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Date: Mon, 1 Mar 2010 19:23:25 +0100
From: Andrea Arcangeli <aarcange@redhat.com>
Subject: Re: [PATCH 04 of 32] update futex compound knowledge
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To: Peter Zijlstra <peterz@infradead.org>
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>, Rik van Riel <riel@redhat.com>, Mel Gorman <mel@csn.ul.ie>, 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, KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>, Balbir Singh <balbir@linux.vnet.ibm.com>, Arnd Bergmann <arnd@arndb.de>
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On Mon, Mar 01, 2010 at 07:07:29PM +0100, Peter Zijlstra wrote:
> On Mon, 2010-03-01 at 18:58 +0100, Andrea Arcangeli wrote:
> > 
> > But nothing risks to break at build time, simply any arch with
> > transparent hugepage support also has to implement
> > __get_user_pages_fast. Disabling irq and using __get_user_pages_fast
> > looked the best way to serialize against split_huge_page here (rather
> > than taking locks). 
> 
> PowerPC uses rcu-free'd pagetables for gup_fast() iirc, so no need to
> disable IRQs there.

btw, powerpc right now doesn't support transparent hugepages. IRQs
have to be disabled to stop pmdp_splitting_flush_notify on x86, for
the same reason why x86 has to disable irq in gup-fast.

I surely can try to abstract it before there is a second arch
involved, but it'd be more natural in the process of porting to an
arch that wants rcu_read_lock instead of local_irq_disable.

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