On Thu, Nov 16, 2023 at 03:32:34PM -0500, Jeff Layton wrote: > One thing that might help would be make the nfsd threadpool more > dynamic. If it could just spin up another thread, we'd be OK. > > Maybe the server could keep an emergency thread around that is just for > processing LAYOUTRETURN/DELEGRETURN (maybe also CLOSE, etc.) calls? > Sometimes these ops are mixed into compounds with other sorts of calls > though, so we'd need to deal with that somehow. I think having an extra thread just for LAYOUTRETURN/DELEGRETURN is fundamentally the right thing to do, as they need to be processed to allow everyone else to progress. > > If nfsd threads are waiting indefinitely, that's a potential DoS > > vector. Ideally the thread should preserve the waiting request > > somehow (or return NFS4ERR_DELAY, maybe?). At some later point > > when the lease conflict is resolved, the requests can be reprocessed. > > > > That's my naive 800,000 ft view. > > That is probably a good idea on top of the above. > So you can always get stuck in that inner break_layout call. We could > try to use IOCB_NOWAIT for I/Os coming from nfsd, which would prevent > that, but that seems like it could change the I/O behavior in other ways > we don't want. It's not clear to me how that would work alongside > IOCB_SYNC anyway. So I definitively think using IOCB_NOWAIT from nfsd and not block the thread for trivial I/O is a good thing. This might even enable not offloading I/O to threads until the IOCB_NOWAIT failed. But any IOCB_NOWAIT that returned -EAGAIN needs to eventually fall back to doing blocking I/O as there is no progress guarantee for IOCB_NOWAIT, and some I/O simply is entirely impossible with IOCB_NOWAIT. > It's not immediately clear to me how we'd do this with the existing > IOCB_* flags, so we might need a new one (IOCB_NFSD?). Then, we could > just make sure that break_layout call is always nonblocking if that flag > is set. I'd name it about the behavior that it controls and not the callers, but otherwise this too seems like a good idea.
