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

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Message-ID: <54ACFE3A.4070502@suse.cz>
Date:	Wed, 07 Jan 2015 10:36:58 +0100
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Michal Hocko <mhocko@...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 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.

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.


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