Thanks Jeff.
Unfortunately the early unhash easily leads to leaks:
crash> kmem -S nfsd_file | grep '\[ffff' | sed -e 's|\[||' -e 's|\]||'
| xargs -i echo struct nfsd_file.nf_flags,nf_ref.refs.counter '{}' >
/var/tmp/nfsd_files
crash> !wc -l /var/tmp/nfsd_files
19 /var/tmp/nfsd_files
crash> < /var/tmp/nfsd_files
crash> struct nfsd_file.nf_flags,nf_ref.refs.counter ffff88865c778900
nf_flags = 8,
nf_ref.refs.counter = 2
crash> struct nfsd_file.nf_flags,nf_ref.refs.counter ffff88865c778cc0
nf_flags = 8,
nf_ref.refs.counter = 3
crash> struct nfsd_file.nf_flags,nf_ref.refs.counter ffff8885d5f35e00
nf_flags = 8,
nf_ref.refs.counter = 1
crash> struct nfsd_file.nf_flags,nf_ref.refs.counter ffff88817443e780
nf_flags = 8,
nf_ref.refs.counter = 3
crash> struct nfsd_file.nf_flags,nf_ref.refs.counter ffff88818b3f0600
nf_flags = 8,
nf_ref.refs.counter = 2
crash> struct nfsd_file.nf_flags,nf_ref.refs.counter ffff88a4490f8300
nf_flags = 8,
nf_ref.refs.counter = 1
crash> struct nfsd_file.nf_flags,nf_ref.refs.counter ffff88a0dab183c0
nf_flags = 8,
nf_ref.refs.counter = 40
...
crash> struct nfsd_file.nf_flags,nf_ref.refs.counter ffff89209535f200
nf_flags = 8,
nf_ref.refs.counter = 2
crash> struct nfsd_file.nf_flags,nf_ref.refs.counter ffff8980e15138c0
nf_flags = 8,
nf_ref.refs.counter = 7
crash> struct nfsd_file.nf_flags,nf_ref.refs.counter ffff898107b95800
nf_flags = 8,
nf_ref.refs.counter = 3
crash> struct nfsd_file.nf_flags,nf_ref.refs.counter ffff898107b95980
nf_flags = 12,
nf_ref.refs.counter = 1
nfsd_file_do_acquire() -> nfsd_file_lookup_locked() relies on the hash
table to find the nfsd_file,
but I am still scratching my head why and how this happens.
FYI, here is the patch I applied for testing:
diff --git a/fs/nfsd/filecache.c b/fs/nfsd/filecache.c
index ad9083ca144b..22ebd7fb8639 100644
--- a/fs/nfsd/filecache.c
+++ b/fs/nfsd/filecache.c
@@ -216,6 +216,7 @@ nfsd_file_alloc(struct net *net, struct inode
*inode, unsigned char need,
return NULL;
INIT_LIST_HEAD(&nf->nf_lru);
+ INIT_LIST_HEAD(&nf->nf_gc);
nf->nf_birthtime = ktime_get();
nf->nf_file = NULL;
nf->nf_cred = get_current_cred();
@@ -393,8 +394,8 @@ 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);
+ nf = list_first_entry(dispose, struct nfsd_file, nf_gc);
+ list_del_init(&nf->nf_gc);
nfsd_file_free(nf);
}
}
@@ -411,12 +412,12 @@ nfsd_file_dispose_list_delayed(struct list_head *dispose)
{
while(!list_empty(dispose)) {
struct nfsd_file *nf = list_first_entry(dispose,
- struct nfsd_file, nf_lru);
+ struct nfsd_file, nf_gc);
struct nfsd_net *nn = net_generic(nf->nf_net, nfsd_net_id);
struct nfsd_fcache_disposal *l = nn->fcache_disposal;
spin_lock(&l->lock);
- list_move_tail(&nf->nf_lru, &l->freeme);
+ list_move_tail(&nf->nf_gc, &l->freeme);
spin_unlock(&l->lock);
svc_wake_up(nn->nfsd_serv);
}
@@ -503,7 +504,8 @@ nfsd_file_lru_cb(struct list_head *item, struct
list_lru_one *lru,
/* 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);
+ list_lru_isolate(lru, &nf->nf_lru);
+ list_add(&nf->nf_gc, head);
this_cpu_inc(nfsd_file_evictions);
trace_nfsd_file_gc_disposed(nf);
return LRU_REMOVED;
@@ -578,7 +580,7 @@ nfsd_file_cond_queue(struct nfsd_file *nf, struct
list_head *dispose)
/* 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);
+ list_add(&nf->nf_gc, dispose);
trace_nfsd_file_closing(nf);
}
}
@@ -654,8 +656,8 @@ nfsd_file_close_inode_sync(struct inode *inode)
nfsd_file_queue_for_close(inode, &dispose);
while (!list_empty(&dispose)) {
- nf = list_first_entry(&dispose, struct nfsd_file, nf_lru);
- list_del_init(&nf->nf_lru);
+ nf = list_first_entry(&dispose, struct nfsd_file, nf_gc);
+ list_del_init(&nf->nf_gc);
nfsd_file_free(nf);
}
}
diff --git a/fs/nfsd/filecache.h b/fs/nfsd/filecache.h
index c61884def906..3fbec24eea6c 100644
--- a/fs/nfsd/filecache.h
+++ b/fs/nfsd/filecache.h
@@ -44,6 +44,7 @@ struct nfsd_file {
struct nfsd_file_mark *nf_mark;
struct list_head nf_lru;
+ struct list_head nf_gc;
struct rcu_head nf_rcu;
ktime_t nf_birthtime;
};
On Mon, Jul 8, 2024 at 10:55 AM Jeff Layton <jlayton@xxxxxxxxxx> wrote:
>
> On Mon, 2024-07-08 at 10:23 -0400, Youzhong Yang wrote:
> > Thanks Jeff.
> >
> > I am ok with reverting the unhash/dispose list reordering in
> > nfsd_file_lru_cb(), as it doesn't make much of a difference,
> > but for nfsd_file_cond_queue(), imagining this:
> >
> > - A nfsd_file is hashed
> > - In nfsd_file_cond_queue(), [if (!nfsd_file_unhash(nf))] will get it
> > unhashed, doesn't it?
> > - It continues to get a reference by nfsd_file_get()
> > - It continues to remove itself from LRU by nfsd_file_lru_remove() if
> > it is on the LRU.
> > - Now it runs refcount_sub_and_test(), what happens if the refcnt
> > does
> > not go to 0? How can this nfsd_file be found again? Through the hash
> > table? Through the LRU walk? how?
> >
> > Thanks again.
> >
> > -Youzhong
> >
>
> It won't need to be found again. The holders of the extra references
> will put those references when they are finished. Since the object is
> no longer HASHED, nfsd_file_put just does this:
>
> if (refcount_dec_and_test(&nf->nf_ref))
> nfsd_file_free(nf);
>
> So that should be fine.
>
> > On Mon, Jul 8, 2024 at 9:35 AM Jeff Layton <jlayton@xxxxxxxxxx>
> > wrote:
> > >
> > > On Mon, 2024-07-08 at 08:58 -0400, Youzhong Yang wrote:
> > > > Thank you Jeff for your invaluable insights. I was leaning
> > > > towards
> > > > adding a new list_head too, and tested this approach on kernel
> > > > 6.6 by
> > > > continuously hammering the server with heavy nfs load for the
> > > > last few
> > > > days, not a single leak.
> > > >
> > > > Here goes the patch (based on Linux kernel master branch), please
> > > > review:
> > > >
> > > > From: Youzhong Yang <youzhong@xxxxxxxxx>
> > > > Date: Thu, 4 Jul 2024 11:25:40 -0400
> > > > Subject: [PATCH] nfsd: fix nfsd_file leaking due to mixed use of
> > > > nf->nf_lru
> > > >
> > > > nfsd_file_put() in one thread can race with another thread doing
> > > > garbage collection (running nfsd_file_gc() -> list_lru_walk() ->
> > > > nfsd_file_lru_cb()):
> > > >
> > > > * In nfsd_file_put(), nf->nf_ref is 1, so it tries to do
> > > > nfsd_file_lru_add().
> > > > * nfsd_file_lru_add() returns true (with NFSD_FILE_REFERENCED
> > > > bit set)
> > > > * garbage collector kicks in, nfsd_file_lru_cb() clears
> > > > REFERENCED bit and
> > > > returns LRU_ROTATE.
> > > > * garbage collector kicks in again, nfsd_file_lru_cb() now
> > > > decrements nf->nf_ref
> > > > to 0, runs nfsd_file_unhash(), removes it from the LRU and
> > > > adds to
> > > > the dispose
> > > > list [list_lru_isolate_move(lru, &nf->nf_lru, head)]
> > > > * nfsd_file_put() detects NFSD_FILE_HASHED bit is cleared, so
> > > > it
> > > > tries to remove
> > > > the 'nf' from the LRU [if (!nfsd_file_lru_remove(nf))]. The
> > > > 'nf'
> > > > has been added
> > > > to the 'dispose' list by nfsd_file_lru_cb(), so
> > > > nfsd_file_lru_remove(nf) simply
> > > > treats it as part of the LRU and removes it, which leads to
> > > > its removal from
> > > > the 'dispose' list.
> > > > * At this moment, 'nf' is unhashed with its nf_ref being 0, and
> > > > not
> > > > on the LRU.
> > > > nfsd_file_put() continues its execution [if
> > > > (refcount_dec_and_test(&nf->nf_ref))],
> > > > as nf->nf_ref is already 0, nf->nf_ref is set to
> > > > REFCOUNT_SATURATED, and the 'nf'
> > > > gets no chance of being freed.
> > > >
> > > > nfsd_file_put() can also race with nfsd_file_cond_queue():
> > > > * In nfsd_file_put(), nf->nf_ref is 1, so it tries to do
> > > > nfsd_file_lru_add().
> > > > * nfsd_file_lru_add() sets REFERENCED bit and returns true.
> > > > * Some userland application runs 'exportfs -f' or something
> > > > like
> > > > that, which triggers
> > > > __nfsd_file_cache_purge() -> nfsd_file_cond_queue().
> > > > * In nfsd_file_cond_queue(), it runs [if
> > > > (!nfsd_file_unhash(nf))],
> > > > unhash is done
> > > > successfully.
> > > > * nfsd_file_cond_queue() runs [if (!nfsd_file_get(nf))], now
> > > > nf->nf_ref goes to 2.
> > > > * nfsd_file_cond_queue() runs [if (nfsd_file_lru_remove(nf))],
> > > > it succeeds.
> > > > * nfsd_file_cond_queue() runs [if
> > > > (refcount_sub_and_test(decrement,
> > > > &nf->nf_ref))]
> > > > (with "decrement" being 2), so the nf->nf_ref goes to 0, the
> > > > 'nf'
> > > > is added to the
> > > > dispose list [list_add(&nf->nf_lru, dispose)]
> > > > * nfsd_file_put() detects NFSD_FILE_HASHED bit is cleared, so
> > > > it
> > > > tries to remove
> > > > the 'nf' from the LRU [if (!nfsd_file_lru_remove(nf))],
> > > > although
> > > > the 'nf' is not
> > > > in the LRU, but it is linked in the 'dispose' list,
> > > > nfsd_file_lru_remove() simply
> > > > treats it as part of the LRU and removes it. This leads to
> > > > its removal from
> > > > the 'dispose' list!
> > > > * Now nf->ref is 0, unhashed. nfsd_file_put() continues its
> > > > execution and set
> > > > nf->nf_ref to REFCOUNT_SATURATED.
> > > >
> > > > As shown in the above analysis, using nf_lru for both the LRU
> > > > list and
> > > > dispose list
> > > > can cause the leaks. This patch adds a new list_head nf_gc in
> > > > struct
> > > > nfsd_file, and uses
> > > > it for the dispose list. It's not expected to have a nfsd_file
> > > > unhashed but it's not
> > > > added to the dispose list, so in nfsd_file_cond_queue() and
> > > > nfsd_file_lru_cb() nfsd_file
> > > > is unhashed after being added to the dispose list.
> > > >
> > >
> > > I don't see where we require the object to be either hashed or on
> > > the
> > > dispose list. I think you probably just want to do a patch that
> > > changes the dispose list to use a dedicated list_head without
> > > reordering when the these things are unhashed.
> > >
> > > > Signed-off-by: Youzhong Yang <youzhong@xxxxxxxxx>
> > > > ---
> > > > fs/nfsd/filecache.c | 23 ++++++++++++++---------
> > > > fs/nfsd/filecache.h | 1 +
> > > > 2 files changed, 15 insertions(+), 9 deletions(-)
> > > >
> > > > diff --git a/fs/nfsd/filecache.c b/fs/nfsd/filecache.c
> > > > index ad9083ca144b..3aef2ddfce94 100644
> > > > --- a/fs/nfsd/filecache.c
> > > > +++ b/fs/nfsd/filecache.c
> > > > @@ -216,6 +216,7 @@ nfsd_file_alloc(struct net *net, struct inode
> > > > *inode, unsigned char need,
> > > > return NULL;
> > > >
> > > > INIT_LIST_HEAD(&nf->nf_lru);
> > > > + INIT_LIST_HEAD(&nf->nf_gc);
> > > > nf->nf_birthtime = ktime_get();
> > > > nf->nf_file = NULL;
> > > > nf->nf_cred = get_current_cred();
> > > > @@ -393,8 +394,8 @@ 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);
> > > > + nf = list_first_entry(dispose, struct nfsd_file,
> > > > nf_gc);
> > > > + list_del_init(&nf->nf_gc);
> > > > nfsd_file_free(nf);
> > > > }
> > > > }
> > > > @@ -411,12 +412,12 @@ nfsd_file_dispose_list_delayed(struct
> > > > list_head *dispose)
> > > > {
> > > > while(!list_empty(dispose)) {
> > > > struct nfsd_file *nf = list_first_entry(dispose,
> > > > - struct nfsd_file,
> > > > nf_lru);
> > > > + struct nfsd_file,
> > > > nf_gc);
> > > > struct nfsd_net *nn = net_generic(nf->nf_net,
> > > > nfsd_net_id);
> > > > struct nfsd_fcache_disposal *l = nn-
> > > > >fcache_disposal;
> > > >
> > > > spin_lock(&l->lock);
> > > > - list_move_tail(&nf->nf_lru, &l->freeme);
> > > > + list_move_tail(&nf->nf_gc, &l->freeme);
> > > > spin_unlock(&l->lock);
> > > > svc_wake_up(nn->nfsd_serv);
> > > > }
> > > > @@ -502,8 +503,10 @@ nfsd_file_lru_cb(struct list_head *item,
> > > > struct
> > > > list_lru_one *lru,
> > > > }
> > > >
> > > > /* Refcount went to zero. Unhash it and queue it to the
> > > > dispose list */
> > > > + list_lru_isolate(lru, &nf->nf_lru);
> > > > + list_add(&nf->nf_gc, head);
> > > > + /* Unhash after removing from LRU and adding to dispose
> > > > list */
> > > > nfsd_file_unhash(nf);
> > > > - list_lru_isolate_move(lru, &nf->nf_lru, head);
> > >
> > > I don't see the point in reordering these operations. Hashing is
> > > all
> > > about making the thing findable by nfsd operations. The _last_
> > > thing we
> > > want to do is put it on the dispose list while the thing can still
> > > be
> > > found by nfsd threads doing operations.
> > >
> > > > this_cpu_inc(nfsd_file_evictions);
> > > > trace_nfsd_file_gc_disposed(nf);
> > > > return LRU_REMOVED;
> > > > @@ -565,7 +568,7 @@ nfsd_file_cond_queue(struct nfsd_file *nf,
> > > > struct
> > > > list_head *dispose)
> > > > int decrement = 1;
> > > >
> > > > /* If we raced with someone else unhashing, ignore it */
> > > > - if (!nfsd_file_unhash(nf))
> > > > + if (!test_bit(NFSD_FILE_HASHED, &nf->nf_flags))
> > >
> > > The above change looks wrong. I don't think we need to change this.
> > >
> > > > return;
> > > >
> > > > /* If we can't get a reference, ignore it */
> > > > @@ -578,7 +581,9 @@ nfsd_file_cond_queue(struct nfsd_file *nf,
> > > > struct
> > > > list_head *dispose)
> > > >
> > > > /* 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);
> > > > + list_add(&nf->nf_gc, dispose);
> > > > + /* Unhash after adding to dispose list */
> > > > + nfsd_file_unhash(nf);
> > >
> > > This too looks wrong? Maybe I'm unclear on the race you're trying
> > > to
> > > fix with this? What's the harm in unhashing it early?
> > >
> > > > trace_nfsd_file_closing(nf);
> > > > }
> > > > }
> > > > @@ -654,8 +659,8 @@ nfsd_file_close_inode_sync(struct inode
> > > > *inode)
> > > >
> > > > nfsd_file_queue_for_close(inode, &dispose);
> > > > while (!list_empty(&dispose)) {
> > > > - nf = list_first_entry(&dispose, struct nfsd_file,
> > > > nf_lru);
> > > > - list_del_init(&nf->nf_lru);
> > > > + nf = list_first_entry(&dispose, struct nfsd_file,
> > > > nf_gc);
> > > > + list_del_init(&nf->nf_gc);
> > > > nfsd_file_free(nf);
> > > > }
> > > > }
> > > > diff --git a/fs/nfsd/filecache.h b/fs/nfsd/filecache.h
> > > > index c61884def906..3fbec24eea6c 100644
> > > > --- a/fs/nfsd/filecache.h
> > > > +++ b/fs/nfsd/filecache.h
> > > > @@ -44,6 +44,7 @@ struct nfsd_file {
> > > >
> > > > struct nfsd_file_mark *nf_mark;
> > > > struct list_head nf_lru;
> > > > + struct list_head nf_gc;
> > > > struct rcu_head nf_rcu;
> > > > ktime_t nf_birthtime;
> > > > };
> > > > --
> > > > 2.34.1
> > > >
> > > > On Thu, Jul 4, 2024 at 7:14 AM Jeff Layton
> > > > <jlayton@xxxxxxxxxxxxxxx> wrote:
> > > > >
> > > > > On Wed, 2024-07-03 at 16:46 -0400, Youzhong Yang wrote:
> > > > > > Thank you Chuck. Here are my quick answers to your comments:
> > > > > >
> > > > > > - I don't have a quick reproducer. I reproduced it by using
> > > > > > hundreds
> > > > > > of nfs clients generating +600K ops under our workload in the
> > > > > > testing
> > > > > > environment. Theoretically it should be possible to simplify
> > > > > > the
> > > > > > reproduction but I am still working on it.
> > > > > >
> > > > > > - I understand zfs is an out-of-tree file system. That's
> > > > > > fine. But
> > > > > > this leaking can happen to any file system, and leaking is
> > > > > > not a good
> > > > > > thing no matter what file system it is.
> > > > > >
> > > > > > - I will try to come up with a reproducer using xfs or btrfs
> > > > > > if possible.
> > > > > >
> > > > > > Now back to the problem itself, here are my findings:
> > > > > >
> > > > > > - nfsd_file_put() in one thread can race with another thread
> > > > > > doing
> > > > > > garbage collection (running nfsd_file_gc() -> list_lru_walk()
> > > > > > ->
> > > > > > nfsd_file_lru_cb()):
> > > > > >
> > > > > > * In nfsd_file_put(), nf->nf_ref is 1, so it tries to do
> > > > > > nfsd_file_lru_add().
> > > > > > * nfsd_file_lru_add() returns true (thus
> > > > > > NFSD_FILE_REFERENCED bit
> > > > > > set for nf->nf_flags)
> > > > > > * garbage collector kicks in, nfsd_file_lru_cb() clears
> > > > > > REFERENCED
> > > > > > bit and returns LRU_ROTATE.
> > > > > > * garbage collector kicks in again, nfsd_file_lru_cb() now
> > > > > > decrements nf->nf_ref to 0, runs nfsd_file_unhash(), removes
> > > > > > it from
> > > > > > the LRU and adds to the dispose list
> > > > > > [list_lru_isolate_move(lru,
> > > > > > &nf->nf_lru, head);]
> > > > > > * nfsd_file_put() detects NFSD_FILE_HASHED bit is cleared,
> > > > > > so it
> > > > > > tries to remove the 'nf' from the LRU [if
> > > > > > (!nfsd_file_lru_remove(nf))]
> > > > > > * The 'nf' has been added to the 'dispose' list by
> > > > > > nfsd_file_lru_cb(), so nfsd_file_lru_remove(nf) simply treats
> > > > > > it as
> > > > > > part of the LRU and removes it, which leads it to be removed
> > > > > > from the
> > > > > > 'dispose' list.
> > > > > > * At this moment, nf->nf_ref is 0, it's unhashed, and not
> > > > > > on the
> > > > > > LRU. nfsd_file_put() continues its execution [if
> > > > > > (refcount_dec_and_test(&nf->nf_ref))], as nf->nf_ref is
> > > > > > already 0, now
> > > > > > bad thing happens: nf->nf_ref is set to REFCOUNT_SATURATED,
> > > > > > and the
> > > > > > 'nf' is leaked.
> > > > > >
> > > > > > To make this happen, the right timing is crucial. It can be
> > > > > > reproduced
> > > > > > by adding artifical delays in filecache.c, or hammering the
> > > > > > nfsd with
> > > > > > tons of ops.
> > > > > >
> > > > > > - Let's see how nfsd_file_put() can race with
> > > > > > nfsd_file_cond_queue():
> > > > > > * In nfsd_file_put(), nf->nf_ref is 1, so it tries to do
> > > > > > nfsd_file_lru_add().
> > > > > > * nfsd_file_lru_add() sets REFERENCED bit and returns true.
> > > > > > * 'exportfs -f' or something like that triggers
> > > > > > __nfsd_file_cache_purge() -> nfsd_file_cond_queue().
> > > > > > * In nfsd_file_cond_queue(), it runs [if
> > > > > > (!nfsd_file_unhash(nf))],
> > > > > > unhash is done successfully.
> > > > > > * nfsd_file_cond_queue() runs [if (!nfsd_file_get(nf))],
> > > > > > now
> > > > > > nf->nf_ref goes to 2.
> > > > > > * nfsd_file_cond_queue() runs [if
> > > > > > (nfsd_file_lru_remove(nf))], it succeeds.
> > > > > > * nfsd_file_cond_queue() runs [if
> > > > > > (refcount_sub_and_test(decrement,
> > > > > > &nf->nf_ref))] (with "decrement" being 2), so the nf->nf_ref
> > > > > > goes to
> > > > > > 0, the 'nf' is added to the dispost list [list_add(&nf-
> > > > > > >nf_lru,
> > > > > > dispose)]
> > > > > > * nfsd_file_put() detects NFSD_FILE_HASHED bit is cleared,
> > > > > > so it
> > > > > > tries to remove the 'nf' from the LRU [if
> > > > > > (!nfsd_file_lru_remove(nf))], although the 'nf' is not in the
> > > > > > LRU, but
> > > > > > it is linked in the 'dispose' list, nfsd_file_lru_remove()
> > > > > > simply
> > > > > > treats it as part of the LRU and removes it. This leads to
> > > > > > its removal
> > > > > > from the 'dispose' list!
> > > > > > * Now nf->ref is 0, unhashed. nfsd_file_put() continues its
> > > > > > execution and sets nf->nf_ref to REFCOUNT_SATURATED.
> > > > > >
> > > > > > The purpose of nf->nf_lru is problematic. As you can see, it
> > > > > > is used
> > > > > > for the LRU list, and also the 'dispose' list. Adding another
> > > > > > 'struct
> > > > > > list_head' specifically for the 'dispose' list seems to be a
> > > > > > better
> > > > > > way of fixing this race condition. Either way works for me.
> > > > > >
> > > > > > Would you agree my above analysis makes sense? Thanks.
> > > > > >
> > > > >
> > > > > I think so. It's been a while since I've done much work in this
> > > > > code,
> > > > > but it does sound like there is a race in the LRU handling.
> > > > >
> > > > >
> > > > > Like Chuck said, the nf->nf_lru list should be safe to use for
> > > > > multiple
> > > > > purposes, but that's only the case if we're not using that list
> > > > > as an
> > > > > indicator.
> > > > >
> > > > > The list_lru code does check this:
> > > > >
> > > > > if (!list_empty(item)) {
> > > > >
> > > > > ...so if we ever check this while it's sitting on the dispose
> > > > > list, it
> > > > > will handle it incorrectly. It sounds like that's the root
> > > > > cause of the
> > > > > problem you're seeing?
> > > > >
> > > > > If so, then maybe a separate list_head for disposal would be
> > > > > better.
> > > > >
> > > > > > Here is my patch with signed-off-by:
> > > > > >
> > > > > > From: Youzhong Yang <youzhong@xxxxxxxxx>
> > > > > > Date: Mon, 1 Jul 2024 06:45:22 -0400
> > > > > > Subject: [PATCH] nfsd: fix nfsd_file leaking due to race
> > > > > > condition and early
> > > > > > unhash
> > > > > >
> > > > > > Signed-off-by: Youzhong Yang <youzhong@xxxxxxxxx>
> > > > > > ---
> > > > > > fs/nfsd/filecache.c | 14 +++++++++++++-
> > > > > > 1 file changed, 13 insertions(+), 1 deletion(-)
> > > > > >
> > > > > > diff --git a/fs/nfsd/filecache.c b/fs/nfsd/filecache.c
> > > > > > index 1a6d5d000b85..2323829f7208 100644
> > > > > > --- a/fs/nfsd/filecache.c
> > > > > > +++ b/fs/nfsd/filecache.c
> > > > > > @@ -389,6 +389,17 @@ nfsd_file_put(struct nfsd_file *nf)
> > > > > > if (!nfsd_file_lru_remove(nf))
> > > > > > return;
> > > > > > }
> > > > > > + /*
> > > > > > + * Racing with nfsd_file_cond_queue() or
> > > > > > nfsd_file_lru_cb(),
> > > > > > + * it's unhashed but then removed from the
> > > > > > dispose list,
> > > > > > + * so we need to free it.
> > > > > > + */
> > > > > > + if (refcount_read(&nf->nf_ref) == 0 &&
> > > > >
> > > > > A refcount_read in this path is a red flag to me. Anytime
> > > > > you're just
> > > > > looking at the refcount without changing anything just screams
> > > > > out
> > > > > "race condition".
> > > > >
> > > > > In this case, what guarantee is there that this won't run afoul
> > > > > of the
> > > > > timing? We could check this and find out it's 1 just before it
> > > > > goes to
> > > > > 0 and you check the other conditions.
> > > > >
> > > > > Does anything prevent that?
> > > > >
> > > > > > + !test_bit(NFSD_FILE_HASHED, &nf-
> > > > > > >nf_flags) &&
> > > > > > + list_empty(&nf->nf_lru)) {
> > > > > > + nfsd_file_free(nf);
> > > > > > + return;
> > > > > > + }
> > > > > > }
> > > > > > if (refcount_dec_and_test(&nf->nf_ref))
> > > > > > nfsd_file_free(nf);
> > > > > > @@ -576,7 +587,7 @@ nfsd_file_cond_queue(struct nfsd_file
> > > > > > *nf, struct
> > > > > > list_head *dispose)
> > > > > > int decrement = 1;
> > > > > >
> > > > > > /* If we raced with someone else unhashing, ignore it
> > > > > > */
> > > > > > - if (!nfsd_file_unhash(nf))
> > > > > > + if (!test_bit(NFSD_FILE_HASHED, &nf->nf_flags))
> > > > > > return;
> > > > >
> > > > > Same here: you're just testing for the HASHED bit, but could
> > > > > this also
> > > > > race with someone who is setting it just after you get here.
> > > > > Why is
> > > > > that not a problem?
> > > > >
> > > > > >
> > > > > > /* If we can't get a reference, ignore it */
> > > > > > @@ -590,6 +601,7 @@ nfsd_file_cond_queue(struct nfsd_file
> > > > > > *nf, struct
> > > > > > list_head *dispose)
> > > > > > /* 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);
> > > > > > + nfsd_file_unhash(nf);
> > > > > > trace_nfsd_file_closing(nf);
> > > > > > }
> > > > > > }
> > > > > > --
> > > > > > 2.43.0
> > > > > >
> > > > > > On Wed, Jul 3, 2024 at 2:21 PM Chuck Lever
> > > > > > <chuck.lever@xxxxxxxxxx> wrote:
> > > > > > >
> > > > > > > On Wed, Jul 03, 2024 at 10:12:33AM -0400, Youzhong Yang
> > > > > > > wrote:
> > > > > > > > Hello,
> > > > > > > >
> > > > > > > > I'd like to report a nfsd_file leaking issue and propose
> > > > > > > > a fix for it.
> > > > > > > >
> > > > > > > > When I tested Linux kernel 6.8 and 6.6, I noticed
> > > > > > > > nfsd_file leaks
> > > > > > > > which led to undestroyable file systems (zfs),
> > > > > > >
> > > > > > > Thanks for the report. Some initial comments:
> > > > > > >
> > > > > > > - Do you have a specific reproducer? In other words, what
> > > > > > > is the
> > > > > > > simplest program that can run on an NFS client that will
> > > > > > > trigger
> > > > > > > this leak, and can you post it?
> > > > > > >
> > > > > > > - "zfs" is an out-of-tree file system, so it's not directly
> > > > > > > supported for NFSD.
> > > > > > >
> > > > > > > - The guidelines for patch submission require us to fix
> > > > > > > issues in
> > > > > > > upstream Linux first (currently that's v6.10-rc6). Then
> > > > > > > that fix
> > > > > > > can be backported to older stable kernels like 6.6.
> > > > > > >
> > > > > > > Can you reproduce the leak with one of the in-kernel
> > > > > > > filesystems
> > > > > > > (either xfs or btrfs would be great) and with NFSD in 6.10-
> > > > > > > rc6?
> > > > > > >
> > > > > > > One more comment below.
> > > > > > >
> > > > > > >
> > > > > > > > here are some examples:
> > > > > > > >
> > > > > > > > crash> struct nfsd_file -x ffff88e160db0460
> > > > > > > > struct nfsd_file {
> > > > > > > > nf_rlist = {
> > > > > > > > rhead = {
> > > > > > > > next = 0xffff8921fa2392f1
> > > > > > > > },
> > > > > > > > next = 0x0
> > > > > > > > },
> > > > > > > > nf_inode = 0xffff8882bc312ef8,
> > > > > > > > nf_file = 0xffff88e2015b1500,
> > > > > > > > nf_cred = 0xffff88e3ab0e7800,
> > > > > > > > nf_net = 0xffffffff83d41600 <init_net>,
> > > > > > > > nf_flags = 0x8,
> > > > > > > > nf_ref = {
> > > > > > > > refs = {
> > > > > > > > counter = 0xc0000000
> > > > > > > > }
> > > > > > > > },
> > > > > > > > nf_may = 0x4,
> > > > > > > > nf_mark = 0xffff88e1bddfb320,
> > > > > > > > nf_lru = {
> > > > > > > > next = 0xffff88e160db04a8,
> > > > > > > > prev = 0xffff88e160db04a8
> > > > > > > > },
> > > > > > > > nf_rcu = {
> > > > > > > > next = 0x10000000000,
> > > > > > > > func = 0x0
> > > > > > > > },
> > > > > > > > nf_birthtime = 0x73d22fc1728
> > > > > > > > }
> > > > > > > >
> > > > > > > > crash> struct
> > > > > > > > nfsd_file.nf_flags,nf_ref.refs.counter,nf_lru,nf_file -x
> > > > > > > > ffff88839a53d850
> > > > > > > > nf_flags = 0x8,
> > > > > > > > nf_ref.refs.counter = 0x0
> > > > > > > > nf_lru = {
> > > > > > > > next = 0xffff88839a53d898,
> > > > > > > > prev = 0xffff88839a53d898
> > > > > > > > },
> > > > > > > > nf_file = 0xffff88810ede8700,
> > > > > > > >
> > > > > > > > crash> struct
> > > > > > > > nfsd_file.nf_flags,nf_ref.refs.counter,nf_lru,nf_file -x
> > > > > > > > ffff88c32b11e850
> > > > > > > > nf_flags = 0x8,
> > > > > > > > nf_ref.refs.counter = 0x0
> > > > > > > > nf_lru = {
> > > > > > > > next = 0xffff88c32b11e898,
> > > > > > > > prev = 0xffff88c32b11e898
> > > > > > > > },
> > > > > > > > nf_file = 0xffff88c20a701c00,
> > > > > > > >
> > > > > > > > crash> struct
> > > > > > > > nfsd_file.nf_flags,nf_ref.refs.counter,nf_lru,nf_file -x
> > > > > > > > ffff88e372709700
> > > > > > > > nf_flags = 0xc,
> > > > > > > > nf_ref.refs.counter = 0x0
> > > > > > > > nf_lru = {
> > > > > > > > next = 0xffff88e372709748,
> > > > > > > > prev = 0xffff88e372709748
> > > > > > > > },
> > > > > > > > nf_file = 0xffff88e0725e6400,
> > > > > > > >
> > > > > > > > crash> struct
> > > > > > > > nfsd_file.nf_flags,nf_ref.refs.counter,nf_lru,nf_file -x
> > > > > > > > ffff8982864944d0
> > > > > > > > nf_flags = 0xc,
> > > > > > > > nf_ref.refs.counter = 0x0
> > > > > > > > nf_lru = {
> > > > > > > > next = 0xffff898286494518,
> > > > > > > > prev = 0xffff898286494518
> > > > > > > > },
> > > > > > > > nf_file = 0xffff89803c0ff700,
> > > > > > > >
> > > > > > > > The leak occurs when nfsd_file_put() races with
> > > > > > > > nfsd_file_cond_queue()
> > > > > > > > or nfsd_file_lru_cb(). With the following patch, I
> > > > > > > > haven't observed
> > > > > > > > any leak after a few days heavy nfs load:
> > > > > > >
> > > > > > > Our patch submission guidelines require a Signed-off-by:
> > > > > > > line at the end of the patch description. See the "Sign
> > > > > > > your work -
> > > > > > > the Developer's Certificate of Origin" section of
> > > > > > >
> > > > > > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/process/submitting-patches.rst?h=v6.10-rc6
> > > > > > >
> > > > > > > (Needed here in case your fix is acceptable).
> > > > > > >
> > > > > > >
> > > > > > > > diff --git a/fs/nfsd/filecache.c b/fs/nfsd/filecache.c
> > > > > > > > index 1a6d5d000b85..2323829f7208 100644
> > > > > > > > --- a/fs/nfsd/filecache.c
> > > > > > > > +++ b/fs/nfsd/filecache.c
> > > > > > > > @@ -389,6 +389,17 @@ nfsd_file_put(struct nfsd_file *nf)
> > > > > > > > if (!nfsd_file_lru_remove(nf))
> > > > > > > > return;
> > > > > > > > }
> > > > > > > > + /*
> > > > > > > > + * Racing with nfsd_file_cond_queue() or
> > > > > > > > nfsd_file_lru_cb(),
> > > > > > > > + * it's unhashed but then removed from the dispose list,
> > > > > > > > + * so we need to free it.
> > > > > > > > + */
> > > > > > > > + if (refcount_read(&nf->nf_ref) == 0 &&
> > > > > > > > + !test_bit(NFSD_FILE_HASHED, &nf->nf_flags) &&
> > > > > > > > + list_empty(&nf->nf_lru)) {
> > > > > > > > + nfsd_file_free(nf);
> > > > > > > > + return;
> > > > > > > > + }
> > > > > > > > }
> > > > > > > > if (refcount_dec_and_test(&nf->nf_ref))
> > > > > > > > nfsd_file_free(nf);
> > > > > > > > @@ -576,7 +587,7 @@ nfsd_file_cond_queue(struct nfsd_file
> > > > > > > > *nf, struct
> > > > > > > > list_head *dispose)
> > > > > > > > int decrement = 1;
> > > > > > > >
> > > > > > > > /* If we raced with someone else unhashing, ignore it
> > > > > > > > */
> > > > > > > > - if (!nfsd_file_unhash(nf))
> > > > > > > > + if (!test_bit(NFSD_FILE_HASHED, &nf->nf_flags))
> > > > > > > > return;
> > > > > > > >
> > > > > > > > /* If we can't get a reference, ignore it */
> > > > > > > > @@ -590,6 +601,7 @@ nfsd_file_cond_queue(struct nfsd_file
> > > > > > > > *nf, struct
> > > > > > > > list_head *dispose)
> > > > > > > > /* 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);
> > > > > > > > + nfsd_file_unhash(nf);
> > > > > > > > trace_nfsd_file_closing(nf);
> > > > > > > > }
> > > > > > > > }
> > > > > > > >
> > > > > > > > Please kindly review the patch and let me know if it
> > > > > > > > makes sense.
> > > > > > > >
> > > > > > > > Thanks,
> > > > > > > >
> > > > > > > > -Youzhong
> > > > > > > >
> > > > > > >
> > > > > > > --
> > > > > > > Chuck Lever
> > > > > >
> > > > >
> > > > > --
> > > > > Jeff Layton <jlayton@xxxxxxxxxxxxxxx>
> > >
> > > --
> > > Jeff Layton <jlayton@xxxxxxxxxxxxxxx>
> > >
> > > --
> > > Jeff Layton <jlayton@xxxxxxxxxx>
>
> --
> Jeff Layton <jlayton@xxxxxxxxxx>
