Re: [PATCH] mm, memcg: avoid oom if cgroup is not populated

[Michal Hocko <mhocko@kernel.org>]

On Tue 26-11-19 08:02:49, Yafang Shao wrote:
> There's one case that the processes in a memcg are all exit (due to OOM
> group or some other reasons), but the file page caches are still exist.
> These file page caches may be protected by memory.min so can't be
> reclaimed. If we can't success to restart the processes in this memcg or
> don't want to make this memcg offline, then we want to drop the file page
> caches.
> The advantage of droping this file caches is it can avoid the reclaimer
> (either kswapd or direct) scanning and reclaiming pages from all memcgs
> exist in this system, because currently the reclaimer will fairly reclaim
> pages from all memcgs if the system is under memory pressure.
> The possible method to drop these file page caches is setting the
> hard limit of this memcg to 0. Unfortunately this may invoke the OOM killer
> and generates lots of misleading outputs, that should not happen.

I disagree that the output is misleading. Quite contrary, it provides a
useful lead on the unreclaimable memory.

> One misleading output is "Out of memory and no killable processes...",
> while really there is no tasks rather than no killable tasks.

Again, this is nothing misleading. No task is a trivial subset of no
killable task. I do not see why we should treat one differently than the
other.

> Furthermore,
> the OOM output is not expected by the admin if he or she only wants to drop
> the cahes and knows there're no processes running in this memcg.

But this is not what hard limit reduced to 0 really does. No matter
whether there is some task or not. It simply reclaims _all_ the memory
as explained in other email.
 
> If memcg is not populated, we should not invoke the OOM killer.

I have already explained why I believe this is not correct in other
email and this description doesn't provide any real justification. It is
merely your intepretation of what should happen and I believe you
haven't thought through it really.
 
> Fixes: b6e6edcf ("mm: memcontrol: reclaim and OOM kill when shrinking memory.max below usage")
> Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Michal Hocko <mhocko@suse.com>
> ---
>  mm/memcontrol.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 1c4c08b..4e08905 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -6139,9 +6139,13 @@ static ssize_t memory_max_write(struct kernfs_open_file *of,
>  			continue;
>  		}
>  
> -		memcg_memory_event(memcg, MEMCG_OOM);
> -		if (!mem_cgroup_out_of_memory(memcg, GFP_KERNEL, 0))
> +		if (cgroup_is_populated(memcg->css.cgroup)) {
> +			memcg_memory_event(memcg, MEMCG_OOM);
> +			if (!mem_cgroup_out_of_memory(memcg, GFP_KERNEL, 0))
> +				break;
> +		} else  {
>  			break;
> +		}
>  	}
>  
>  	memcg_wb_domain_size_changed(memcg);
> -- 
> 1.8.3.1

-- 
Michal Hocko
SUSE Labs