linux-kernel - Re: [PATCH] xen-netfront: Remove BUGs on paged skb data which crosses a page boundary

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Date:	Wed, 26 Nov 2014 21:53:50 -0600
From:	Seth Forshee <seth.forshee@...onical.com>
To:	David Miller <davem@...emloft.net>
Cc:	konrad.wilk@...cle.com, boris.ostrovsky@...cle.com,
	david.vrabel@...rix.com, zoltan.kiss@...aro.org,
	eric.dumazet@...il.com, stefan.bader@...onical.com,
	xen-devel@...ts.xenproject.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] xen-netfront: Remove BUGs on paged skb data which
 crosses a page boundary

On Wed, Nov 26, 2014 at 12:28:12PM -0500, David Miller wrote:
> From: Seth Forshee <seth.forshee@...onical.com>
> Date: Tue, 25 Nov 2014 20:28:24 -0600
> 
> > These BUGs can be erroneously triggered by frags which refer to
> > tail pages within a compound page. The data in these pages may
> > overrun the hardware page while still being contained within the
> > compound page, but since compound_order() evaluates to 0 for tail
> > pages the assertion fails. The code already iterates through
> > subsequent pages correctly in this scenario, so the BUGs are
> > unnecessary and can be removed.
> > 
> > Fixes: f36c374782e4 ("xen/netfront: handle compound page fragments on transmit")
> > Cc: <stable@...r.kernel.org> # 3.7+
> > Signed-off-by: Seth Forshee <seth.forshee@...onical.com>
> 
> Can I get some Xen developer reviews?

Fwiw this issue was discussed previously and this was the recommended
fix.

 http://article.gmane.org/gmane.linux.kernel/1825381

Since then I got some feedback from a tester that he didn't see any
problems with the BUGs removed (actually replaced with a WARN so I know
that he actually saw the condition which triggered the BUG).

Thanks,
Seth
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