RE: [Xen-devel] Xen-unstable Linux 3.14-rc3 and 3.13 Network troubles "bisected"

[Paul Durrant]

> -----Original Message-----
> From: Sander Eikelenboom [mailto:linux@xxxxxxxxxxxxxx]
> Sent: 26 March 2014 18:08
> To: Paul Durrant
> Cc: Wei Liu; annie li; Zoltan Kiss; xen-devel@xxxxxxxxxxxxx; Ian Campbell; linux-
> kernel; netdev@xxxxxxxxxxxxxxx
> Subject: Re: [Xen-devel] Xen-unstable Linux 3.14-rc3 and 3.13 Network
> troubles "bisected"
> 
> 
> Wednesday, March 26, 2014, 6:46:06 PM, you wrote:
> 
> > Re-send shortened version...
> 
> >> -----Original Message-----
> >> From: Sander Eikelenboom [mailto:linux@xxxxxxxxxxxxxx]
> >> Sent: 26 March 2014 16:54
> >> To: Paul Durrant
> >> Cc: Wei Liu; annie li; Zoltan Kiss; xen-devel@xxxxxxxxxxxxx; Ian Campbell;
> linux-
> >> kernel; netdev@xxxxxxxxxxxxxxx
> >> Subject: Re: [Xen-devel] Xen-unstable Linux 3.14-rc3 and 3.13 Network
> >> troubles "bisected"
> >>
> > [snip]
> >> >>
> >> >> - When processing an SKB we end up in "xenvif_gop_frag_copy" while
> >> prod
> >> >> == cons ... but we still have bytes and size left ..
> >> >> - start_new_rx_buffer() has returned true ..
> >> >> - so we end up in get_next_rx_buffer
> >> >> - this does a RING_GET_REQUEST and ups cons ..
> >> >> - and we end up with a bad grant reference.
> >> >>
> >> >> Sometimes we are saved by the bell .. since additional slots have
> become
> >> >> free (you see cons become > prod in "get_next_rx_buffer" but shortly
> >> after
> >> >> that prod is increased ..
> >> >> just in time to not cause a overrun).
> >> >>
> >>
> >> > Ah, but hang on... There's a BUG_ON meta_slots_used >
> >> max_slots_needed, so if we are overflowing the worst-case calculation
> then
> >> why is that BUG_ON not firing?
> >>
> >> You mean:
> >>                 sco = (struct skb_cb_overlay *)skb->cb;
> >>                 sco->meta_slots_used = xenvif_gop_skb(skb, &npo);
> >>                 BUG_ON(sco->meta_slots_used > max_slots_needed);
> >>
> >> in "get_next_rx_buffer" ?
> >>
> 
> > That code excerpt is from net_rx_action(),isn't it?
> 
> Yes
> 
> >> I don't know .. at least now it doesn't crash dom0 and therefore not my
> >> complete machine and since tcp is recovering from a failed packet  :-)
> >>
> 
> > Well, if the code calculating max_slots_needed were underestimating then
> the BUG_ON() should fire. If it is not firing in your case then this suggests
> your problem lies elsewhere, or that meta_slots_used is not equal to the
> number of ring slots consumed.
> 
> It's seem to be the last ..
> 
> [ 1157.188908] vif vif-7-0 vif7.0: ?!? xenvif_gop_skb Me here 5 npo-
> >meta_prod:40 old_meta_prod:36 vif->rx.sring->req_prod:2105867 vif-
> >rx.req_cons:2105868 meta->gso_type:1 meta->gso_size:1448 nr_frags:1
> req->gref:657 req->id:7 estimated_slots_needed:4 j(data):1
> reserved_slots_left:-1    used in funcstart: 0 + 1 .. used_dataloop:1 ..
> used_fragloop:3
> [ 1157.244975] vif vif-7-0 vif7.0: ?!? xenvif_rx_action me here 2 ..  vif-
> >rx.sring->req_prod:2105867 vif->rx.req_cons:2105868 sco-
> >meta_slots_used:4 max_upped_gso:1 skb_is_gso(skb):1
> max_slots_needed:4 j:6 is_gso:1 nr_frags:1 firstpart:1 secondpart:2
> reserved_slots_left:-1
> 
> net_rx_action() calculated we would need 4 slots .. and sco-
> >meta_slots_used == 4 when we return so it doesn't trigger you BUG_ON ..
> 
> The 4 slots we calculated are:
>   1 slot for the data part: DIV_ROUND_UP(offset_in_page(skb->data) +
> skb_headlen(skb), PAGE_SIZE)
>   2 slots for the single frag in this SKB from: DIV_ROUND_UP(size, PAGE_SIZE)
>   1 slot since GSO
> 
> In the debug code i annotated all cons++, and the code uses 1 slot to process
> the data from the SKB as expected but uses 3 slots in the frag chopping loop.
> And when it reaches the state  were cons > prod it is always in
> "get_next_rx_buffer".
> 
> >> But probably because "npo->copy_prod++" seems to be used for the
> frags ..
> >> and it isn't added to  npo->meta_prod ?
> >>
> 
> > meta_slots_used is calculated as the value of meta_prod at return (from
> xenvif_gop_skb()) minus the value on entry ,
> > and if you look back up the code then you can see that meta_prod is
> incremented every time RING_GET_REQUEST() is evaluated.
> > So, we must be consuming a slot without evaluating RING_GET_REQUEST()
> and I think that's exactly what's happening...
> > Right at the bottom of xenvif_gop_frag_copy() req_cons is simply
> incremented in the case of a GSO. So the BUG_ON() is indeed off by one.
> 
> That is probably only done on first iteration / frag ?

Yes, the extra slot is accounted for right after the head frag is processed.

  Paul

> Because i don't see my warn there trigger .. but it could be that's because at
> that moment we still have cons <= prod.
> 
> 
> >   Paul
> 
> 

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