On Wed, Nov 20, 2024 at 09:27:51AM +1100, NeilBrown wrote: > On Wed, 20 Nov 2024, Chuck Lever wrote: > > On Tue, Nov 19, 2024 at 11:41:31AM +1100, NeilBrown wrote: > > > If a client ever uses the highest available slot for a given session, > > > attempt to allocate another slot so there is room for the client to use > > > more slots if wanted. GFP_NOWAIT is used so if there is not plenty of > > > free memory, failure is expected - which is what we want. It also > > > allows the allocation while holding a spinlock. > > > > > > We would expect to stablise with one more slot available than the client > > > actually uses. > > > > Which begs the question "why have a 2048 slot maximum session slot > > table size?" 1025 might work too. But is there a need for any > > maximum at all, or is this just a sanity check? > > Linux NFS presumably isn't the only client, and it might change in the > future. Maybe there is no need for a maximum. It was mostly as a > sanity check. > > It wouldn't take much to convince me to remove the limit. What's the worse that might happen if there is no cap? Can this be used as a DoS vector? If a maximum should be necessary, its value should be clearly labeled as "not an architectural limit -- for sanity checking only". > > > Now that we grow the slot table on demand we can start with a smaller > > > allocation. Define NFSD_MAX_INITIAL_SLOTS and allocate at most that > > > many when session is created. > > > > Maybe NFSD_DEFAULT_INITIAL_SLOTS is more descriptive? > > I don't think "DEFAULT" is the right word. The client requests a number > of slots. That is the "Default". The server can impose a limit - a > maximum. > Maybe we don't need a limit here either? I see. Well I don't think there needs to be a "maximum" number of initial slots. NFSD can try to allocate the number the client requested as best it can, until it hits our sane maximum above. I think sessions should have a minimum number of slots to guarantee forward progress (or IOW prevent a deadlock). I would say that number should be larger than 1 -- perhaps 2 or even 4. The problem with a small initial slot count is that means the session has a slow start heuristic. That might or might not be desirable here. -- Chuck Lever
