netdev - Re: [Xen-devel] compound skb frag pages appearing in start

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Date:	Wed, 10 Oct 2012 14:09:58 +0100
From:	Ian Campbell <Ian.Campbell@...rix.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Eric Dumazet <edumazet@...gle.com>,
	xen-devel <xen-devel@...ts.xen.org>,
	"Konrad Rzeszutek Wilk" <konrad@...nel.org>,
	Sander Eikelenboom <linux@...elenboom.it>
Subject: Re: [Xen-devel] compound skb frag pages appearing in start_xmit

On Wed, 2012-10-10 at 11:13 +0100, Ian Campbell wrote:
> I haven't tackled netfront yet. 

I seem to be totally unable to reproduce the equivalent issue on the
netfront xmit side, even though it seems like the loop in
xennet_make_frags ought to be obviously susceptible to it.

Konrad, Sander, are either of you able to repro, e.g. with:

diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
index b06ef81..8a3f770 100644
--- a/drivers/net/xen-netfront.c
+++ b/drivers/net/xen-netfront.c
@@ -462,6 +462,8 @@ static void xennet_make_frags(struct sk_buff *skb, struct net_device *dev,
 		ref = gnttab_claim_grant_reference(&np->gref_tx_head);
 		BUG_ON((signed short)ref < 0);
 
+		BUG_ON(PageCompound(skb_frag_page(frag)));
+
 		mfn = pfn_to_mfn(page_to_pfn(skb_frag_page(frag)));
 		gnttab_grant_foreign_access_ref(ref, np->xbdev->otherend_id,
 						mfn, GNTMAP_readonly);

My repro for netback was just to netcat a wodge of data from dom0->domU
but going the other way doesn't seem to trigger.

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