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

[Sander Eikelenboom <linux@xxxxxxxxxxxxxx>]

Thursday, March 27, 2014, 3:00:14 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)
>> 
>> 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."
>> 

> Assuming it is similar, then the tx queue should be stalled. It's possible 
> the frontend is not posting enough rx requests into the ring to take the 
> packet. Can you tell if this is happening?

How would i do that ?
(and it seems unlikely since that case that triggers is .. is swamping the 
interface with both inbound and outbound traffic)

>   Paul

>> --
>> Sander




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