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

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Message-ID: <20120814083320.GA3597@redhat.com>
Date:	Tue, 14 Aug 2012 11:33:20 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Rafael Aquini <aquini@...hat.com>, 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 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?

-- 
MST
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