Re: [PATCH] nfsd: don't free files unconditionally in __nfsd_file_cache_purge

[Jeff Layton <jlayton@kernel.org>]

On Sat, 2023-01-21 at 17:04 +0000, Chuck Lever III wrote:
> 
> > On Jan 20, 2023, at 3:54 PM, Jeff Layton <jlayton@kernel.org> wrote:
> > 
> > On Fri, 2023-01-20 at 20:21 +0000, Chuck Lever III wrote:
> > > 
> > > > On Jan 20, 2023, at 2:52 PM, Jeff Layton <jlayton@kernel.org> wrote:
> > > > 
> > > > nfsd_file_cache_purge is called when the server is shutting down, in
> > > > which case, tearing things down is generally fine, but it also gets
> > > > called when the exports cache is flushed.
> > > 
> > > Yeah... cache flush is probably the case we've been missing.
> > > 
> > > 
> > > > Instead of walking the cache and freeing everything unconditionally,
> > > > handle it the same as when we have a notification of conflicting access.
> > > > 
> > > > Fixes: ac3a2585f018 ("nfsd: rework refcounting in filecache")
> > > > Reported-by: Ruben Vestergaard <rubenv@drcmr.dk>
> > > > Reported-by: Torkil Svensgaard <torkil@drcmr.dk>
> > > > Reported-by: Shachar Kagan <skagan@nvidia.com>
> > > > Signed-off-by: Jeff Layton <jlayton@kernel.org>
> > > > ---
> > > > fs/nfsd/filecache.c | 61 +++++++++++++++++++++++++++------------------
> > > > 1 file changed, 37 insertions(+), 24 deletions(-)
> > > > 
> > > > v2: use the same method to purge entries from the cache as we do when
> > > >   there is a notification of conflicting access.
> > > > 
> > > > diff --git a/fs/nfsd/filecache.c b/fs/nfsd/filecache.c
> > > > index 58ac93e7e680..397ae212b98d 100644
> > > > --- a/fs/nfsd/filecache.c
> > > > +++ b/fs/nfsd/filecache.c
> > > > @@ -661,6 +661,40 @@ static struct shrinker	nfsd_file_shrinker = {
> > > > 	.seeks = 1,
> > > > };
> > > > 
> > > > +/**
> > > > + * maybe_queue_nfsd_file - try to unhash and queue a nfsd_file to be freed
> > > > + * @nf: nfsd_file to attempt to queue
> > > > + * @dispose: private list to queue successfully-put objects
> > > > + *
> > > > + * Unhash an nfsd_file, try to get a reference to it, and then put that
> > > > + * reference. If it's the last reference, queue it to the dispose list.
> > > > + *
> > > > + * The caller MUST hold the rcu_read_lock() !
> > > 
> > > __nfsd_file_cache_purge() isn't holding rcu_read_lock(), it's
> > > holding the nfsd_mutex. Is this comment incorrect, or is it just
> > > too specific? Or did I miss something obvious?
> > > 
> > 
> > It's implicitly taken by rhashtable_walk_start and released by
> > rhashtable_walk_stop.
> > 
> > FWIW, it'd be nice if there were a lockdep_assert_held equivalent for
> > the rcu_read_lock() here, but I didn't see one of those.
> 
> Would
> 
> static void
> nfsd_file_cond_queue(struct nfsd_file *nf, struct list_head *dispose)   
> 	__must_hold(RCU)
> {
> 
> do the trick?
> 

Seems like a reasonable thing to add.

> 
> > There is a
> > rcu_read_lock_held(), but I didn't see a good way to get that to compile
> > out when lockdep was disabled.
> > 
> > > > + */
> > > > +static void
> > > > +maybe_queue_nfsd_file(struct nfsd_file *nf, struct list_head *dispose)
> > > 
> > > I prefer the name nfsd_file_try_to_queue() or nfsd_file_try_to_dispose().
> > > nfsd_file_ should be the prefix where possible. Unless you're
> > > redriving, I can fix that.
> > > 
> > > 
> > 
> > Rename at will.
> 
> Done and applied to nfsd-fixes, replacing v1 of this patch.
> 

LGTM. Thanks!

> 
> > > > +{
> > > > +	int decrement = 1;
> > > > +
> > > > +	/* If we raced with someone else unhashing, ignore it */
> > > > +	if (!nfsd_file_unhash(nf))
> > > > +		return;
> > > > +
> > > > +	/* If we can't get a reference, ignore it */
> > > > +	if (!nfsd_file_get(nf))
> > > > +		return;
> > > > +
> > > > +	/* Extra decrement if we remove from the LRU */
> > > > +	if (nfsd_file_lru_remove(nf))
> > > > +		++decrement;
> > > > +
> > > > +	/* If refcount goes to 0, then put on the dispose list */
> > > > +	if (refcount_sub_and_test(decrement, &nf->nf_ref)) {
> > > > +		list_add(&nf->nf_lru, dispose);
> > > > +		trace_nfsd_file_closing(nf);
> > > > +	}
> > > > +}
> > > > +
> > > > /**
> > > > * nfsd_file_queue_for_close: try to close out any open nfsd_files for an inode
> > > > * @inode:   inode on which to close out nfsd_files
> > > > @@ -688,30 +722,12 @@ nfsd_file_queue_for_close(struct inode *inode, struct list_head *dispose)
> > > > 
> > > > 	rcu_read_lock();
> > > > 	do {
> > > > -		int decrement = 1;
> > > > -
> > > > 		nf = rhashtable_lookup(&nfsd_file_rhash_tbl, &key,
> > > > 				       nfsd_file_rhash_params);
> > > > 		if (!nf)
> > > > 			break;
> > > > 
> > > > -		/* If we raced with someone else unhashing, ignore it */
> > > > -		if (!nfsd_file_unhash(nf))
> > > > -			continue;
> > > > -
> > > > -		/* If we can't get a reference, ignore it */
> > > > -		if (!nfsd_file_get(nf))
> > > > -			continue;
> > > > -
> > > > -		/* Extra decrement if we remove from the LRU */
> > > > -		if (nfsd_file_lru_remove(nf))
> > > > -			++decrement;
> > > > -
> > > > -		/* If refcount goes to 0, then put on the dispose list */
> > > > -		if (refcount_sub_and_test(decrement, &nf->nf_ref)) {
> > > > -			list_add(&nf->nf_lru, dispose);
> > > > -			trace_nfsd_file_closing(nf);
> > > > -		}
> > > > +		maybe_queue_nfsd_file(nf, dispose);
> > > > 	} while (1);
> > > > 	rcu_read_unlock();
> > > > }
> > > > @@ -928,11 +944,8 @@ __nfsd_file_cache_purge(struct net *net)
> > > > 
> > > > 		nf = rhashtable_walk_next(&iter);
> > > > 		while (!IS_ERR_OR_NULL(nf)) {
> > > > -			if (!net || nf->nf_net == net) {
> > > > -				nfsd_file_unhash(nf);
> > > > -				nfsd_file_lru_remove(nf);
> > > > -				list_add(&nf->nf_lru, &dispose);
> > > > -			}
> > > > +			if (!net || nf->nf_net == net)
> > > > +				maybe_queue_nfsd_file(nf, &dispose);
> > > > 			nf = rhashtable_walk_next(&iter);
> > > > 		}
> > > > 
> > > > -- 
> > > > 2.39.0
> > > > 
> > > 
> > > --
> > > Chuck Lever
> > > 
> > > 
> > > 
> > 
> > -- 
> > Jeff Layton <jlayton@kernel.org>
> 
> --
> Chuck Lever
> 
> 
> 

-- 
Jeff Layton <jlayton@kernel.org>