linux-kernel - Re: [PATCH 2/2] mm: do not loop over ALLOC_NO

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Message-ID: <20151118151119.GG19145@dhcp22.suse.cz>
Date:	Wed, 18 Nov 2015 16:11:19 +0100
From:	Michal Hocko <mhocko@...nel.org>
To:	Vlastimil Babka <vbabka@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>,
	David Rientjes <rientjes@...gle.com>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] mm: do not loop over ALLOC_NO_WATERMARKS without
 triggering reclaim

On Wed 18-11-15 15:57:45, Vlastimil Babka wrote:
[...]
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -3046,32 +3046,36 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> >  		 * allocations are system rather than user orientated
> >  		 */
> >  		ac->zonelist = node_zonelist(numa_node_id(), gfp_mask);
> > -		do {
> > -			page = get_page_from_freelist(gfp_mask, order,
> > -							ALLOC_NO_WATERMARKS, ac);
> > -			if (page)
> > -				goto got_pg;
> > -
> > -			if (gfp_mask & __GFP_NOFAIL)
> > -				wait_iff_congested(ac->preferred_zone,
> > -						   BLK_RW_ASYNC, HZ/50);
> 
> I've been thinking if the lack of unconditional wait_iff_congested() can affect
> something negatively. I guess not?

Considering that the wait_iff_congested is removed only for PF_MEMALLOC
with __GFP_NOFAIL which should be non-existent in the kernel then I
think the risk is really low. Even if there was a caller _and_ there
was a congestion then the behavior wouldn't be much more worse than
what we have currently. The system is out of memory hoplessly if
ALLOC_NO_WATERMARKS allocation fails.
-- 
Michal Hocko
SUSE Labs
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