linux-kernel - Re: [PATCH 1/2] mm, oom: Give __GFP_NOFAIL allocations access to memory reserves

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Date:	Wed, 25 Nov 2015 12:57:08 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Michal Hocko <mhocko@...nel.org>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>,
	Johannes Weiner <hannes@...xchg.org>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] mm, oom: Give __GFP_NOFAIL allocations access to
 memory reserves

On Wed, 25 Nov 2015, Michal Hocko wrote:

> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 8034909faad2..94b04c1e894a 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2766,8 +2766,13 @@ __alloc_pages_may_oom(gfp_t gfp_mask, unsigned int order,
>  			goto out;
>  	}
>  	/* Exhausted what can be done so it's blamo time */
> -	if (out_of_memory(&oc) || WARN_ON_ONCE(gfp_mask & __GFP_NOFAIL))
> +	if (out_of_memory(&oc) || WARN_ON_ONCE(gfp_mask & __GFP_NOFAIL)) {
>  		*did_some_progress = 1;
> +
> +		if (gfp_mask & __GFP_NOFAIL)
> +			page = get_page_from_freelist(gfp_mask, order,
> +					ALLOC_NO_WATERMARKS, ac);
> +	}
>  out:
>  	mutex_unlock(&oom_lock);
>  	return page;

Well, sure, that's one way to do it, but for cpuset users, wouldn't this 
lead to a depletion of the first system zone since you've dropped 
ALLOC_CPUSET and are doing ALLOC_NO_WATERMARKS in the same call?  
get_page_from_freelist() shouldn't be doing any balancing over the set of 
allowed zones.  Can you justify depleting memory reserves on a zone 
outside of the set of allowed cpuset mems rather than trying to drop 
ALLOC_CPUSET first?
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