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Date: Tue, 29 Jul 2014 16:51:38 -0700 (PDT)
From: David Rientjes <rientjes@google.com>
Subject: Re: [PATCH v5 05/14] mm, compaction: move pageblock checks up from
 isolate_migratepages_range()
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References: <1406553101-29326-1-git-send-email-vbabka@suse.cz> <1406553101-29326-6-git-send-email-vbabka@suse.cz> <alpine.DEB.2.02.1407281709050.8998@chino.kir.corp.google.com> <53D7690D.5070307@suse.cz> <alpine.DEB.2.02.1407291559130.20991@chino.kir.corp.google.com>
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To: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Vlastimil Babka <vbabka@suse.cz>, Andrew Morton <akpm@linux-foundation.org>, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Minchan Kim <minchan@kernel.org>, Joonsoo Kim <iamjoonsoo.kim@lge.com>, Michal Nazarewicz <mina86@mina86.com>, Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>, Christoph Lameter <cl@linux.com>, Rik van Riel <riel@redhat.com>, Mel Gorman <mgorman@suse.de>, Zhang Yanfei <zhangyanfei@cn.fujitsu.com>

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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