linux-kernel - Re: [PATCH 23 of 66] add pmd_huge_pte to mm

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Date:	Thu, 18 Nov 2010 13:13:40 +0000
From:	Mel Gorman <mel@....ul.ie>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	linux-mm@...ck.org, Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Adam Litke <agl@...ibm.com>, Avi Kivity <avi@...hat.com>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Rik van Riel <riel@...hat.com>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Ingo Molnar <mingo@...e.hu>, Mike Travis <travis@....com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Chris Wright <chrisw@...s-sol.org>, bpicco@...hat.com,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	Chris Mason <chris.mason@...cle.com>,
	Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH 23 of 66] add pmd_huge_pte to mm_struct

On Wed, Nov 03, 2010 at 04:27:58PM +0100, Andrea Arcangeli wrote:
> From: Andrea Arcangeli <aarcange@...hat.com>
> 
> This increase the size of the mm struct a bit but it is needed to preallocate
> one pte for each hugepage so that split_huge_page will not require a fail path.
> Guarantee of success is a fundamental property of split_huge_page to avoid
> decrasing swapping reliability and to avoid adding -ENOMEM fail paths that
> would otherwise force the hugepage-unaware VM code to learn rolling back in the
> middle of its pte mangling operations (if something we need it to learn
> handling pmd_trans_huge natively rather being capable of rollback). When
> split_huge_page runs a pte is needed to succeed the split, to map the newly
> splitted regular pages with a regular pte.  This way all existing VM code
> remains backwards compatible by just adding a split_huge_page* one liner. The
> memory waste of those preallocated ptes is negligible and so it is worth it.
> 
> Signed-off-by: Andrea Arcangeli <aarcange@...hat.com>
> Acked-by: Rik van Riel <riel@...hat.com>

Acked-by: Mel Gorman <mel@....ul.ie>

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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