From: Chunxin Zang <redacted>

In the cgroup v1, we have 'force_mepty' interface. This is very
useful for userspace to actively release memory. But the cgroup
v2 does not.

This patch reuse cgroup v1's function, but have a new name for
the interface. Because I think 'drop_cache' may be is easier to
understand :)

Signed-off-by: Chunxin Zang <redacted>
---
 Documentation/admin-guide/cgroup-v2.rst | 11 +++++++++++
 mm/memcontrol.c                         |  5 +++++
 2 files changed, 16 insertions(+)

diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
index ce3e05e41724..fbff959c8116 100644
--- a/Documentation/admin-guide/cgroup-v2.rst
+++ b/Documentation/admin-guide/cgroup-v2.rst

@@ -1181,6 +1181,17 @@ PAGE_SIZE multiple when read back.
        high limit is used and monitored properly, this limit's
        utility is limited to providing the final safety net.

+  memory.drop_cache
+    A write-only single value file which exists on non-root
+    cgroups.
+
+    Provide a mechanism for users to actively trigger memory
+    reclaim. The cgroup will be reclaimed and as many pages
+    reclaimed as possible.
+
+    It will broke low boundary. Because it tries to reclaim the
+    memory many times, until the memory drops to a certain level.
+