On 01/11/2018 01:42 PM, Michal Hocko wrote:

On Wed 10-01-18 15:43:17, Andrey Ryabinin wrote:
[...]

quoted

@@ -2506,15 +2480,13 @@ static int mem_cgroup_resize_limit(struct mem_cgroup *memcg,
 		if (!ret)
 			break;
 
-		try_to_free_mem_cgroup_pages(memcg, 1, GFP_KERNEL, !memsw);
-
-		curusage = page_counter_read(counter);
-		/* Usage is reduced ? */
-		if (curusage >= oldusage)
-			retry_count--;
-		else
-			oldusage = curusage;
-	} while (retry_count);
+		usage = page_counter_read(counter);
+		if (!try_to_free_mem_cgroup_pages(memcg, usage - limit,
+						GFP_KERNEL, !memsw)) {

If the usage drops below limit in the meantime then you get underflow
and reclaim the whole memcg. I do not think this is a good idea. This
can also lead to over reclaim. Why don't you simply stick with the
original SWAP_CLUSTER_MAX (aka 1 for try_to_free_mem_cgroup_pages)?

Because, if new limit is gigabytes bellow the current usage, retrying to set
new limit after reclaiming only 32 pages seems unreasonable.

So, I made this:


From: Andrey Ryabinin <redacted>
Subject: mm-memcg-try-harder-to-decrease-limit_in_bytes-fix

Protect from overreclaim if usage become lower than limit.

Signed-off-by: Andrey Ryabinin <redacted>
---
 mm/memcontrol.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 4671ae8a8b1a..6120bb619547 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c

@@ -2455,7 +2455,7 @@ static DEFINE_MUTEX(memcg_limit_mutex);
 static int mem_cgroup_resize_limit(struct mem_cgroup *memcg,
 				   unsigned long limit, bool memsw)
 {
-	unsigned long usage;
+	unsigned long nr_pages;
 	bool enlarge = false;
 	int ret;
 	bool limits_invariant;

@@ -2487,8 +2487,8 @@ static int mem_cgroup_resize_limit(struct mem_cgroup *memcg,
 		if (!ret)
 			break;
 
-		usage = page_counter_read(counter);
-		if (!try_to_free_mem_cgroup_pages(memcg, usage - limit,
+		nr_pages = max_t(long, 1, page_counter_read(counter) - limit);
+		if (!try_to_free_mem_cgroup_pages(memcg, nr_pages,
 						GFP_KERNEL, !memsw)) {
 			ret = -EBUSY;
 			break;

-- 
2.13.6