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Re: [Xen-devel] Error with NFS and XEN (High network load)

To: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>, Roberto Gonzalez Azevedo <rgonzalez@xxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] Error with NFS and XEN (High network load)
From: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
Date: Thu, 17 Aug 2006 02:04:23 +1000
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On Tue, Aug 15, 2006 at 10:58:15PM +0000, Roberto Gonzalez Azevedo wrote:
> On domU machine:
> root@domU:# mount -t nfs real-machine-not-xen-or-any-virtual:/mnt /data
> [ok]
> Crash:
> root@domU:# time dd if=/dev/zero of=/data/test.txt bs=16k count=16384
> [NOT OK]

Thanks for the report.  This is what I needed to reproduce the problem.
I forgot to initialise the first fragment to a proper value in the first
SG patch.

[NET] back: Initialise first fragment properly

The first fragment is used to store the pending_idx of the leading
txreq if it doesn't fit in the head area.  When it does fit into
the head we need to ensure that the first fragment contains a value
that is not equal to pending_idx as that's what we use to distinguish
between the two cases in a a number of places.

This patch sets the first fragment to ~0 which is not equal to any
valid pending_idx.  Without this initialisation, we may double-free
a pending_idx if the first fragment happened to contain a value
equal to it (this usually happened with pending_idx 0).

Signed-off-by: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>

Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@xxxxxxxxxxxxxxxxxxx>
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diff -r ec03b24a2d83 linux-2.6-xen-sparse/drivers/xen/netback/netback.c
--- a/linux-2.6-xen-sparse/drivers/xen/netback/netback.c        Tue Aug 15 
19:53:55 2006 +0100
+++ b/linux-2.6-xen-sparse/drivers/xen/netback/netback.c        Thu Aug 17 
01:57:54 2006 +1000
@@ -1147,6 +1147,8 @@ static void net_tx_action(unsigned long 
                __skb_put(skb, data_len);
                skb_shinfo(skb)->nr_frags = ret;
+               skb_shinfo(skb)->frags[0].page = (void *)~0UL;
                if (data_len < txreq.size) {
                        skb_shinfo(skb)->frags[0].page =

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