[PATCH] mm, memcg: reset memcg's memory.{min, low} for reclaiming itself

[Yafang Shao <laoar.shao@gmail.com>]

memory.{emin, elow} are set in mem_cgroup_protected(), and the values of
them won't be changed until next recalculation in this function. After
either or both of them are set, the next reclaimer to relcaim this memcg
may be a different reclaimer, e.g. this memcg is also the root memcg of
the new reclaimer, and then in mem_cgroup_protection() in get_scan_count()
the old values of them will be used to calculate scan count, that is not
proper. We should reset them to zero in this case.

Here's an example of this issue.

    root_mem_cgroup
         /
        A   memory.max=1024M memory.min=512M memory.current=800M

Once kswapd is waked up, it will try to scan all MEMCGs, including
this A, and it will assign memory.emin of A with 512M.
After that, A may reach its hard limit(memory.max), and then it will
do memcg reclaim. Because A is the root of this reclaimer, so it will
not calculate its memory.emin. So the memory.emin is the old value
512M, and then this old value will be used in
mem_cgroup_protection() in get_scan_count() to get the scan count.
That is not proper.

Fixes: 9783aa9917f8 ("mm, memcg: proportional memory.{low,min} reclaim")
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Cc: Chris Down <chris@chrisdown.name>
Cc: Roman Gushchin <guro@fb.com>
Cc: stable@vger.kernel.org
---
 mm/memcontrol.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 601405b..bb3925d 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6287,8 +6287,17 @@ enum mem_cgroup_protection mem_cgroup_protected(struct mem_cgroup *root,
 
 	if (!root)
 		root = root_mem_cgroup;
-	if (memcg == root)
+	if (memcg == root) {
+		/*
+		 * Reset memory.(emin, elow) for reclaiming the memcg
+		 * itself.
+		 */
+		if (memcg != root_mem_cgroup) {
+			memcg->memory.emin = 0;
+			memcg->memory.elow = 0;
+		}
 		return MEMCG_PROT_NONE;
+	}
 
 	usage = page_counter_read(&memcg->memory);
 	if (!usage)
-- 
1.8.3.1