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

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Message-ID: <20120821175502.GC12294@t510.redhat.com>
Date:	Tue, 21 Aug 2012 14:55:03 -0300
From:	Rafael Aquini <aquini@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	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 04:52:23PM +0300, Michael S. Tsirkin wrote:
> > + * address_space_operations utilized methods for ballooned pages:
> > + *   .migratepage    - used to perform balloon's page migration (as is)
> > + *   .launder_page   - used to isolate a page from balloon's page list
> > + *   .freepage       - used to reinsert an isolated page to balloon's page list
> > + */
> 
> It would be a good idea to document the assumptions here.
> Looks like .launder_page and .freepage are called in rcu critical
> section.
> But migratepage isn't - why is that safe?
> 

The migratepage callback for virtio_balloon can sleep, and IIUC we cannot sleep
within a RCU critical section. 

Also, The migratepage callback is called at inner migration's circle function
move_to_new_page(), and I don't think embedding it in a RCU critical section
would be a good idea, for the same understanding aforementioned.

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