Re: panic on 4.20 server exporting xfs filesystem

[Christoph Hellwig <hch@lst.de>]

On Fri, Mar 27, 2015 at 11:41:35AM +0100, Christoph Hellwig wrote:
> FYI, I small update on tracking down the recall issue:  this seems to
> be very much something in the callback channel on the server.  When tracing
> the client all the recalls it gets they are handled fine, but we do get
> error back in the layout recall ->done handler, which most of the time
> but not always are local Linux errnos and not nfs error numbers, indicating
> something went wrong, probably in the RPC code.

I think I've tracked down the major issue here (I think there are some more
hidding in the backchannel error handling as well):

 - the Linux NFS server completely ignores the limits the client specifies
   for the backchannel in CREATE_SESSION, most importantly the ca_maxrequests
   value.  Thus it will happily send lots of callback requests that can
   overflow the clients callback slot table.
 - even worse the Linux client has a callback slot table with just a single
   entry, so this is pretty easy to trigger.

I can try to dive into this, but it might make sense if someone more familar
with the sessions implementation could look into this issue.