Re: [PATCH] sunrpc: make warning in svc_check_conn_limits() more generic

[Neil Brown <neilb@suse.de>]

On Thursday October 16, jlayton@redhat.com wrote:
> 
> Thanks for the info Neil, that helps clarify this...
> 
> Using RLIMIT_NOFILE is an interesting idea. From a cursory look at the
> code, the default for RLIMIT_NOFILE looks like it's generally 1024.
> We'll have to figure that this limit will effectively act as a limit on
> the number of concurrent lockd clients. It's not too hard to imagine a
> server with more clients than this (think of a large compute cluster).

If all those clients used UDP, this would not be a problem.  While I
see the value of TCP for NFS, it doesn't seem as convincing for NLM.

But I don't expect we have the luxury of insisting that clients use
UDP for locking :-(

> 
> The problem as you mention, is that that limit won't be easily tunable.
> I think we need some mechanism for an admin to tune this limit. It
> doesn't have to be tunable on the fly, but shouldn't require a kernel
> rebuild. We could eliminate this check for single-threaded services
> entirely, but I suppose that leaves the door open for DoS attacks
> against those services.
> 
> Maybe the best thing is to go with Bruce's idea and add a sv_maxconn
> field to the svc_serv struct. We could make that default to the max of
> RLIMIT_NOFILE rlim_cur value or the currently calculated value.
> Eventually we could add a mechanism to allow  someone to tune that
> value. A module parameter would probably be fine for lockd. We might
> even want to set the limit lower for things like the nfsv4 callback
> thread.
> 
> Thoughts?

A per-service setting that defaults to something reasonable like your
suggestions and can be over-ridden by a module parameter sounds like a
good idea.
If you change the module parameter via
 /sys/modules/lockd/parameters/max_connections
then it wouldn't take effect until the service were stopped and restarted,
but I expect that is acceptable (and could probably be 'fixed' if really
needed).

NeilBrown