Date: Thu, 22 Apr 2010 20:39:33 -0700
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Christoph Lameter <cl@linux.com>
Cc: Minchan Kim <minchan.kim@gmail.com>,
       KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
       Mel Gorman <mel@csn.ul.ie>, Andrew Morton <akpm@linux-foundation.org>,
       Andrea Arcangeli <aarcange@redhat.com>, Adam Litke <agl@us.ibm.com>,
       Avi Kivity <avi@redhat.com>, David Rientjes <rientjes@google.com>,
       KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
       Rik van Riel <riel@redhat.com>, linux-kernel@vger.kernel.org,
       linux-mm@kvack.org
Subject: Re: [PATCH 04/14] mm,migration: Allow the migration of
 PageSwapCache pages
Message-ID: <20100423033933.GA2619@linux.vnet.ibm.com>
Reply-To: paulmck@linux.vnet.ibm.com
References: <alpine.DEB.2.00.1004211027120.4959@router.home>
 <20100421153421.GM30306@csn.ul.ie>
 <alpine.DEB.2.00.1004211038020.4959@router.home>
 <20100422092819.GR30306@csn.ul.ie>
 <20100422184621.0aaaeb5f.kamezawa.hiroyu@jp.fujitsu.com>
 <x2l28c262361004220313q76752366l929a8959cd6d6862@mail.gmail.com>
 <20100422193106.9ffad4ec.kamezawa.hiroyu@jp.fujitsu.com>
 <20100422195153.d91c1c9e.kamezawa.hiroyu@jp.fujitsu.com>
 <1271946226.2100.211.camel@barrios-desktop>
 <alpine.DEB.2.00.1004221009150.32107@router.home>
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On Thu, Apr 22, 2010 at 10:14:04AM -0500, Christoph Lameter wrote:
> On Thu, 22 Apr 2010, Minchan Kim wrote:
> 
> > For further optimization, we can hold vma->adjust_lock if vma_address
> > returns -EFAULT. But I hope we redesigns it without new locking.
> > But I don't have good idea, now. :(
> 
> You could make it atomic through the use of RCU.
> 
> Create a new vma entry with the changed parameters and then atomically
> switch to the new vma.
> 
> Problem is that you have some list_heads in there.

Indeed, it would be necessary to update -all- pointers to the old
vma entry to point to the new vma entry.  The question at that point
will be "is it OK to switch the pointers over one at a time?"

In many situations, the answer is "yes", but it is necessary to check
carefully.

						Thanx, Paul
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