linux-kernel - Re: [patch -mm] mm, thp: avoid excessive compaction latency during fault fix

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Message-ID: <537DB89B.8080301@suse.cz>
Date:	Thu, 22 May 2014 10:43:07 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	David Rientjes <rientjes@...gle.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Greg Thelen <gthelen@...gle.com>,
	Hugh Dickins <hughd@...gle.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [patch -mm] mm, thp: avoid excessive compaction latency during
 fault fix

On 05/22/2014 04:49 AM, David Rientjes wrote:
> On Tue, 13 May 2014, Vlastimil Babka wrote:
>
>> I wonder what about a process doing e.g. mmap() with MAP_POPULATE. It seems to
>> me that it would get only MIGRATE_ASYNC here, right? Since gfp_mask would
>> include __GFP_NO_KSWAPD and it won't have PF_KTHREAD.
>> I think that goes against the idea that with MAP_POPULATE you say you are
>> willing to wait to have everything in place before you actually use the
>> memory. So I guess you are also willing to wait for hugepages in that
>> situation?
>>
>
> I don't understand the distinction you're making between MAP_POPULATE and
> simply a prefault of the anon memory.  What is the difference in semantics
> between using MAP_POPULATE and touching a byte every page size along the
> range?  In the latter, you'd be faulting thp with MIGRATE_ASYNC, so I
> don't understand how MAP_POPULATE is any different or implies any
> preference for hugepages.

Hm, OK. It's right we cannot distinguish populating by touching the 
pages manually. Nevermind then.

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