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From:   "NeilBrown" <neilb@suse.de>
To:     "Jeff Layton" <jlayton@kernel.org>
Cc:     chuck.lever@oracle.com, linux-nfs@vger.kernel.org
Subject: Re: [PATCH v2 1/3] nfsd: rework refcounting in filecache
In-reply-to: <20221027215213.138304-2-jlayton@kernel.org>
References: <20221027215213.138304-1-jlayton@kernel.org>,
 <20221027215213.138304-2-jlayton@kernel.org>
Date:   Fri, 28 Oct 2022 09:51:28 +1100
Message-id: <166691108885.13915.13997292493521001238@noble.neil.brown.name>
Precedence: bulk

On Fri, 28 Oct 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.
>=20
> 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 the handling

-EINTR ??? (you got interrupted and didn't finish the sentence?)

>=20
> 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.
>=20
> Signed-off-by: Jeff Layton <jlayton@kernel.org>
> ---
>  fs/nfsd/filecache.c | 291 +++++++++++++++++++++-----------------------
>  fs/nfsd/trace.h     |   5 +-
>  2 files changed, 144 insertions(+), 152 deletions(-)
>=20
> diff --git a/fs/nfsd/filecache.c b/fs/nfsd/filecache.c
> index 98c6b5f51bc8..e63534f4b9f8 100644
> --- a/fs/nfsd/filecache.c
> +++ b/fs/nfsd/filecache.c
> @@ -1,6 +1,12 @@
>  // SPDX-License-Identifier: GPL-2.0
>  /*
>   * The NFSD open file cache.
> + *
> + * Each nfsd_file is created in response to client activity -- either regu=
lar
> + * file I/O for v2/v3, or opening a file for v4. Files opened via v4 are
> + * cleaned up as soon as their refcount goes to 0.  Entries for v2/v3 are
> + * flagged with NFSD_FILE_GC. On their last put, they are added to the LRU=
 for
> + * eventual disposal if they aren't used again within a short time period.
>   */
> =20
>  #include <linux/hash.h>
> @@ -302,31 +308,43 @@ nfsd_file_alloc(struct nfsd_file_lookup_key *key, uns=
igned int may)
>  		if (key->gc)
>  			__set_bit(NFSD_FILE_GC, &nf->nf_flags);
>  		nf->nf_inode =3D 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 =3D key->need;
>  		nf->nf_mark =3D NULL;
>  	}
>  	return nf;
>  }
> =20
> -static bool
> +static void
> +nfsd_file_flush(struct nfsd_file *nf)
> +{
> +	struct file *file =3D nf->nf_file;
> +
> +	if (!file || !(file->f_mode & FMODE_WRITE))
> +		return;
> +	this_cpu_add(nfsd_file_pages_flushed, file->f_mapping->nrpages);
> +	if (vfs_fsync(file, 1) !=3D 0)
> +		nfsd_reset_write_verifier(net_generic(nf->nf_net, nfsd_net_id));
> +}
> +
> +static void
>  nfsd_file_free(struct nfsd_file *nf)
>  {
>  	s64 age =3D ktime_to_ms(ktime_sub(ktime_get(), nf->nf_birthtime));
> -	bool flush =3D false;
> +
> +	trace_nfsd_file_free(nf);
> =20
>  	this_cpu_inc(nfsd_file_releases);
>  	this_cpu_add(nfsd_file_total_age, age);
> =20
> -	trace_nfsd_file_put_final(nf);
> +	nfsd_file_flush(nf);
> +
>  	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 =3D true;
>  	}
> =20
>  	/*
> @@ -334,10 +352,9 @@ 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;
> =20
>  	call_rcu(&nf->nf_rcu, nfsd_file_slab_free);
> -	return flush;
>  }
> =20
>  static bool
> @@ -363,29 +380,23 @@ nfsd_file_check_write_error(struct nfsd_file *nf)
>  	return filemap_check_wb_err(file->f_mapping, READ_ONCE(file->f_wb_err));
>  }
> =20
> -static void
> -nfsd_file_flush(struct nfsd_file *nf)
> -{
> -	struct file *file =3D nf->nf_file;
> -
> -	if (!file || !(file->f_mode & FMODE_WRITE))
> -		return;
> -	this_cpu_add(nfsd_file_pages_flushed, file->f_mapping->nrpages);
> -	if (vfs_fsync(file, 1) !=3D 0)
> -		nfsd_reset_write_verifier(net_generic(nf->nf_net, nfsd_net_id));
> -}
> -
> -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;
>  }
> =20
> -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;
>  }
> =20
>  static void
> @@ -409,94 +420,89 @@ nfsd_file_unhash(struct nfsd_file *nf)
>  	return false;
>  }
> =20
> -static void
> -nfsd_file_unhash_and_dispose(struct nfsd_file *nf, struct list_head *dispo=
se)
> +struct nfsd_file *
> +nfsd_file_get(struct nfsd_file *nf)
>  {
> -	trace_nfsd_file_unhash_and_dispose(nf);
> +	if (likely(refcount_inc_not_zero(&nf->nf_ref)))
> +		return nf;
> +	return NULL;
> +}
> +
> +/**
> + * 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 co=
me
> + * 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;
>  }
> =20
> -static void
> -nfsd_file_put_noref(struct nfsd_file *nf)
> +static bool
> +__nfsd_file_put(struct nfsd_file *nf)

The return value of this function is never tested.
Maybe it should return void.

Further, I don't think this is a useful abstraction.
I would rather move the refcount_dec_and_test to the caller, and move
the lru_remove and unash into nfsd_file_free.

>  {
> -	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_unhash(nf);
>  		nfsd_file_free(nf);
> +		return true;
>  	}
> +	return false;
>  }
> =20
> -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 v2/3 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();
> -
> -	if (test_bit(NFSD_FILE_GC, &nf->nf_flags))
> -		nfsd_file_lru_add(nf);
> -	else if (refcount_read(&nf->nf_ref) =3D=3D 2)
> -		nfsd_file_unhash_and_put(nf);
> -
> -	if (!test_bit(NFSD_FILE_HASHED, &nf->nf_flags)) {
> -		nfsd_file_flush(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);
> -}
> -
> -struct nfsd_file *
> -nfsd_file_get(struct nfsd_file *nf)
> -{
> -	if (likely(refcount_inc_not_zero(&nf->nf_ref)))
> -		return nf;
> -	return NULL;
> -}
> -
> -static void
> -nfsd_file_dispose_list(struct list_head *dispose)
> -{
> -	struct nfsd_file *nf;
> +	trace_nfsd_file_put(nf);
> =20
> -	while(!list_empty(dispose)) {
> -		nf =3D list_first_entry(dispose, struct nfsd_file, nf_lru);
> -		list_del_init(&nf->nf_lru);
> -		nfsd_file_flush(nf);
> -		nfsd_file_put_noref(nf);
> +	if (test_bit(NFSD_FILE_GC, &nf->nf_flags)) {

I would prefer this included a test on NFSD_FILE_HASHED as well so that
if the file isn't hashed, we don't consider it for the lru.
This would me we can simple called nfsd_file_put() for things on the
dispose list, rather then needing __nfsd_file_put()

> +		/*
> +		 * If this is the last reference (nf_ref =3D=3D 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))
> +			return;
>  	}
> +	__nfsd_file_put(nf);

As suggested above, this would become
   if (refcount_dec_and_test(&nf->nf_ref))
	nfsd_file_free(nf);

>  }
> =20
>  static void
> -nfsd_file_dispose_list_sync(struct list_head *dispose)
> +nfsd_file_dispose_list(struct list_head *dispose)
>  {
> -	bool flush =3D false;
>  	struct nfsd_file *nf;
> =20
>  	while(!list_empty(dispose)) {
>  		nf =3D list_first_entry(dispose, struct nfsd_file, nf_lru);
>  		list_del_init(&nf->nf_lru);
> -		nfsd_file_flush(nf);
> -		if (!refcount_dec_and_test(&nf->nf_ref))
> -			continue;
> -		if (nfsd_file_free(nf))
> -			flush =3D true;
> +		nfsd_file_free(nf);
>  	}
> -	if (flush)
> -		flush_delayed_fput();
>  }
> =20
>  static void
> @@ -566,21 +572,8 @@ nfsd_file_lru_cb(struct list_head *item, struct list_l=
ru_one *lru,
>  	struct list_head *head =3D arg;
>  	struct nfsd_file *nf =3D list_entry(item, struct nfsd_file, nf_lru);
> =20
> -	/*
> -	 * 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 v2/3 entries here */
> +	WARN_ON_ONCE(!test_bit(NFSD_FILE_GC, &nf->nf_flags));
> =20
>  	/*
>  	 * Don't throw out files that are still undergoing I/O or
> @@ -591,40 +584,30 @@ nfsd_file_lru_cb(struct list_head *item, struct list_=
lru_one *lru,
>  		return LRU_SKIP;
>  	}
> =20
> +	/* 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;
>  	}
> =20
> -	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;
>  	}
> =20
> +	/* 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;
>  }
> =20
> -/*
> - * 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)
>  {
> @@ -634,7 +617,7 @@ nfsd_file_gc(void)
>  	ret =3D 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);
>  }
> =20
>  static void
> @@ -659,7 +642,7 @@ nfsd_file_lru_scan(struct shrinker *s, struct shrink_co=
ntrol *sc)
>  	ret =3D 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;
>  }
> =20
> @@ -670,8 +653,11 @@ static struct shrinker	nfsd_file_shrinker =3D {
>  };
> =20
>  /*
> - * 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 success=
ful.
> + *
> + * 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)
> @@ -689,52 +675,58 @@ __nfsd_file_close_inode(struct inode *inode, struct l=
ist_head *dispose)
>  				       nfsd_file_rhash_params);
>  		if (!nf)
>  			break;
> -		nfsd_file_unhash_and_dispose(nf, dispose);
> -		count++;
> +
> +		if (nfsd_file_unhash_and_queue(nf, dispose))
> +			count++;
>  	} while (1);
>  	rcu_read_unlock();
>  	return count;
>  }
> =20
>  /**
> - * nfsd_file_close_inode_sync - attempt to forcibly close a nfsd_file
> + * nfsd_file_close_inode - attempt a delayed close of a nfsd_file
>   * @inode: inode of the file to attempt to remove
>   *
> - * Unhash and put, then flush and fput all cache items associated with @in=
ode.
> + * Unhash and put all cache item associated with @inode.
>   */
> -void
> -nfsd_file_close_inode_sync(struct inode *inode)
> +static unsigned int
> +nfsd_file_close_inode(struct inode *inode)
>  {
> -	LIST_HEAD(dispose);
> +	struct nfsd_file *nf;
>  	unsigned int count;
> +	LIST_HEAD(dispose);
> =20
>  	count =3D __nfsd_file_close_inode(inode, &dispose);
> -	trace_nfsd_file_close_inode_sync(inode, count);
> -	nfsd_file_dispose_list_sync(&dispose);
> +	trace_nfsd_file_close_inode(inode, count);
> +	if (count) {
> +		while(!list_empty(&dispose)) {
> +			nf =3D list_first_entry(&dispose, struct nfsd_file, nf_lru);
> +			list_del_init(&nf->nf_lru);
> +			trace_nfsd_file_closing(nf);
> +			__nfsd_file_put(nf);

If nfsd_file_put() didn't add unhashed files to the lru, this can be
nfsd_file_put().=20

> +		}
> +	}
> +	return count;
>  }
> =20
>  /**
> - * nfsd_file_close_inode - attempt a delayed close of a nfsd_file
> + * nfsd_file_close_inode_sync - attempt to forcibly close a nfsd_file
>   * @inode: inode of the file to attempt to remove
>   *
> - * Unhash and put all cache item associated with @inode.
> + * Unhash and put, then flush and fput all cache items associated with @in=
ode.
>   */
> -static void
> -nfsd_file_close_inode(struct inode *inode)
> +void
> +nfsd_file_close_inode_sync(struct inode *inode)
>  {
> -	LIST_HEAD(dispose);
> -	unsigned int count;
> -
> -	count =3D __nfsd_file_close_inode(inode, &dispose);
> -	trace_nfsd_file_close_inode(inode, count);
> -	nfsd_file_dispose_list_delayed(&dispose);
> +	if (nfsd_file_close_inode(inode))
> +		flush_delayed_fput();
>  }
> =20
>  /**
>   * 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
> @@ -892,7 +884,7 @@ __nfsd_file_cache_purge(struct net *net)
>  		while (!IS_ERR_OR_NULL(nf)) {
>  			if (net && nf->nf_net !=3D net)
>  				continue;
> -			nfsd_file_unhash_and_dispose(nf, &dispose);
> +			nfsd_file_unhash_and_queue(nf, &dispose);
>  			nf =3D rhashtable_walk_next(&iter);
>  		}
> =20
> @@ -1093,11 +1085,10 @@ nfsd_file_do_acquire(struct svc_rqst *rqstp, struct=
 svc_fh *fhp,
>  			goto out;
>  		}
>  		open_retry =3D false;
> -		nfsd_file_put_noref(nf);
> +		__nfsd_file_put(nf);

This nf is not hashed, and I think it has no other reference.  So we
could use nfsd_file_free() - but nfsd_file_put() would be just as good
and safer.

>  		goto retry;
>  	}
> =20
> -	nfsd_file_lru_remove(nf);

Hmmm...  why not remove from the lru.  I guess this justifies patch 2/3,
but it might be cleaner to make this

  if (nfsd_file_lru_remove(nf))
        nffsd_file_put(nf);
??

>  	this_cpu_inc(nfsd_file_cache_hits);
> =20
>  	status =3D nfserrno(nfsd_open_break_lease(file_inode(nf->nf_file), may_fl=
ags));
> @@ -1107,7 +1098,7 @@ nfsd_file_do_acquire(struct svc_rqst *rqstp, struct s=
vc_fh *fhp,
>  			this_cpu_inc(nfsd_file_acquisitions);
>  		*pnf =3D nf;
>  	} else {
> -		nfsd_file_put(nf);
> +		__nfsd_file_put(nf);

I don't see the justification for this change.
If status =3D=3D nfserr_jukebox, then it is OK.
If status is whatever we might get from break_lease(), then it seems
wrong.=20
If we modify nfsd_file_put() as I suggest, it will handle both cases.


>  		nf =3D NULL;
>  	}
> =20
> @@ -1134,7 +1125,7 @@ nfsd_file_do_acquire(struct svc_rqst *rqstp, struct s=
vc_fh *fhp,
>  	 * then unhash.
>  	 */
>  	if (status !=3D nfs_ok || key.inode->i_nlink =3D=3D 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 b09ab4f92d43..a44ded06af87 100644
> --- a/fs/nfsd/trace.h
> +++ b/fs/nfsd/trace.h
> @@ -903,10 +903,11 @@ DEFINE_EVENT(nfsd_file_class, name, \
>  	TP_PROTO(struct nfsd_file *nf), \
>  	TP_ARGS(nf))
> =20
> -DEFINE_NFSD_FILE_EVENT(nfsd_file_put_final);
> +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_unhash_and_dispose);
> +DEFINE_NFSD_FILE_EVENT(nfsd_file_closing);
> +DEFINE_NFSD_FILE_EVENT(nfsd_file_unhash_and_queue);
> =20
>  TRACE_EVENT(nfsd_file_alloc,
>  	TP_PROTO(
> --=20
> 2.37.3
>=20
>=20

Thanks,
NeilBrown