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

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Date:	Tue, 14 Aug 2012 22:31:09 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Rafael Aquini <aquini@...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 03:44:09PM -0300, Rafael Aquini wrote:
> On Tue, Aug 14, 2012 at 11:33:20AM +0300, Michael S. Tsirkin wrote:
> > On Tue, Aug 14, 2012 at 09:29:49AM +0930, Rusty Russell wrote:
> > > On Mon, 13 Aug 2012 11:41:23 +0300, "Michael S. Tsirkin" <mst@...hat.com> wrote:
> > > > On Fri, Aug 10, 2012 at 02:55:15PM -0300, Rafael Aquini wrote:
> > > > > +/*
> > > > > + * Populate balloon_mapping->a_ops->freepage method to help compaction on
> > > > > + * re-inserting an isolated page into the balloon page list.
> > > > > + */
> > > > > +void virtballoon_putbackpage(struct page *page)
> > > > > +{
> > > > > +	spin_lock(&pages_lock);
> > > > > +	list_add(&page->lru, &vb_ptr->pages);
> > > > > +	spin_unlock(&pages_lock);
> > > > 
> > > > Could the following race trigger:
> > > > migration happens while module unloading is in progress,
> > > > module goes away between here and when the function
> > > > returns, then code for this function gets overwritten?
> > > > If yes we need locking external to module to prevent this.
> > > > Maybe add a spinlock to struct address_space?
> > > 
> > > The balloon module cannot be unloaded until it has leaked all its pages,
> > > so I think this is safe:
> > > 
> > >         static void remove_common(struct virtio_balloon *vb)
> > >         {
> > >         	/* There might be pages left in the balloon: free them. */
> > >         	while (vb->num_pages)
> > >         		leak_balloon(vb, vb->num_pages);
> > > 
> > > Cheers,
> > > Rusty.
> > 
> > I know I meant something else.
> > Let me lay this out:
> > 
> > CPU1 executes:
> > void virtballoon_putbackpage(struct page *page)
> > {
> > 	spin_lock(&pages_lock);
> > 	list_add(&page->lru, &vb_ptr->pages);
> > 	spin_unlock(&pages_lock);
> > 
> > 
> > 		at this point CPU2 unloads module:
> > 						leak_balloon
> > 						......
> > 
> > 		next CPU2 loads another module so code memory gets overwritten
> > 
> > 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.
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