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 16:34:39 -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 10:16:12PM +0300, Michael S. Tsirkin wrote:
> On Tue, Aug 21, 2012 at 02:55:03PM -0300, Rafael Aquini wrote:
> > 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.
> 
> Yes but this means it is still exposed to the module unloading
> races that RCU was supposed to fix.
> So need to either rework that code so it won't sleep
> or switch to some other synchronization.
>
Can you refactor tell_host() to not sleep? Or, can I get rid of calling it at
virtballoon_migratepage()? If 'no' is the answer for both questions, that's the
way that code has to remain, even if we find a way around to hack the
migratepage callback and have it embedded into a RCU crit section.

That's why I believe once the balloon driver is commanded to unload, we must
flag virtballoon_migratepage to skip it's work. By doing this, the thread
performing memory compaction will have to recur to the 'putback' path which is
RCU protected. (IMHO).

As the module will not uload utill it leaks all pages on its list, that unload
race you pointed before will be covered.

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