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

[Jeff Layton <jlayton@kernel.org>]

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.

> 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>