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 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.

  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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