[PATCH 1/2] mm/memcg: try harder to decrease [memory,memsw].limit_in_bytes

[Andrey Ryabinin]

mem_cgroup_resize_[memsw]_limit() tries to free only 32 (SWAP_CLUSTER_MAX)
pages on each iteration. This makes practically impossible to decrease
limit of memory cgroup. Tasks could easily allocate back 32 pages,
so we can't reduce memory usage, and once retry_count reaches zero we return
-EBUSY.

It's easy to reproduce the problem by running the following commands:

  mkdir /sys/fs/cgroup/memory/test
  echo $$ >> /sys/fs/cgroup/memory/test/tasks
  cat big_file > /dev/null &
  sleep 1 && echo $((100*1024*1024)) > /sys/fs/cgroup/memory/test/memory.limit_in_bytes
  -bash: echo: write error: Device or resource busy

Instead of trying to free small amount of pages, it's much more
reasonable to free 'usage - limit' pages.

Signed-off-by: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
---
 mm/memcontrol.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index f40b5ad3f959..09ee052cf684 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2476,7 +2476,7 @@ static int mem_cgroup_resize_limit(struct mem_cgroup *memcg,
 	retry_count = MEM_CGROUP_RECLAIM_RETRIES *
 		      mem_cgroup_count_children(memcg);
 
-	oldusage = page_counter_read(&memcg->memory);
+	curusage = oldusage = page_counter_read(&memcg->memory);
 
 	do {
 		if (signal_pending(current)) {
@@ -2498,7 +2498,8 @@ static int mem_cgroup_resize_limit(struct mem_cgroup *memcg,
 		if (!ret)
 			break;
 
-		try_to_free_mem_cgroup_pages(memcg, 1, GFP_KERNEL, true);
+		try_to_free_mem_cgroup_pages(memcg, curusage - limit,
+					GFP_KERNEL, true);
 
 		curusage = page_counter_read(&memcg->memory);
 		/* Usage is reduced ? */
@@ -2527,7 +2528,7 @@ static int mem_cgroup_resize_memsw_limit(struct mem_cgroup *memcg,
 	retry_count = MEM_CGROUP_RECLAIM_RETRIES *
 		      mem_cgroup_count_children(memcg);
 
-	oldusage = page_counter_read(&memcg->memsw);
+	curusage = oldusage = page_counter_read(&memcg->memsw);
 
 	do {
 		if (signal_pending(current)) {
@@ -2549,7 +2550,8 @@ static int mem_cgroup_resize_memsw_limit(struct mem_cgroup *memcg,
 		if (!ret)
 			break;
 
-		try_to_free_mem_cgroup_pages(memcg, 1, GFP_KERNEL, false);
+		try_to_free_mem_cgroup_pages(memcg, curusage - limit,
+					GFP_KERNEL, false);
 
 		curusage = page_counter_read(&memcg->memsw);
 		/* Usage is reduced ? */
-- 
2.13.6