linux-kernel - Re: [patch] mm: memcg: do not declare OOM from __GFP

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Message-ID: <alpine.DEB.2.02.1311261658170.21003@chino.kir.corp.google.com>
Date:	Tue, 26 Nov 2013 17:01:43 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Johannes Weiner <hannes@...xchg.org>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Michal Hocko <mhocko@...e.cz>, linux-mm@...ck.org,
	cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [patch] mm: memcg: do not declare OOM from __GFP_NOFAIL
 allocations

On Fri, 22 Nov 2013, Johannes Weiner wrote:

> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 13b9d0f..cc4f9cb 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2677,6 +2677,9 @@ static int __mem_cgroup_try_charge(struct mm_struct *mm,
>  	if (unlikely(task_in_memcg_oom(current)))
>  		goto bypass;
>  
> +	if (gfp_mask & __GFP_NOFAIL)
> +		oom = false;
> +
>  	/*
>  	 * We always charge the cgroup the mm_struct belongs to.
>  	 * The mm_struct's mem_cgroup changes on task migration if the

Sorry, I don't understand this.  What happens in the following scenario:

 - memory.usage_in_bytes == memory.limit_in_bytes,

 - memcg reclaim fails to reclaim memory, and

 - all processes (perhaps only one) attached to the memcg are doing one of
   the over dozen __GFP_NOFAIL allocations in the kernel?

How do we make forward progress if you cannot oom kill something?
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