linux-kernel - Re: [PATCH v5 05/14] mm, compaction: move pageblock checks up from isolate_migratepages

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Message-ID: <alpine.DEB.2.02.1407291646590.961@chino.kir.corp.google.com>
Date:	Tue, 29 Jul 2014 16:51:38 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
cc:	Vlastimil Babka <vbabka@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Minchan Kim <minchan@...nel.org>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Michal Nazarewicz <mina86@...a86.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Christoph Lameter <cl@...ux.com>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Zhang Yanfei <zhangyanfei@...fujitsu.com>
Subject: Re: [PATCH v5 05/14] mm, compaction: move pageblock checks up from
 isolate_migratepages_range()

On Wed, 30 Jul 2014, Kirill A. Shutemov wrote:

> > Hmm, I'm confused at how that could be true, could you explain what 
> > memory other than thp can return true for PageTransHuge()?
> 
> PageTransHuge() will be true for any head of compound page if THP is
> enabled compile time: hugetlbfs, slab, whatever.
> 

I was meaning in the context of the patch :)  Since PageLRU is set, that 
discounts slab so we're left with thp or hugetlbfs.  Logically, both 
should have sizes that are >= the size of the pageblock itself so I'm not 
sure why we don't unconditionally align up to pageblock_nr_pages here.  Is 
there a legitimiate configuration where a pageblock will span multiple 
pages of HPAGE_PMD_ORDER?
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