Re: [Xen-devel] [PATCH net v2 1/3] xen-netback: remove pointless clause from if statement

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Sander Eikelenboom [mailto:linux@xxxxxxxxxxxxxx]
> Sent: 27 March 2014 14:03
> To: Paul Durrant
> Cc: xen-devel@xxxxxxxxxxxxx; netdev@xxxxxxxxxxxxxxx; Ian Campbell; Wei Liu
> Subject: Re: [PATCH net v2 1/3] xen-netback: remove pointless clause from if
> statement
> 
> 
> Thursday, March 27, 2014, 2:54:46 PM, you wrote:
> 
> >> -----Original Message-----
> >> From: Sander Eikelenboom [mailto:linux@xxxxxxxxxxxxxx]
> >> Sent: 27 March 2014 13:46
> >> To: Paul Durrant
> >> Cc: xen-devel@xxxxxxxxxxxxx; netdev@xxxxxxxxxxxxxxx; Ian Campbell; Wei
> Liu
> >> Subject: Re: [PATCH net v2 1/3] xen-netback: remove pointless clause
> from if
> >> statement
> >>
> >>
> >> Thursday, March 27, 2014, 1:56:11 PM, you wrote:
> >>
> >> > This patch removes a test in start_new_rx_buffer() that checks whether
> >> > a copy operation is less than MAX_BUFFER_OFFSET in length, since
> >> > MAX_BUFFER_OFFSET is defined to be PAGE_SIZE and the only caller of
> >> > start_new_rx_buffer() already limits copy operations to PAGE_SIZE or
> less.
> >>
> >> > Signed-off-by: Paul Durrant <paul.durrant@xxxxxxxxxx>
> >> > Cc: Ian Campbell <ian.campbell@xxxxxxxxxx>
> >> > Cc: Wei Liu <wei.liu2@xxxxxxxxxx>
> >> > Cc: Sander Eikelenboom <linux@xxxxxxxxxxxxxx>
> >> > ---
> >>
> >> > v2:
> >> >  - Add BUG_ON() as suggested by Ian Campbell
> >>
> >> >  drivers/net/xen-netback/netback.c |    4 ++--
> >> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >>
> >> > diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-
> >> netback/netback.c
> >> > index 438d0c0..72314c7 100644
> >> > --- a/drivers/net/xen-netback/netback.c
> >> > +++ b/drivers/net/xen-netback/netback.c
> >> > @@ -192,8 +192,8 @@ static bool start_new_rx_buffer(int offset,
> >> unsigned long size, int head)
> >> >          * into multiple copies tend to give large frags their
> >> >          * own buffers as before.
> >> >          */
> >> > -       if ((offset + size > MAX_BUFFER_OFFSET) &&
> >> > -           (size <= MAX_BUFFER_OFFSET) && offset && !head)
> >> > +       BUG_ON(size > MAX_BUFFER_OFFSET);
> >> > +       if ((offset + size > MAX_BUFFER_OFFSET) && offset && !head)
> >> >                 return true;
> >> >
> >> >         return false;
> >>
> >> Hi Paul,
> >>
> >> Unfortunately .. no good ..
> >>
> >> With these patches (v2) applied to 3.14-rc8 it all seems to work well,
> >> until i do my test case .. it still chokes and now effectively permanently
> stalls
> >> network traffic to that guest.
> >>
> >> No error messages or anything in either xl dmesg or dmesg on the host ..
> and
> >> nothing in dmesg in the guest either.
> >>
> >> But in the guest the TX bytes ifconfig reports for eth0 still increase but 
> >> RX
> >> bytes does nothing, so it seems only the RX path is effected)
> >>
> 
> > But you're not getting ring overflow, right? So that suggests this series is
> working and you're now hitting another problem? I don't see how these
> patches could directly cause the new behaviour you're seeing.
> 
> Don't know  .. how ever .. i previously tested:
>         - unconditionally doing "max_slots_needed + 1"  in "net_rx_action()",
> and that circumvented the problem reliably without causing anything else
>         - reverting the calculation of "max_slots_needed + 1"  in
> "net_rx_action()" to what it was before :
>                 int max = DIV_ROUND_UP(vif->dev->mtu, PAGE_SIZE);
>                 if (vif->can_sg || vif->gso_mask || vif->gso_prefix_mask)
>                         max += MAX_SKB_FRAGS + 1; /* extra_info + frags */
> 

So, it may be that the worse-case estimate is now too bad. In the case where 
it's failing for you it would be nice to know what the estimate was. Looking at 
netfront, we could be in trouble if it ever goes above 64.

  Paul

> So that leads me to think it's something caused by this patch set.
> Patch1 could be a candidate .. perhaps that check was needed for some
> reason .. will see what not applying that one does
> 
> --
> Sander
> 
> 
> >   Paul
> 
> >> So it now seems i now have the situation which you described in the
> commit
> >> message from "ca2f09f2b2c6c25047cfc545d057c4edfcfe561c",
> >> "Without this patch I can trivially stall netback permanently by just 
> >> doing a
> >> large guest to guest file copy between two Windows Server 2008R2 VMs
> on a
> >> single host."
> >>
> >> --
> >> Sander
> 
> 


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