core: add cgroup memory controller support on the unified hierarchy (#3315)

On the unified hierarchy, memory controller implements three control knobs -
low, high and max which enables more useable and versatile control over memory
usage.  This patch implements support for the three control knobs.

* MemoryLow, MemoryHigh and MemoryMax are added for memory.low, memory.high and
  memory.max, respectively.

* As all absolute limits on the unified hierarchy use "max" for no limit, make
  memory limit parse functions accept "max" in addition to "infinity" and
  document "max" for the new knobs.

* Implement compatibility translation between MemoryMax and MemoryLimit.

v2:

- Fixed missing else's in config_parse_memory_limit().
- Fixed missing newline when writing out drop-ins.
- Coding style updates to use "val > 0" instead of "val".
- Minor updates to documentation.

1 files changed, 71 insertions, 0 deletions

diff --git a/man/systemd.resource-control.xml b/man/systemd.resource-control.xml
index 066f2cc19b..570619a743 100644
--- a/man/systemd.resource-control.xml
+++ b/man/systemd.resource-control.xml
@@ -114,6 +114,13 @@
             prefixed ones. On unified hierarchy, IO resource control also applies to buffered writes.</para>
           </listitem>
         </varlistentry>
+        <varlistentry>
+          <term><option>Memory</option></term>
+          <listitem>
+            <para><varname>MemoryMax</varname> replaces <varname>MemoryLimit</varname>. <varname>MemoryLow</varname>
+            and <varname>MemoryHigh</varname> are effective only on unified hierarchy.</para>
+          </listitem>
+        </varlistentry>
       </variablelist>
     </para>
 
@@ -213,6 +220,67 @@
       </varlistentry>
 
       <varlistentry>
+        <term><varname>MemoryLow=<replaceable>bytes</replaceable></varname></term>
+
+        <listitem>
+          <para>Specify the best-effort memory usage protection of the executed processes in this unit. If the memory
+          usages of this unit and all its ancestors are below their low boundaries, this unit's memory won't be
+          reclaimed as long as memory can be reclaimed from unprotected units.</para>
+
+          <para>Takes a memory size in bytes. If the value is suffixed with K, M, G or T, the specified memory size is
+          parsed as Kilobytes, Megabytes, Gigabytes, or Terabytes (with the base 1024), respectively. This controls the
+          <literal>memory.low</literal> control group attribute. For details about this control group attribute, see
+          <ulink url="https://www.kernel.org/doc/Documentation/cgroup-v2.txt">cgroup-v2.txt</ulink>.</para>
+
+          <para>Implies <literal>MemoryAccounting=true</literal>.</para>
+
+          <para>This setting is supported only if the unified control group hierarchy is used.</para>
+        </listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>MemoryHigh=<replaceable>bytes</replaceable></varname></term>
+
+        <listitem>
+          <para>Specify the high limit on memory usage of the executed processes in this unit. Memory usage may go
+          above the limit if unavoidable, but the processes are heavily slowed down and memory is taken away
+          aggressively in such cases. This is the main mechanism to control memory usage of a unit.</para>
+
+          <para>Takes a memory size in bytes. If the value is suffixed with K, M, G or T, the specified memory size is
+          parsed as Kilobytes, Megabytes, Gigabytes, or Terabytes (with the base 1024), respectively. If assigned the
+          special value <literal>max</literal>, no memory limit is applied. This controls the
+          <literal>memory.high</literal> control group attribute. For details about this control group attribute, see
+          <ulink url="https://www.kernel.org/doc/Documentation/cgroup-v2.txt">cgroup-v2.txt</ulink>.</para>
+
+          <para>Implies <literal>MemoryAccounting=true</literal>.</para>
+
+          <para>This setting is supported only if the unified control group hierarchy is used.</para>
+        </listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>MemoryMax=<replaceable>bytes</replaceable></varname></term>
+
+        <listitem>
+          <para>Specify the absolute limit on memory usage of the executed processes in this unit. If memory usage
+          cannot be contained under the limit, out-of-memory killer is invoked inside the unit. It is recommended to
+          use <varname>MemoryHigh=</varname> as the main control mechanism and use <varname>MemoryMax=</varname> as the
+          last line of defense.</para>
+
+          <para>Takes a memory size in bytes. If the value is suffixed with K, M, G or T, the specified memory size is
+          parsed as Kilobytes, Megabytes, Gigabytes, or Terabytes (with the base 1024), respectively. If assigned the
+          special value <literal>max</literal>, no memory limit is applied. This controls the
+          <literal>memory.max</literal> control group attribute. For details about this control group attribute, see
+          <ulink url="https://www.kernel.org/doc/Documentation/cgroup-v2.txt">cgroup-v2.txt</ulink>.</para>
+
+          <para>Implies <literal>MemoryAccounting=true</literal>.</para>
+
+          <para>This setting is supported only if the unified control group hierarchy is used. Use
+          <varname>MemoryLimit=</varname> on systems using the legacy control group hierarchy.</para>
+        </listitem>
+      </varlistentry>
+
+      <varlistentry>
         <term><varname>MemoryLimit=<replaceable>bytes</replaceable></varname></term>
 
         <listitem>
@@ -230,6 +298,9 @@
           url="https://www.kernel.org/doc/Documentation/cgroup-v1/memory.txt">memory.txt</ulink>.</para>
 
           <para>Implies <literal>MemoryAccounting=true</literal>.</para>
+
+          <para>This setting is supported only if the legacy control group hierarchy is used. Use
+          <varname>MemoryMax=</varname> on systems using the unified control group hierarchy.</para>
         </listitem>
       </varlistentry>