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

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Message-ID: <alpine.DEB.2.02.1405211945140.13243@chino.kir.corp.google.com>
Date:	Wed, 21 May 2014 19:49:11 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Vlastimil Babka <vbabka@...e.cz>
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 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.
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