linux-kernel - Re: [PATCH] memcg: enable memcg oom-kill for __GFP

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Date:   Wed, 24 Feb 2021 10:14:31 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     David Rientjes <rientjes@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <guro@...com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        cgroups@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] memcg: enable memcg oom-kill for __GFP_NOFAIL

On Tue 23-02-21 12:43:37, Shakeel Butt wrote:
> In the era of async memcg oom-killer, the commit a0d8b00a3381 ("mm:
> memcg: do not declare OOM from __GFP_NOFAIL allocations") added the code
> to skip memcg oom-killer for __GFP_NOFAIL allocations. The reason was
> that the __GFP_NOFAIL callers will not enter aync oom synchronization
> path and will keep the task marked as in memcg oom. At that time the
> tasks marked in memcg oom can bypass the memcg limits and the oom
> synchronization would have happened later in the later userspace
> triggered page fault. Thus letting the task marked as under memcg oom
> bypass the memcg limit for arbitrary time.
> 
> With the synchronous memcg oom-killer (commit 29ef680ae7c21 ("memcg,
> oom: move out_of_memory back to the charge path")) and not letting the
> task marked under memcg oom to bypass the memcg limits (commit
> 1f14c1ac19aa4 ("mm: memcg: do not allow task about to OOM kill to bypass
> the limit")), we can again allow __GFP_NOFAIL allocations to trigger
> memcg oom-kill. This will make memcg oom behavior closer to page
> allocator oom behavior.

The patch is correct, I just do follow why 1f14c1ac19aa4 is really
relevant here. There nomem label wouldn't make any difference for
__GFP_NOFAIL requests. The code has has changed quite a lot since then.
 
> Signed-off-by: Shakeel Butt <shakeelb@...gle.com>

This is a clear overlook when I moved the oom handling back to the
charge path. Thanks for the fixup.
Acked-by: Michal Hocko <mhocko@...e.com>

> ---
>  mm/memcontrol.c | 3 ---
>  1 file changed, 3 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 2db2aeac8a9e..dcb5665aeb69 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2797,9 +2797,6 @@ static int try_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
>  	if (gfp_mask & __GFP_RETRY_MAYFAIL)
>  		goto nomem;
>  
> -	if (gfp_mask & __GFP_NOFAIL)
> -		goto force;
> -
>  	if (fatal_signal_pending(current))
>  		goto force;
>  
> -- 
> 2.30.0.617.g56c4b15f3c-goog

-- 
Michal Hocko
SUSE Labs