> 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. >>> 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
