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 20:18
> 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, 7:15:30 PM, you wrote:
> 
> >> -----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
> 
> Ok so we are talking about:
> 
> if (*head && ((1 << gso_type) & vif->gso_mask)){
>                         vif->rx.req_cons++;
> 
> Well it had some debug code in place to detect if that path is taken as well:
> 
> [ 1157.095216] vif vif-7-0 vif7.0: ?!? xenvif_gop_frag_copy Me here end npo-
> >meta_prod:40 vif->rx.sring->req_prod:2105867 vif->rx.req_cons:2105868
> npo->copy_gref:4325379 npo->copy_off:560  MAX_BUFFER_OFFSET:4096
> bytes:560 size:0  offset:560 head:1273462060 i:3 vif->rx.sring-
> >req_event:2105868 gso_gaps:0 estimated_slots_needed:4
> reserved_slots_left:-1
> 
> Well "gso_gaps:0" indicates that in this case that path in
> "xenvif_gop_frag_copy()" has not been taken in any iteration of that frag.
> 
> However i=3 .. so we have done 3 iterations of the loop while we expected
> to do only 2 ...
> 
> So that would mean that somehow the code in "xenvif_gop_frag_copy()"
> needs more slots
> to brake this frag down than the loop with DIV_ROUND_UP(size, PAGE_SIZE)
> in net_rx_action() has accounted for.
> 

Yes. The code is not assuming worst-case page-spanning and it looks like it needs to.

I also notice a bogus clause in this if statement in start_new_rx_buffer():

        if ((offset + size > MAX_BUFFER_OFFSET) &&
            (size <= MAX_BUFFER_OFFSET) && offset && !head)
                return true;

MAX_BUFFER_OFFSET is defined to be PAGE_SIZE and xenvif_gop_frag_copy() never passes a value of size > PAGE_SIZE, so that 2nd clause is completely pointless.
I'll come up with some patches shortly.

  Paul

> 
> 
> 
> 
> 

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