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@xxxxxxxxxx>
