Re: [PATCH v2 1/3] nfsd: rework refcounting in filecache

Jeff Layton
2022-10-28T09:58:24Z

On Fri, 2022-10-28 at 09:51 +1100, NeilBrown wrote:
> 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.
> >
> > 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?)
>

Yes, I meant to go back and flesh that out, and forgot before posting.

> >
> > 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.
> >
> > 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(-)
> >
> > 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 regular
> > + * 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.
> > */
> >
> > #include <linux/hash.h>
> > @@ -302,31 +308,43 @@ 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;
> > }
> > return nf;
> > }
> >
> > -static bool
> > +static void
> > +nfsd_file_flush(struct nfsd_file *nf)
> > +{
> > + struct file *file = 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) != 0)
> > + nfsd_reset_write_verifier(net_generic(nf->nf_net, nfsd_net_id));
> > +}
> > +
> > +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);
> >
> > - 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 = true;
> > }
> >
> > /*
> > @@ -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;
> >
> > call_rcu(&nf->nf_rcu, nfsd_file_slab_free);
> > - return flush;
> > }
> >
> > 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));
> > }
> >
> > -static void
> > -nfsd_file_flush(struct nfsd_file *nf)
> > -{
> > - struct file *file = 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) != 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;
> > }
> >
> > -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;
> > }
> >
> > static void
> > @@ -409,94 +420,89 @@ nfsd_file_unhash(struct nfsd_file *nf)
> > return false;
> > }
> >
> > -static void
> > -nfsd_file_unhash_and_dispose(struct nfsd_file *nf, struct list_head *dispose)
> > +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 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)
> > +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.
>

Ok, sounds reasonable.

> > {
> > - 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;
> > }
> >
> > -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) == 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);
> >
> > - while(!list_empty(dispose)) {
> > - nf = 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()
>

I had an incorrectly reversed test for that in the previous version in
nfsd_file_lru_add and you mentioned that it was racy. Why would that not
be the case here?

> > + /*
> > + * 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))
> > + return;
> > }
> > + __nfsd_file_put(nf);
>
> As suggested above, this would become
> if (refcount_dec_and_test(&nf->nf_ref))
> nfsd_file_free(nf);
>

Ok.

> > }
> >
> > 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_flush(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
> > @@ -566,21 +572,8 @@ 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 v2/3 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
> > @@ -591,40 +584,30 @@ 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)
> > {
> > @@ -634,7 +617,7 @@ 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
> > @@ -659,7 +642,7 @@ 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;
> > }
> >
> > @@ -670,8 +653,11 @@ 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)
> > @@ -689,52 +675,58 @@ __nfsd_file_close_inode(struct inode *inode, struct list_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;
> > }
> >
> > /**
> > - * 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 @inode.
> > + * 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);
> >
> > count = __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 = 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().
>
> > + }
> > + }
> > + return count;
> > }
> >
> > /**
> > - * 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 @inode.
> > */
> > -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 = __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();
> > }
> >
> > /**
> > * 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 != net)
> > continue;
> > - nfsd_file_unhash_and_dispose(nf, &dispose);
> > + nfsd_file_unhash_and_queue(nf, &dispose);
> > nf = rhashtable_walk_next(&iter);
> > }
> >
> > @@ -1093,11 +1085,10 @@ nfsd_file_do_acquire(struct svc_rqst *rqstp, struct svc_fh *fhp,
> > goto out;
> > }
> > open_retry = 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;
> > }
> >
> > - 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);
> ??
>

Removing from the LRU means putting a reference now. The last "put" of a
nfsd_file can be rather expensive (you might need to flush data, and
issue a close()).

In this particular codepath, that's not so much a danger, but avoiding
excess "put" calls is still a good thing to do. That's the main reason
I've tried to "transfer" references to and from the LRU where possible.

> > this_cpu_inc(nfsd_file_cache_hits);
> >
> > status = nfserrno(nfsd_open_break_lease(file_inode(nf->nf_file), may_flags));
> > @@ -1107,7 +1098,7 @@ 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);
> > + __nfsd_file_put(nf);
>
> I don't see the justification for this change.
> If status == nfserr_jukebox, then it is OK.
> If status is whatever we might get from break_lease(), then it seems
> wrong.
> If we modify nfsd_file_put() as I suggest, it will handle both cases.
>
>

The justification is that when we're dealing with an error from an open,
we don't want to put the nfsd_file onto the LRU. So, a direct call to
__nfsd_file_put is what's needed here.

I'll plan to open-code those like you suggest in the next iteration.

> > nf = NULL;
> > }
> >
> > @@ -1134,7 +1125,7 @@ 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 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))
> >
> > -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);
> >
> > TRACE_EVENT(nfsd_file_alloc,
> > TP_PROTO(
> > --
> > 2.37.3
> >
> >
>
> Thanks,
> NeilBrown

--
Jeff Layton <jlayton@kernel.org>