linux-kernel - Re: [PATCH v7 2/4] virtio_balloon: introduce migration primitives to balloon pages

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Message-ID: <20120815123457.GA2175@t510.redhat.com>
Date:	Wed, 15 Aug 2012 09:34:58 -0300
From:	Rafael Aquini <aquini@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Rusty Russell <rusty@...tcorp.com.au>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	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 v7 2/4] virtio_balloon: introduce migration primitives to
 balloon pages

On Tue, Aug 14, 2012 at 10:31:09PM +0300, Michael S. Tsirkin wrote:
> > > now CPU1 executes the next instruction:
> > > 
> > > }
> > > 
> > > which would normally return to function's caller,
> > > but it has been overwritten by CPU2 so we get corruption.
> > > 
> > > No?
> > 
> > At the point CPU2 is unloading the module, it will be kept looping at the
> > snippet Rusty pointed out because the isolation / migration steps do not mess
> > with 'vb->num_pages'. The driver will only unload after leaking the total amount
> > of balloon's inflated pages, which means (for this hypothetical case) CPU2 will
> > wait until CPU1 finishes the putaback procedure.
> > 
> 
> Yes but only until unlock finishes. The last return from function
> is not guarded and can be overwritten.

CPU1 will be returning to putback_balloon_page() which code is located at core
mm/compaction.c, outside the driver.
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