Re: [Xen-devel] xen-netfront possibly rides the rocket too often

[David Laight <David.Laight@xxxxxxxxxx>]

From: Wei Liu
> On Thu, May 15, 2014 at 09:46:45AM +0100, Ian Campbell wrote:
> > On Wed, 2014-05-14 at 20:49 +0100, Zoltan Kiss wrote:
> > > On 13/05/14 19:21, Stefan Bader wrote:
> > > > We had reports about this message being seen on EC2 for a while but 
> > > > finally a
> > > > reporter did notice some details about the guests and was able to 
> > > > provide a
> > > > simple way to reproduce[1].
> > > >
> > > > For my local experiments I use a Xen-4.2.2 based host (though I would 
> > > > say the
> > > > host versions are not important). The host has one NIC which is used as 
> > > > the
> > > > outgoing port of a Linux based (not openvswitch) bridge. And the PV 
> > > > guests use
> > > > that bridge. I set the mtu to 9001 (which was seen on affected instance 
> > > > types)
> > > > and also inside the guests. As described in the report one guests runs
> > > > redis-server and the other nodejs through two scripts (for me I had to 
> > > > do the
> > > > two sub.js calls in separate shells). After a bit the error messages 
> > > > appear on
> > > > the guest running the redis-server.
> > > >
> > > > I added some debug printk's to show a bit more detail about the skb and 
> > > > got the
> > > > following (<length>@<offset (after masking off complete pages)>):
> > > >
> > > > [ 698.108119] xen_netfront: xennet: skb rides the rocket: 19 slots
> > > > [ 698.108134] header 1490@238 -> 1 slots
> > > > [ 698.108139] frag #0 1614@2164 -> + 1 pages
> > > > [ 698.108143] frag #1 3038@1296 -> + 2 pages
> > > > [ 698.108147] frag #2 6076@1852 -> + 2 pages
> > > > [ 698.108151] frag #3 6076@292 -> + 2 pages
> > > > [ 698.108156] frag #4 6076@2828 -> + 3 pages
> > > > [ 698.108160] frag #5 3038@1268 -> + 2 pages
> > > > [ 698.108164] frag #6 2272@1824 -> + 1 pages
> > > > [ 698.108168] frag #7 3804@0 -> + 1 pages
> > > > [ 698.108172] frag #8 6076@264 -> + 2 pages
> > > > [ 698.108177] frag #9 3946@2800 -> + 2 pages
> > > > [ 698.108180] frags adding 18 slots
> > > >
> > > > Since I am not deeply familiar with the networking code, I wonder about 
> > > > two things:
> > > > - is there something that should limit the skb data length from all 
> > > > frags
> > > >    to stay below the 64K which the definition of MAX_SKB_FRAGS hints?
> > > I think netfront should be able to handle 64K packets at most.
> >
> > Ah, maybe this relates to this fix from Wei?
> >
> 
> Yes, below patch limits SKB size to 64KB.  However the problem here is
> not SKB exceeding 64KB. The said SKB is acutally 43KB in size. The
> problem is that guest kernel is  using compound page so a frag which can
> be fit into one 4K page spans two 4K pages.  The fix seems to be
> coalescing SKB in frontend, but it will degrade performance.

Presumably the thing to do is copy the frag list into a locally allocated
list of 4k fragments.
Unless you can support having some fragments mapped and some local
there probably isn't much point in trying to maintain the fragment
boundaries.

        David




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