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Date: Tue, 26 Jan 2010 12:48:43 -0500
From: Rik van Riel <riel@redhat.com>
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Subject: Re: [PATCH 20 of 31] add pmd_huge_pte to mm_struct
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To: Andrea Arcangeli <aarcange@redhat.com>
Cc: linux-mm@kvack.org, Marcelo Tosatti <mtosatti@redhat.com>, Adam Litke <agl@us.ibm.com>, Avi Kivity <avi@redhat.com>, Izik Eidus <ieidus@redhat.com>, Hugh Dickins <hugh.dickins@tiscali.co.uk>, Nick Piggin <npiggin@suse.de>, Mel Gorman <mel@csn.ul.ie>, Andi Kleen <andi@firstfloor.org>, Dave Hansen <dave@linux.vnet.ibm.com>, Benjamin Herrenschmidt <benh@kernel.crashing.org>, Ingo Molnar <mingo@elte.hu>, Mike Travis <travis@sgi.com>, KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>, Christoph Lameter <cl@linux-foundation.org>, Chris Wright <chrisw@sous-sol.org>, Andrew Morton <akpm@linux-foundation.org>, bpicco@redhat.com, Christoph Hellwig <chellwig@redhat.com>, KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
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On 01/26/2010 08:52 AM, Andrea Arcangeli wrote:
> From: Andrea Arcangeli<aarcange@redhat.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@redhat.com>

Acked-by: Rik van Riel <riel@redhat.com>

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