linux-kernel - Re: [PATCH 1/3] mm: Fix dropped memcg from mem cgroup soft limit tree

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Date:   Wed, 10 Feb 2021 10:47:44 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Tim Chen <tim.c.chen@...ux.intel.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Ying Huang <ying.huang@...el.com>, linux-mm@...ck.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] mm: Fix dropped memcg from mem cgroup soft limit tree

On Tue 09-02-21 12:29:45, Tim Chen wrote:
> During soft limit memory reclaim, we will temporarily remove the target
> mem cgroup from the cgroup soft limit tree.  We then perform memory
> reclaim, update the memory usage excess count and re-insert the mem
> cgroup back into the mem cgroup soft limit tree according to the new
> memory usage excess count.
> 
> However, when memory reclaim failed for a maximum number of attempts
> and we bail out of the reclaim loop, we forgot to put the target mem
> cgroup chosen for next reclaim back to the soft limit tree. This prevented
> pages in the mem cgroup from being reclaimed in the future even though
> the mem cgroup exceeded its soft limit.  Fix the logic and put the mem
> cgroup back on the tree when page reclaim failed for the mem cgroup.
> 
> Reviewed-by: Ying Huang <ying.huang@...el.com>
> Signed-off-by: Tim Chen <tim.c.chen@...ux.intel.com>

It seems this goes all the way to when it has been introduced by 
4e41695356fb ("memory controller: soft limit reclaim on contention").
Please add a Fixes tag pointing to the above one. While this looks like
a rare event to happen because there should be some reclaimable memory
usually.

Acked-by: Michal Hocko <mhocko@...e.com>

Thanks!

> ---
>  mm/memcontrol.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index ed5cc78a8dbf..a51bf90732cb 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -3505,8 +3505,12 @@ unsigned long mem_cgroup_soft_limit_reclaim(pg_data_t *pgdat, int order,
>  			loop > MEM_CGROUP_MAX_SOFT_LIMIT_RECLAIM_LOOPS))
>  			break;
>  	} while (!nr_reclaimed);
> -	if (next_mz)
> +	if (next_mz) {
> +		spin_lock_irq(&mctz->lock);
> +		__mem_cgroup_insert_exceeded(next_mz, mctz, excess);
> +		spin_unlock_irq(&mctz->lock);
>  		css_put(&next_mz->memcg->css);
> +	}
>  	return nr_reclaimed;
>  }
>  
> -- 
> 2.20.1

-- 
Michal Hocko
SUSE Labs