any context.</p>
</div>
<dl class="c function">
<dt class="sig sig-object c" id="c.kunmap_high">
<span class="kt"><span class="pre">void</span></span><span class="w"> </span><span class="sig-name descname"><span class="n"><span class="pre">kunmap_high</span></span></span><span class="sig-paren">(</span><span class="k"><span class="pre">const</span></span><span class="w"> </span><span class="k"><span class="pre">struct</span></span><span class="w"> </span><a class="reference internal" href="#c.kunmap_high" title="page"><span class="n"><span class="pre">page</span></span></a><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">page</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.kunmap_high" title="Link to this definition">¶</a><br /></dt>
<dd><p>unmap a highmem page into memory</p>
</dd></dl>
<div class="kernelindent docutils container">
<p><strong>Parameters</strong></p>
<dl class="simple">
<dt><code class="docutils literal notranslate"><span class="pre">const</span> <span class="pre">struct</span> <span class="pre">page</span> <span class="pre">*page</span></code></dt><dd><p><code class="xref c c-type docutils literal notranslate"><span class="pre">struct</span> <span class="pre">page</span></code> to unmap</p>
</dd>
</dl>
<p><strong>Description</strong></p>
<p>If ARCH_NEEDS_KMAP_HIGH_GET is not defined then this may be called
only from user context.</p>
</div>
<dl class="c function">
<dt class="sig sig-object c" id="c.page_address">
<span class="kt"><span class="pre">void</span></span><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="sig-name descname"><span class="n"><span class="pre">page_address</span></span></span><span class="sig-paren">(</span><span class="k"><span class="pre">const</span></span><span class="w"> </span><span class="k"><span class="pre">struct</span></span><span class="w"> </span><a class="reference internal" href="#c.page_address" title="page"><span class="n"><span class="pre">page</span></span></a><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">page</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.page_address" title="Link to this definition">¶</a><br /></dt>
<dd><p>get the mapped virtual address of a page</p>
</dd></dl>
<div class="kernelindent docutils container">
<p><strong>Parameters</strong></p>
<dl class="simple">
<dt><code class="docutils literal notranslate"><span class="pre">const</span> <span class="pre">struct</span> <span class="pre">page</span> <span class="pre">*page</span></code></dt><dd><p><code class="xref c c-type docutils literal notranslate"><span class="pre">struct</span> <span class="pre">page</span></code> to get the virtual address of</p>
</dd>
</dl>
<p><strong>Description</strong></p>
<p>Returns the page’s virtual address.</p>
</div>
<dl class="c function">
<dt class="sig sig-object c" id="c.set_page_address">
<span class="kt"><span class="pre">void</span></span><span class="w"> </span><span class="sig-name descname"><span class="n"><span class="pre">set_page_address</span></span></span><span class="sig-paren">(</span><span class="k"><span class="pre">struct</span></span><span class="w"> </span><a class="reference internal" href="#c.set_page_address" title="page"><span class="n"><span class="pre">page</span></span></a><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">page</span></span>, <span class="kt"><span class="pre">void</span></span><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">virtual</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.set_page_address" title="Link to this definition">¶</a><br /></dt>
<dd><p>set a page’s virtual address</p>
</dd></dl>
<div class="kernelindent docutils container">
<p><strong>Parameters</strong></p>
<dl class="simple">
<dt><code class="docutils literal notranslate"><span class="pre">struct</span> <span class="pre">page</span> <span class="pre">*page</span></code></dt><dd><p><code class="xref c c-type docutils literal notranslate"><span class="pre">struct</span> <span class="pre">page</span></code> to set</p>
</dd>
<dt><code class="docutils literal notranslate"><span class="pre">void</span> <span class="pre">*virtual</span></code></dt><dd><p>virtual address to use</p>
</dd>
</dl>
</div>
<dl class="c macro">
<dt class="sig sig-object c" id="c.kunmap_atomic">
<span class="sig-name descname"><span class="n"><span class="pre">kunmap_atomic</span></span></span><a class="headerlink" href="#c.kunmap_atomic" title="Link to this definition">¶</a><br /></dt>
<dd></dd></dl>
<p><code class="docutils literal notranslate"><span class="pre">kunmap_atomic</span> <span class="pre">(__addr)</span></code></p>
<blockquote>
<div><p>Unmap the virtual address mapped by <a class="reference internal" href="#c.kmap_atomic" title="kmap_atomic"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap_atomic()</span></code></a> - deprecated!</p>
</div></blockquote>
<div class="kernelindent docutils container">
<p><strong>Parameters</strong></p>
<dl class="simple">
<dt><code class="docutils literal notranslate"><span class="pre">__addr</span></code></dt><dd><p>Virtual address to be unmapped</p>
</dd>
</dl>
<p><strong>Description</strong></p>
<p>Unmaps an address previously mapped by <a class="reference internal" href="#c.kmap_atomic" title="kmap_atomic"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap_atomic()</span></code></a> and re-enables
pagefaults. Depending on PREEMP_RT configuration, re-enables also
migration and preemption. Users should not count on these side effects.</p>
<p>Mappings should be unmapped in the reverse order that they were mapped.
See <a class="reference internal" href="#c.kmap_local_page" title="kmap_local_page"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap_local_page()</span></code></a> for details on nesting.</p>
<p><strong>__addr</strong> can be any address within the mapped page, so there is no need
to subtract any offset that has been added. In contrast to <a class="reference internal" href="#c.kunmap" title="kunmap"><code class="xref c c-func docutils literal notranslate"><span class="pre">kunmap()</span></code></a>,
this function takes the address returned from <a class="reference internal" href="#c.kmap_atomic" title="kmap_atomic"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap_atomic()</span></code></a>, not the
page passed to it. The compiler will warn you if you pass the page.</p>
</div>
<dl class="c macro">
<dt class="sig sig-object c" id="c.kunmap_local">
<span class="sig-name descname"><span class="n"><span class="pre">kunmap_local</span></span></span><a class="headerlink" href="#c.kunmap_local" title="Link to this definition">¶</a><br /></dt>
<dd></dd></dl>
<p><code class="docutils literal notranslate"><span class="pre">kunmap_local</span> <span class="pre">(__addr)</span></code></p>
<blockquote>
<div><p>Unmap a page mapped via <a class="reference internal" href="#c.kmap_local_page" title="kmap_local_page"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap_local_page()</span></code></a>.</p>
</div></blockquote>
<div class="kernelindent docutils container">
<p><strong>Parameters</strong></p>
<dl class="simple">
<dt><code class="docutils literal notranslate"><span class="pre">__addr</span></code></dt><dd><p>An address within the page mapped</p>
</dd>
</dl>
<p><strong>Description</strong></p>
<p><strong>__addr</strong> can be any address within the mapped page. Commonly it is the
address return from <a class="reference internal" href="#c.kmap_local_page" title="kmap_local_page"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap_local_page()</span></code></a>, but it can also include offsets.</p>
<p>Unmapping should be done in the reverse order of the mapping. See
<a class="reference internal" href="#c.kmap_local_page" title="kmap_local_page"><code class="xref c c-func docutils literal notranslate"><span class="pre">kmap_local_page()</span></code></a> for details.</p>
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