Re: [PATCH v5 3/5] nfsd: rework refcounting in filecache

[Jeff Layton <jlayton@kernel.org>]

On Wed, 2022-11-02 at 18:07 +0000, Chuck Lever III wrote:
> 
> > On Nov 2, 2022, at 12:58 PM, Jeff Layton <jlayton@kernel.org> 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@kernel.org>
> 
> --
> Chuck Lever
> 
> 
> 

-- 
Jeff Layton <jlayton@kernel.org>