linux-kernel - Re: [PATCH v8 1/5] mm: introduce a common interface for balloon pages mobility

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Date:	Tue, 21 Aug 2012 18:41:42 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Rafael Aquini <aquini@...hat.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	Rusty Russell <rusty@...tcorp.com.au>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mel@....ul.ie>,
	Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Minchan Kim <minchan@...nel.org>
Subject: Re: [PATCH v8 1/5] mm: introduce a common interface for balloon
 pages mobility

On Tue, Aug 21, 2012 at 05:16:06PM +0200, Peter Zijlstra wrote:
> On Tue, 2012-08-21 at 16:52 +0300, Michael S. Tsirkin wrote:
> > > +             rcu_read_lock();
> > > +             mapping = rcu_dereference(page->mapping);
> > > +             if (mapping_balloon(mapping))
> > > +                     ret = true;
> > > +             rcu_read_unlock();
> > 
> > This looks suspicious: you drop rcu_read_unlock
> > so can't page switch from balloon to non balloon? 
> 
> RCU read lock is a non-exclusive lock, it cannot avoid anything like
> that.

You are right, of course. So even keeping rcu_read_lock across both test
and operation won't be enough - you need to make this function return
the mapping and pass it to isolate_page/putback_page so that it is only
dereferenced once.

-- 
MST
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