Re: [PATCH v4] mm/memcg: try harder to decrease [memory,memsw].limit_in_bytes

[Andrey Ryabinin <aryabinin@virtuozzo.com>]

On 01/11/2018 01:42 PM, Michal Hocko wrote:
> On Wed 10-01-18 15:43:17, Andrey Ryabinin wrote:
> [...]
>> @@ -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 <aryabinin@virtuozzo.com>
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 <aryabinin@virtuozzo.com>
---
 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