On Wed, 2022-11-02 at 18:07 +0000, Chuck Lever III wrote: > > > On Nov 2, 2022, at 12:58 PM, Jeff Layton <jlayton@xxxxxxxxxx> wrote: > > > > On Tue, 2022-11-01 at 18:42 -0400, Jeff Layton wrote: > > > On Wed, 2022-11-02 at 09:05 +1100, NeilBrown wrote: > > > > On Wed, 02 Nov 2022, Jeff Layton wrote: > > > > > On Wed, 2022-11-02 at 08:23 +1100, NeilBrown wrote: > > > > > > On Wed, 02 Nov 2022, Jeff Layton wrote: > > > > > > > The filecache refcounting is a bit non-standard for something searchable > > > > > > > by RCU, in that we maintain a sentinel reference while it's hashed. This > > > > > > > in turn requires that we have to do things differently in the "put" > > > > > > > depending on whether its hashed, which we believe to have led to races. > > > > > > > > > > > > > > There are other problems in here too. nfsd_file_close_inode_sync can end > > > > > > > up freeing an nfsd_file while there are still outstanding references to > > > > > > > it, and there are a number of subtle ToC/ToU races. > > > > > > > > > > > > > > Rework the code so that the refcount is what drives the lifecycle. When > > > > > > > the refcount goes to zero, then unhash and rcu free the object. > > > > > > > > > > > > > > With this change, the LRU carries a reference. Take special care to > > > > > > > deal with it when removing an entry from the list. > > > > > > > > > > > > The refcounting and lru management all look sane here. > > > > > > > > > > > > You need to have moved the final put (and corresponding fsync) to > > > > > > different threads. I think I see you and Chuck discussing that and I > > > > > > have no sense of what is "right". > > > > > > > > > > > > > > > > Yeah, this is a tough call. I get Chuck's reticence. > > > > > > > > > > One thing we could consider is offloading the SYNC_NONE writeback > > > > > submission to a workqueue. I'm not sure though whether that's a win -- > > > > > it might just add needless context switches. OTOH, that would make it > > > > > fairly simple to kick off writeback when the REFERENCED flag is cleared, > > > > > which would probably be the best time to do it. > > > > > > > > > > An entry that ends up being harvested by the LRU scanner is going to be > > > > > touched by it at least twice: once to clear the REFERENCED flag, and > > > > > again ~2s later to reap it. > > > > > > > > > > If we schedule writeback when we clear the flag then we have a pretty > > > > > good indication that nothing else is going to be using it (though I > > > > > think we need to clear REFERENCED even when nfsd_file_check_writeback > > > > > returns true -- I'll fix that in the coming series). > > > > > > > > > > In any case, I'd probably like to do that sort of change in a separate > > > > > series after we get the first part sorted. > > > > > > > > > > > But it would be nice to explain in > > > > > > the comment what is being moved and why, so I could then confirm that > > > > > > the code matches the intent. > > > > > > > > > > > > > > > > I'm happy to add comments, but I'm a little unclear on what you're > > > > > confused by here. It's a bit too big of a patch for me to give a full > > > > > play-by-play description. Can you elaborate on what you'd like to see? > > > > > > > > > > > > > I don't need blow-by-blow, but all the behavioural changes should at > > > > least be flagged in the intro, and possibly explained. > > > > The one I particularly noticed is in nfsd_file_close_inode() which > > > > previously used nfsd_file_dispose_list() which hands the final close off > > > > to nfsd_filecache_wq. > > > > But this patch now does the final close in-line so an fsnotify event > > > > might now do the fsync. I was assuming that was deliberate and wanted > > > > it to be explained. But maybe it wasn't deliberate? > > > > > > > > > > Good catch! That wasn't a deliberate change, or at least I missed the > > > subtlety that the earlier code attempted to avoid it. fsnotify callbacks > > > are run under the srcu_read_lock. I don't think we want to run a fsync > > > under that if we can at all help it. > > > > > > What we can probably do is unhash it and dequeue it from the LRU, and > > > then do a refcount_dec_and_test. If that comes back true, we can then > > > queue it to the nfsd_fcache_disposal infrastructure to be closed and > > > freed. I'll have a look at that tomorrow. > > > > > > > Ok, I looked over the notification handling in here again and there is a > > bit of a dilemma: > > > > Neil is correct that we currently just put the reference directly in the > > notification callback, and if we put the last reference at that point > > then we could end up waiting on writeback. > > I expect that for an unlink, that is the common case. > > > > There are two notification callbacks: > > > > 1/ fanotify callback to tell us when the link count on a file has gone > > to 0. > > > > 2/ the setlease_notifier which is called when someone wants to do a > > setlease > > > > ...both are called under the srcu_read_lock(), and they are both fairly > > similar situations. We call different functions for them today, but we > > may be OK to unify them since their needs are similar. > > > > When I originally added these, I made them synchronous because it's best > > if nfsd can clean up and get out the way quickly when either of these > > events happen. At that time though, the code didn't call vfs_fsync at > > all, much less always on the last put. > > > > We have a couple of options: > > > > 1/ just continue to do them synchronously: We can sleep under the > > srcu_read_lock, so we can do both of those synchronously, but blocking > > it for a long period of time could cause latency issues elsewhere. > > > > 2/ move them to the delayed freeing infrastructure. That should be fine > > but we could end doing stuff like silly renaming when someone deletes an > > open file on an NFS reexport. > > Isn't the NFS re-export case handled already by nfsd_file_close_inode_sync() ? > In that case, the fsync / close is done synchronously before the unlink, but > the caller is an nfsd thread, so that should be safe. > > > > Thoughts? What's the best approach here? > > > > Maybe we should just leave them synchronous for now, and plan to address > > this in a later set? > > I need to collect some experimental evidence, but we shouldn't be adding > or removing notification calls with your patch set. It ought to be safe > to leave it for a subsequent fix. > > My current plan is to have the notifier callbacks call nfsd_file_close_inode which will unhash the things and put them on the per-net dispose list and schedule the disposal workqueue job. So, those will get cleaned up asynchronously and we shouldn't block the srcu callbacks at all. For the nfsd rename/unlink cases, we'll close things out synchronously, as we always have. I think this is probably the safest compromise for the moment and we can work to optimize other cases later. > > > > The movement of flush_delayed_fput() threw me at first, but I think I > > > > understand it now - the new code for close_inode_sync is much cleaner, > > > > not needing dispose_list_sync. > > > > > > > > > > Yep, I think this is cleaner too. > > > > > > > -- > > Jeff Layton <jlayton@xxxxxxxxxx> > > -- > Chuck Lever > > > -- Jeff Layton <jlayton@xxxxxxxxxx>
