Re: [libvirt] [PATCH v3 5/5] qemu: Introduce memoryBacking/discard

https://bugzilla.redhat.com/show_bug.cgi?id=1480668 QEMU has this new feature memory-backend-file.discard-data=yes which is a nifty optimization. Basically, when qemu is quitting or on memory hotplug it calls munmap() and close() on the file that is backing the memory. However, this does not mean kernel won't stop touching that part of memory. It still might. With this feature enabled we tell kernel: "we don't need this memory nor data stored in it". This makes kernel drop the memory immediately without trying to sync memory with the mapped file. Unfortunately, this cannot be turned on by default because we can't be sure when users really don't care about what happens to data after qemu dies. So it has to be opt-in. As usual, there are three places where one can configure memory attributes. This patch adds the feature to all of them. Signed-off-by: Michal Privoznik <mprivozn(a)redhat.com&gt; --- docs/formatdomain.html.in | 40 +++++++++++++++++++++++++--- docs/schemas/cputypes.rng | 5 ++++ docs/schemas/domaincommon.rng | 10 +++++++ src/conf/domain_conf.c | 39 +++++++++++++++++++++++++-- src/conf/domain_conf.h | 3 +++ src/conf/numa_conf.c | 27 +++++++++++++++++++ src/conf/numa_conf.h | 3 +++ src/libvirt_private.syms | 1 + src/qemu/qemu_command.c | 27 ++++++++++++++++--- tests/qemuxml2argvdata/hugepages-pages7.args | 3 ++- tests/qemuxml2argvdata/hugepages-pages7.xml | 4 +-- tests/qemuxml2argvtest.c | 3 ++- 12 files changed, 152 insertions(+), 13 deletions(-) diff --git a/docs/formatdomain.html.in b/docs/formatdomain.html.in index 5e99884dc5..d40536d06f 100644 --- a/docs/formatdomain.html.in +++ b/docs/formatdomain.html.in @@ -1016,6 +1016,7 @@ &lt;source type="file|anonymous"/&gt; &lt;access mode="shared|private"/&gt; &lt;allocation mode="immediate|ondemand"/&gt; + &lt;discard/&gt; &lt;/memoryBacking&gt; ... &lt;/domain&gt;

@@ -1063,11 +1064,21 @@ suitable for the specific environment at the same time to mitigate the risks described above. <span class="since">Since 1.0.6</span></dd> <dt><code>source</code></dt> - <dd>In this attribute you can switch to file memorybacking or keep default anonymous.</dd> + <dd>In this attribute you can switch to file memorybacking or keep + default anonymous.</dd> <dt><code>access</code></dt> - <dd>Specify if memory is shared or private. This can be overridden per numa node by <code>memAccess</code></dd> + <dd>Specify if memory is shared or private. This can be overridden per + numa node by <code>memAccess</code></dd> <dt><code>allocation</code></dt>

<dd>Specify when allocate the memory</dd> + <dt><code>discard</code></dt> + <dd>When set an supported by hypervisor the memory

+ content is discarded just before guest shuts down (or + when DIMM module is unplugged). Please note that this is + just an optimization and is not guaranteed to work in + all cases (e.g. when hypervisor crashes). + <span class="since">Since 4.3.0</span> (QEMU/KVM only) + </dd> </dl> @@ -1606,7 +1617,7 @@ &lt;cpu&gt; ... &lt;numa&gt; - &lt;cell id='0' cpus='0-3' memory='512000' unit='KiB'/&gt; + &lt;cell id='0' cpus='0-3' memory='512000' unit='KiB' discard='yes'/&gt; &lt;cell id='1' cpus='4-7' memory='512000' unit='KiB' memAccess='shared'/&gt; &lt;/numa&gt; ... @@ -1632,6 +1643,13 @@ <code>memAccess</code> can control whether the memory is to be mapped as "shared" or "private". This is valid only for hugepages-backed memory and nvdimm modules. + + Each <code>cell</code> element can have an optional + attribute <code>discard</code> which fine tunes the discard

+ feature for given numa node as described under + <a href="#elementsMemoryBacking">Memory Backing</a>. + Accepted values are <code>yes</code> and <code>no</code>. + <span class='since'>Since 4.3.0</span> </p> <p>