Re: [Xen-devel] [PATCH] xen-netback: count number required slots for an skb more carefully

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Wed, Sep 04, 2013 at 01:41:25PM +0100, Ian Campbell wrote:
> On Wed, 2013-09-04 at 12:48 +0100, David Vrabel wrote:
> > On 03/09/13 22:53, Wei Liu wrote:
> > > On Tue, Sep 03, 2013 at 06:29:50PM +0100, David Vrabel wrote:
> > >> From: David Vrabel <david.vrabel@xxxxxxxxxx>
> > >>
> > >> When a VM is providing an iSCSI target and the LUN is used by the
> > >> backend domain, the generated skbs for direct I/O writes to the disk
> > >> have large, multi-page skb->data but no frags.
> > >>
> > >> With some lengths and starting offsets, xen_netbk_count_skb_slots()
> > >> would be one short because the simple calculation of
> > >> DIV_ROUND_UP(skb_headlen(), PAGE_SIZE) was not accounting for the
> > >> decisions made by start_new_rx_buffer() which does not guarantee
> > >> responses are fully packed.
> > >>
> > >> For example, a skb with length < 2 pages but which spans 3 pages would
> > >> be counted as requiring 2 slots but would actually use 3 slots.
> > >>
> > >> skb->data:
> > >>
> > >>     |        1111|222222222222|3333        |
> > >>
> > >> Fully packed, this would need 2 slots:
> > >>
> > >>     |111122222222|22223333    |
> > >>
> > >> But because the 2nd page wholy fits into a slot it is not split across
> > >> slots and goes into a slot of its own:
> > >>
> > >>     |1111        |222222222222|3333        |
> > >>
> > >> Miscounting the number of slots means netback may push more responses
> > >> than the number of available requests.  This will cause the frontend
> > >> to get very confused and report "Too many frags/slots".  The frontend
> > >> never recovers and will eventually BUG.
> > >>
> > >> Fix this by counting the number of required slots more carefully.  In
> > >> xen_netbk_count_skb_slots(), more closely follow the algorithm used by
> > >> xen_netbk_gop_skb() by introducing xen_netbk_count_frag_slots() which
> > >> is the dry-run equivalent of netbk_gop_frag_copy().
> > >>
> > > 
> > > Phew! So this is backend miscounting bug. I thought it was a frontend
> > > bug so it didn't ring a bell when we had our face-to-face discussion,
> > > sorry. :-(
> > > 
> > > This bug was discussed back in July among Annie, Matt, Ian and I. We
> > > finally agreed to take Matt's solution. Matt agreed to post final
> > > version within a week but obviously he's too busy to do so. I was away
> > > so I didn't follow closely. Eventually it fell through the crack. :-(
> > 
> > I think I prefer fixing the counting for backporting to stable kernels.
> 
> That's a good argument. I think we should take this patch, or something
> very like it, now and then rebase the more complex thing on top.
> 

It's not as complex as you first see it. David's and Xi's approaches are
different routes to the same destination.

David's approach makes counting fits what netbk_gop_frag_copy actually
does, while Xi's approach is the other way around -- makes
netbk_gop_frag_copy fits what counting returns.

As long as we have them agreed with each other we're fine.

> >  Xi's approach of packing the ring differently is a change in frontend
> > visible behaviour and seems more risky. e.g., possible performance
> > impact so I would like to see some performance analysis of that approach.
> 
> Yes.

The performance impact is more concerning. I have a short analysis in
the reply to David's email.

Wei.

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