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

[Jeff Layton <jlayton@kernel.org>]

On Tue, 2022-11-01 at 17:25 +0000, Chuck Lever III wrote:
> 
> > On Nov 1, 2022, at 10:46 AM, Jeff Layton <jlayton@kernel.org> 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.
> 
> Handful of minor nits below. I'm adding some remarks from the
> other patches here for my own convenience. Nothing earth-
> shattering, the series looks pretty close.
> 
> Test results:
> 
> When I run my test I "watch cat /proc/fs/nfsd/filecache". The
> workload is 12-thread "untar git && make git && make test" on
> NFSv3. It's showing worse eviction behavior than the current
> code.
> 

What do you mean by "worse" here?

> Basically all cached items appear to be immediately placed on
> the LRU. Do you expect this behavior change? We want to keep
> the LRU as short as possible; but maybe the LRU callback is
> stopping after a few items, so it might not matter.
> 

Could be. I'm not sure how that works.

> Can you look into it before I apply? I suspect it's because
> the list_lru_isolate() call is moved after the REFERENCED
> check.
> 

I don't see how we can change the order of operations in the LRU
callback without reintroducing race conditions. The existing check is:

	if (refcount_read(&nf->nf_ref) > 1) {
		list_lru_isolate(lru, &nf->nf_lru);

...which was racy, of course...

> 
> > Signed-off-by: Jeff Layton <jlayton@kernel.org>
> > ---
> > fs/nfsd/filecache.c | 247 ++++++++++++++++++++++----------------------
> > fs/nfsd/trace.h     |   1 +
> > 2 files changed, 123 insertions(+), 125 deletions(-)
> > 
> > diff --git a/fs/nfsd/filecache.c b/fs/nfsd/filecache.c
> > index 0bf3727455e2..e67297ad12bf 100644
> > --- a/fs/nfsd/filecache.c
> > +++ b/fs/nfsd/filecache.c
> > @@ -303,8 +303,7 @@ nfsd_file_alloc(struct nfsd_file_lookup_key *key, unsigned int may)
> > 		if (key->gc)
> > 			__set_bit(NFSD_FILE_GC, &nf->nf_flags);
> > 		nf->nf_inode = key->inode;
> > -		/* nf_ref is pre-incremented for hash table */
> > -		refcount_set(&nf->nf_ref, 2);
> > +		refcount_set(&nf->nf_ref, 1);
> > 		nf->nf_may = key->need;
> > 		nf->nf_mark = NULL;
> > 	}
> > @@ -353,24 +352,35 @@ nfsd_file_unhash(struct nfsd_file *nf)
> > 	return false;
> > }
> > 
> > -static bool
> > +static void
> > nfsd_file_free(struct nfsd_file *nf)
> > {
> > 	s64 age = ktime_to_ms(ktime_sub(ktime_get(), nf->nf_birthtime));
> > -	bool flush = false;
> > 
> > 	trace_nfsd_file_free(nf);
> > 
> > 	this_cpu_inc(nfsd_file_releases);
> > 	this_cpu_add(nfsd_file_total_age, age);
> > 
> > +	nfsd_file_unhash(nf);
> > +
> > +	/*
> > +	 * We call fsync here in order to catch writeback errors. It's not
> > +	 * strictly required by the protocol, but an nfsd_file coule get
> 
> ^coule^could
> 
> > +	 * evicted from the cache before a COMMIT comes in. If another
> > +	 * task were to open that file in the interim and scrape the error,
> > +	 * then the client may never see it. By calling fsync here, we ensure
> > +	 * that writeback happens before the entry is freed, and that any
> > +	 * errors reported result in the write verifier changing.
> > +	 */
> > +	nfsd_file_fsync(nf);
> 
> I'm wondering if this call should be preceded by
> 
> 	if (test_bit(NFSD_FILE_GC, &nf->nf_flags))
> 
> as you suggested last week.
> 
> 
> > +
> > 	if (nf->nf_mark)
> > 		nfsd_file_mark_put(nf->nf_mark);
> > 	if (nf->nf_file) {
> > 		get_file(nf->nf_file);
> > 		filp_close(nf->nf_file, NULL);
> > 		fput(nf->nf_file);
> > -		flush = true;
> > 	}
> > 
> > 	/*
> > @@ -378,10 +388,9 @@ nfsd_file_free(struct nfsd_file *nf)
> > 	 * WARN and leak it to preserve system stability.
> > 	 */
> > 	if (WARN_ON_ONCE(!list_empty(&nf->nf_lru)))
> > -		return flush;
> > +		return;
> > 
> > 	call_rcu(&nf->nf_rcu, nfsd_file_slab_free);
> > -	return flush;
> > }
> > 
> > static bool
> > @@ -397,17 +406,23 @@ nfsd_file_check_writeback(struct nfsd_file *nf)
> > 		mapping_tagged(mapping, PAGECACHE_TAG_WRITEBACK);
> > }
> > 
> > -static void nfsd_file_lru_add(struct nfsd_file *nf)
> > +static bool nfsd_file_lru_add(struct nfsd_file *nf)
> > {
> > 	set_bit(NFSD_FILE_REFERENCED, &nf->nf_flags);
> > -	if (list_lru_add(&nfsd_file_lru, &nf->nf_lru))
> > +	if (list_lru_add(&nfsd_file_lru, &nf->nf_lru)) {
> > 		trace_nfsd_file_lru_add(nf);
> > +		return true;
> > +	}
> > +	return false;
> > }
> > 
> > -static void nfsd_file_lru_remove(struct nfsd_file *nf)
> > +static bool nfsd_file_lru_remove(struct nfsd_file *nf)
> > {
> > -	if (list_lru_del(&nfsd_file_lru, &nf->nf_lru))
> > +	if (list_lru_del(&nfsd_file_lru, &nf->nf_lru)) {
> > 		trace_nfsd_file_lru_del(nf);
> > +		return true;
> > +	}
> > +	return false;
> > }
> > 
> > struct nfsd_file *
> > @@ -418,86 +433,80 @@ nfsd_file_get(struct nfsd_file *nf)
> > 	return NULL;
> > }
> > 
> > -static void
> > +/**
> > + * nfsd_file_unhash_and_queue - unhash a file and queue it to the dispose list
> > + * @nf: nfsd_file to be unhashed and queued
> > + * @dispose: list to which it should be queued
> > + *
> > + * Attempt to unhash a nfsd_file and queue it to the given list. Each file
> > + * will have a reference held on behalf of the list. That reference may come
> > + * from the LRU, or we may need to take one. If we can't get a reference,
> > + * ignore it altogether.
> > + */
> > +static bool
> > nfsd_file_unhash_and_queue(struct nfsd_file *nf, struct list_head *dispose)
> > {
> > 	trace_nfsd_file_unhash_and_queue(nf);
> > 	if (nfsd_file_unhash(nf)) {
> > -		/* caller must call nfsd_file_dispose_list() later */
> > -		nfsd_file_lru_remove(nf);
> > +		/*
> > +		 * If we remove it from the LRU, then just use that
> > +		 * reference for the dispose list. Otherwise, we need
> > +		 * to take a reference. If that fails, just ignore
> > +		 * the file altogether.
> > +		 */
> > +		if (!nfsd_file_lru_remove(nf) && !nfsd_file_get(nf))
> > +			return false;
> > 		list_add(&nf->nf_lru, dispose);
> > +		return true;
> > 	}
> > +	return false;
> > }
> > 
> > -static void
> > -nfsd_file_put_noref(struct nfsd_file *nf)
> > -{
> > -	trace_nfsd_file_put(nf);
> > -
> > -	if (refcount_dec_and_test(&nf->nf_ref)) {
> > -		WARN_ON(test_bit(NFSD_FILE_HASHED, &nf->nf_flags));
> > -		nfsd_file_lru_remove(nf);
> > -		nfsd_file_free(nf);
> > -	}
> > -}
> > -
> > -static void
> > -nfsd_file_unhash_and_put(struct nfsd_file *nf)
> > -{
> > -	if (nfsd_file_unhash(nf))
> > -		nfsd_file_put_noref(nf);
> > -}
> > -
> > +/**
> > + * nfsd_file_put - put the reference to a nfsd_file
> > + * @nf: nfsd_file of which to put the reference
> > + *
> > + * Put a reference to a nfsd_file. In the v4 case, we just put the
> > + * reference immediately. In the GC case, if the reference would be
> > + * the last one, the put it on the LRU instead to be cleaned up later.
> > + */
> > void
> > nfsd_file_put(struct nfsd_file *nf)
> > {
> > 	might_sleep();
> > +	trace_nfsd_file_put(nf);
> > 
> > -	if (test_bit(NFSD_FILE_GC, &nf->nf_flags))
> > -		nfsd_file_lru_add(nf);
> > -	else if (refcount_read(&nf->nf_ref) == 2)
> > -		nfsd_file_unhash_and_put(nf);
> > -
> > -	if (!test_bit(NFSD_FILE_HASHED, &nf->nf_flags)) {
> > -		nfsd_file_fsync(nf);
> > -		nfsd_file_put_noref(nf);
> > -	} else if (nf->nf_file && test_bit(NFSD_FILE_GC, &nf->nf_flags)) {
> > -		nfsd_file_put_noref(nf);
> > -		nfsd_file_schedule_laundrette();
> > -	} else
> > -		nfsd_file_put_noref(nf);
> > -}
> > -
> > -static void
> > -nfsd_file_dispose_list(struct list_head *dispose)
> > -{
> > -	struct nfsd_file *nf;
> > -
> > -	while(!list_empty(dispose)) {
> > -		nf = list_first_entry(dispose, struct nfsd_file, nf_lru);
> > -		list_del_init(&nf->nf_lru);
> > -		nfsd_file_fsync(nf);
> > -		nfsd_file_put_noref(nf);
> > +	/*
> > +	 * The HASHED check is racy. We may end up with the occasional
> > +	 * unhashed entry on the LRU, but they should get cleaned up
> > +	 * like any other.
> > +	 */
> > +	if (test_bit(NFSD_FILE_GC, &nf->nf_flags) &&
> > +	    test_bit(NFSD_FILE_HASHED, &nf->nf_flags)) {
> > +		/*
> > +		 * If this is the last reference (nf_ref == 1), then transfer
> > +		 * it to the LRU. If the add to the LRU fails, just put it as
> > +		 * usual.
> > +		 */
> > +		if (refcount_dec_not_one(&nf->nf_ref) || nfsd_file_lru_add(nf)) {
> > +			nfsd_file_schedule_laundrette();
> > +			return;
> > +		}
> 
> A subsequent patch adds the words "fall through" in a comment, and
> checkscript flags that. I changed that comment to:
> 
> +                       /*
> +                        * We're racing with unhashing, so try to remove it from
> +                        * the LRU. If removal fails, then someone else already
> +                        * has our reference.
> +                        */
> 
> to mollify it.
> 
> 
> > 	}
> > +	if (refcount_dec_and_test(&nf->nf_ref))
> > +		nfsd_file_free(nf);
> > }
> > 
> > static void
> > -nfsd_file_dispose_list_sync(struct list_head *dispose)
> > +nfsd_file_dispose_list(struct list_head *dispose)
> > {
> > -	bool flush = false;
> > 	struct nfsd_file *nf;
> > 
> > 	while(!list_empty(dispose)) {
> > 		nf = list_first_entry(dispose, struct nfsd_file, nf_lru);
> > 		list_del_init(&nf->nf_lru);
> > -		nfsd_file_fsync(nf);
> > -		if (!refcount_dec_and_test(&nf->nf_ref))
> > -			continue;
> > -		if (nfsd_file_free(nf))
> > -			flush = true;
> > +		nfsd_file_free(nf);
> > 	}
> > -	if (flush)
> > -		flush_delayed_fput();
> > }
> > 
> > static void
> > @@ -567,21 +576,8 @@ nfsd_file_lru_cb(struct list_head *item, struct list_lru_one *lru,
> > 	struct list_head *head = arg;
> > 	struct nfsd_file *nf = list_entry(item, struct nfsd_file, nf_lru);
> > 
> > -	/*
> > -	 * Do a lockless refcount check. The hashtable holds one reference, so
> > -	 * we look to see if anything else has a reference, or if any have
> > -	 * been put since the shrinker last ran. Those don't get unhashed and
> > -	 * released.
> > -	 *
> > -	 * Note that in the put path, we set the flag and then decrement the
> > -	 * counter. Here we check the counter and then test and clear the flag.
> > -	 * That order is deliberate to ensure that we can do this locklessly.
> > -	 */
> > -	if (refcount_read(&nf->nf_ref) > 1) {
> > -		list_lru_isolate(lru, &nf->nf_lru);
> > -		trace_nfsd_file_gc_in_use(nf);
> > -		return LRU_REMOVED;
> > -	}
> > +	/* We should only be dealing with GC entries here */
> > +	WARN_ON_ONCE(!test_bit(NFSD_FILE_GC, &nf->nf_flags));
> > 
> > 	/*
> > 	 * Don't throw out files that are still undergoing I/O or
> > @@ -592,40 +588,30 @@ nfsd_file_lru_cb(struct list_head *item, struct list_lru_one *lru,
> > 		return LRU_SKIP;
> > 	}
> > 
> > +	/* If it was recently added to the list, skip it */
> > 	if (test_and_clear_bit(NFSD_FILE_REFERENCED, &nf->nf_flags)) {
> > 		trace_nfsd_file_gc_referenced(nf);
> > 		return LRU_ROTATE;
> > 	}
> > 
> > -	if (!test_and_clear_bit(NFSD_FILE_HASHED, &nf->nf_flags)) {
> > -		trace_nfsd_file_gc_hashed(nf);
> > -		return LRU_SKIP;
> > +	/*
> > +	 * Put the reference held on behalf of the LRU. If it wasn't the last
> > +	 * one, then just remove it from the LRU and ignore it.
> > +	 */
> > +	if (!refcount_dec_and_test(&nf->nf_ref)) {
> > +		trace_nfsd_file_gc_in_use(nf);
> > +		list_lru_isolate(lru, &nf->nf_lru);
> > +		return LRU_REMOVED;
> > 	}
> > 
> > +	/* Refcount went to zero. Unhash it and queue it to the dispose list */
> > +	nfsd_file_unhash(nf);
> > 	list_lru_isolate_move(lru, &nf->nf_lru, head);
> > 	this_cpu_inc(nfsd_file_evictions);
> > 	trace_nfsd_file_gc_disposed(nf);
> > 	return LRU_REMOVED;
> > }
> > 
> > -/*
> > - * Unhash items on @dispose immediately, then queue them on the
> > - * disposal workqueue to finish releasing them in the background.
> > - *
> > - * cel: Note that between the time list_lru_shrink_walk runs and
> > - * now, these items are in the hash table but marked unhashed.
> > - * Why release these outside of lru_cb ? There's no lock ordering
> > - * problem since lru_cb currently takes no lock.
> > - */
> > -static void nfsd_file_gc_dispose_list(struct list_head *dispose)
> > -{
> > -	struct nfsd_file *nf;
> > -
> > -	list_for_each_entry(nf, dispose, nf_lru)
> > -		nfsd_file_hash_remove(nf);
> > -	nfsd_file_dispose_list_delayed(dispose);
> > -}
> > -
> > static void
> > nfsd_file_gc(void)
> > {
> > @@ -635,7 +621,7 @@ nfsd_file_gc(void)
> > 	ret = list_lru_walk(&nfsd_file_lru, nfsd_file_lru_cb,
> > 			    &dispose, list_lru_count(&nfsd_file_lru));
> > 	trace_nfsd_file_gc_removed(ret, list_lru_count(&nfsd_file_lru));
> > -	nfsd_file_gc_dispose_list(&dispose);
> > +	nfsd_file_dispose_list_delayed(&dispose);
> > }
> > 
> > static void
> > @@ -660,7 +646,7 @@ nfsd_file_lru_scan(struct shrinker *s, struct shrink_control *sc)
> > 	ret = list_lru_shrink_walk(&nfsd_file_lru, sc,
> > 				   nfsd_file_lru_cb, &dispose);
> > 	trace_nfsd_file_shrinker_removed(ret, list_lru_count(&nfsd_file_lru));
> > -	nfsd_file_gc_dispose_list(&dispose);
> > +	nfsd_file_dispose_list_delayed(&dispose);
> > 	return ret;
> > }
> > 
> > @@ -671,8 +657,11 @@ static struct shrinker	nfsd_file_shrinker = {
> > };
> > 
> > /*
> > - * Find all cache items across all net namespaces that match @inode and
> > - * move them to @dispose. The lookup is atomic wrt nfsd_file_acquire().
> > + * Find all cache items across all net namespaces that match @inode, unhash
> > + * them, take references and then put them on @dispose if that was successful.
> > + *
> > + * The nfsd_file objects on the list will be unhashed, and each will have a
> > + * reference taken.
> >  */
> > static unsigned int
> > __nfsd_file_close_inode(struct inode *inode, struct list_head *dispose)
> > @@ -690,8 +679,9 @@ __nfsd_file_close_inode(struct inode *inode, struct list_head *dispose)
> > 				       nfsd_file_rhash_params);
> > 		if (!nf)
> > 			break;
> > -		nfsd_file_unhash_and_queue(nf, dispose);
> > -		count++;
> > +
> > +		if (nfsd_file_unhash_and_queue(nf, dispose))
> > +			count++;
> > 	} while (1);
> > 	rcu_read_unlock();
> > 	return count;
> > @@ -703,15 +693,23 @@ __nfsd_file_close_inode(struct inode *inode, struct list_head *dispose)
> >  *
> >  * Unhash and put all cache item associated with @inode.
> >  */
> > -static void
> > +static unsigned int
> > nfsd_file_close_inode(struct inode *inode)
> > {
> > -	LIST_HEAD(dispose);
> > +	struct nfsd_file *nf;
> > 	unsigned int count;
> > +	LIST_HEAD(dispose);
> > 
> > 	count = __nfsd_file_close_inode(inode, &dispose);
> > 	trace_nfsd_file_close_inode(inode, count);
> > -	nfsd_file_dispose_list_delayed(&dispose);
> > +	while(!list_empty(&dispose)) {
> 
> checkpatch complained about wanting whitespace between "while"
> and "(!list_empty())".
> 
> 
> > +		nf = list_first_entry(&dispose, struct nfsd_file, nf_lru);
> > +		list_del_init(&nf->nf_lru);
> > +		trace_nfsd_file_closing(nf);
> > +		if (refcount_dec_and_test(&nf->nf_ref))
> > +			nfsd_file_free(nf);
> > +	}
> > +	return count;
> > }
> > 
> > /**
> > @@ -723,19 +721,15 @@ nfsd_file_close_inode(struct inode *inode)
> > void
> > nfsd_file_close_inode_sync(struct inode *inode)
> > {
> > -	LIST_HEAD(dispose);
> > -	unsigned int count;
> > -
> > -	count = __nfsd_file_close_inode(inode, &dispose);
> > -	trace_nfsd_file_close_inode_sync(inode, count);
> > -	nfsd_file_dispose_list_sync(&dispose);
> > +	if (nfsd_file_close_inode(inode))
> > +		flush_delayed_fput();
> > }
> > 
> > /**
> >  * nfsd_file_delayed_close - close unused nfsd_files
> >  * @work: dummy
> >  *
> > - * Walk the LRU list and close any entries that have not been used since
> > + * Walk the LRU list and destroy any entries that have not been used since
> >  * the last scan.
> >  */
> > static void
> > @@ -1056,8 +1050,10 @@ nfsd_file_do_acquire(struct svc_rqst *rqstp, struct svc_fh *fhp,
> > 	rcu_read_lock();
> > 	nf = rhashtable_lookup(&nfsd_file_rhash_tbl, &key,
> > 			       nfsd_file_rhash_params);
> > -	if (nf)
> > -		nf = nfsd_file_get(nf);
> > +	if (nf) {
> > +		if (!nfsd_file_lru_remove(nf))
> > +			nf = nfsd_file_get(nf);
> > +	}
> > 	rcu_read_unlock();
> > 	if (nf)
> > 		goto wait_for_construction;
> > @@ -1092,11 +1088,11 @@ nfsd_file_do_acquire(struct svc_rqst *rqstp, struct svc_fh *fhp,
> > 			goto out;
> > 		}
> > 		open_retry = false;
> > -		nfsd_file_put_noref(nf);
> > +		if (refcount_dec_and_test(&nf->nf_ref))
> > +			nfsd_file_free(nf);
> > 		goto retry;
> > 	}
> > 
> > -	nfsd_file_lru_remove(nf);
> > 	this_cpu_inc(nfsd_file_cache_hits);
> > 
> > 	status = nfserrno(nfsd_open_break_lease(file_inode(nf->nf_file), may_flags));
> > @@ -1106,7 +1102,8 @@ nfsd_file_do_acquire(struct svc_rqst *rqstp, struct svc_fh *fhp,
> > 			this_cpu_inc(nfsd_file_acquisitions);
> > 		*pnf = nf;
> > 	} else {
> > -		nfsd_file_put(nf);
> > +		if (refcount_dec_and_test(&nf->nf_ref))
> > +			nfsd_file_free(nf);
> > 		nf = NULL;
> > 	}
> > 
> > @@ -1133,7 +1130,7 @@ nfsd_file_do_acquire(struct svc_rqst *rqstp, struct svc_fh *fhp,
> > 	 * then unhash.
> > 	 */
> > 	if (status != nfs_ok || key.inode->i_nlink == 0)
> > -		nfsd_file_unhash_and_put(nf);
> > +		nfsd_file_unhash(nf);
> > 	clear_bit_unlock(NFSD_FILE_PENDING, &nf->nf_flags);
> > 	smp_mb__after_atomic();
> > 	wake_up_bit(&nf->nf_flags, NFSD_FILE_PENDING);
> > diff --git a/fs/nfsd/trace.h b/fs/nfsd/trace.h
> > index 940252482fd4..a44ded06af87 100644
> > --- a/fs/nfsd/trace.h
> > +++ b/fs/nfsd/trace.h
> > @@ -906,6 +906,7 @@ DEFINE_EVENT(nfsd_file_class, name, \
> > DEFINE_NFSD_FILE_EVENT(nfsd_file_free);
> > DEFINE_NFSD_FILE_EVENT(nfsd_file_unhash);
> > DEFINE_NFSD_FILE_EVENT(nfsd_file_put);
> > +DEFINE_NFSD_FILE_EVENT(nfsd_file_closing);
> > DEFINE_NFSD_FILE_EVENT(nfsd_file_unhash_and_queue);
> > 
> > TRACE_EVENT(nfsd_file_alloc,
> > -- 
> > 2.38.1
> > 
> 
> --
> Chuck Lever
> 
> 
> 

-- 
Jeff Layton <jlayton@kernel.org>