linux-kernel - Re: [PATCH 04/14] mm,migration: Allow the migration of PageSwapCache pages

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Date:	Thu, 22 Apr 2010 20:39:33 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Minchan Kim <minchan.kim@...il.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Mel Gorman <mel@....ul.ie>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Adam Litke <agl@...ibm.com>, Avi Kivity <avi@...hat.com>,
	David Rientjes <rientjes@...gle.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH 04/14] mm,migration: Allow the migration of
 PageSwapCache pages

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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