linux-kernel - Re: [PATCH] rmap: fix pgoff calculation to handle hugepage correctly

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Date:	Tue, 1 Jul 2014 23:15:40 +0300
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Hugh Dickins <hughd@...gle.com>,
	Rik van Riel <riel@...hat.com>,
	Hillf Danton <dhillf@...il.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Naoya Horiguchi <nao.horiguchi@...il.com>
Subject: Re: [PATCH] rmap: fix pgoff calculation to handle hugepage correctly

On Tue, Jul 01, 2014 at 02:50:21PM -0400, Naoya Horiguchi wrote:
> On Tue, Jul 01, 2014 at 09:07:39PM +0300, Kirill A. Shutemov wrote:
> > Why do we need this special case for hugetlb page ->index? Why not use
> > PAGE_SIZE units there too? Or I miss something?
> 
> hugetlb pages are never split, so we use larger page cache size for
> hugetlbfs file (to avoid large sparse page cache tree.)

For transparent huge page cache I would like to have native support in
page cache radix-tree: since huge pages are always naturally aligned we
can create a leaf node for it several (RADIX_TREE_MAP_SHIFT -
HPAGE_PMD_ORDER) levels up by tree, which would cover all indexes in the
range the huge page represents. This approach should fit hugetlb too. And
-1 special case for hugetlb.
But I'm not sure when I'll get time to play with this...

-- 
 Kirill A. Shutemov
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