linux-kernel - Re: [PATCH V4 1/4] mm: set page->pfmemalloc in prep_new

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Message-ID: <20150107105445.GB16553@dhcp22.suse.cz>
Date:	Wed, 7 Jan 2015 11:54:45 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Vlastimil Babka <vbabka@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Mel Gorman <mgorman@...e.de>,
	Zhang Yanfei <zhangyanfei@...fujitsu.com>,
	Minchan Kim <minchan@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Rik van Riel <riel@...hat.com>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: Re: [PATCH V4 1/4] mm: set page->pfmemalloc in prep_new_page()

On Wed 07-01-15 10:36:58, Vlastimil Babka wrote:
> On 01/06/2015 10:44 PM, Michal Hocko wrote:
> > On Tue 06-01-15 22:10:55, Vlastimil Babka wrote:
> >> On 01/06/2015 03:30 PM, Michal Hocko wrote:
> > [...]
> >> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> >> > index 1bb65e6f48dd..1682d766cb8e 100644
> >> > --- a/mm/page_alloc.c
> >> > +++ b/mm/page_alloc.c
> >> > @@ -2175,10 +2175,11 @@ zonelist_scan:
> >> >  		}
> >> >  
> >> >  try_this_zone:
> >> > -		page = buffered_rmqueue(preferred_zone, zone, order,
> >> > +		do {
> >> > +			page = buffered_rmqueue(preferred_zone, zone, order,
> >> >  						gfp_mask, migratetype);
> >> > -		if (page)
> >> > -			break;
> >> > +		} while (page && prep_new_page(page, order, gfp_mask,
> >> > +					       alloc_flags));
> >> 
> >> Hm but here we wouldn't return page on success.
> > 
> > Right.
> > 
> >> I wonder if you overlooked the return, hence your "not breaking out of
> >> the loop" remark?
> > 
> > This was merely to show the intention. Sorry for not being clear enough.
> 
> OK, but I don't see other way than to follow this do-while with another
> 
> if (page)
>     return page;
> 
> So I think it would be more complicated than now. We wouldn't even be able to
> remove the 'try_this_zone' label, since it's used for goto from elsewhere as well.

Getting rid of the label wasn't the intention. I just found the
allocation retry easier to follow this way. I have no objection if you
keep the code as is.

> Now that I'm thinking of it, maybe we should have a "goto zonelist_scan" there
> instead. We discard a bad page and might come below the watermarks. But the
> chances of this mattering are tiny I guess.

If anything this would be worth a separate patch.

-- 
Michal Hocko
SUSE Labs
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