Subject: Re: [PATCH] vfs: Don't leak disconnected dentries on umount

From: Jan Kara <[email-masked]>
Date: Fri, 3 Oct 2025 15:33:27 +0200

On Thu 02-10-25 17:36:49, Al Viro wrote:
> On Thu, Oct 02, 2025 at 05:55:07PM +0200, Jan Kara wrote:
> 
> > diff --git a/fs/dcache.c b/fs/dcache.c
> > index 65cc11939654..3ec21f9cedba 100644
> > --- a/fs/dcache.c
> > +++ b/fs/dcache.c
> > @@ -2557,6 +2557,8 @@ struct dentry *d_alloc_parallel(struct dentry *parent,
> >  	spin_lock(&parent->d_lock);
> >  	new->d_parent = dget_dlock(parent);
> >  	hlist_add_head(&new->d_sib, &parent->d_children);
> > +	if (parent->d_flags & DCACHE_DISCONNECTED)
> > +		new->d_flags |= DCACHE_DISCONNECTED;
> >  	spin_unlock(&parent->d_lock);
> 
> Not a good place for that |=, IMO...

The second place I was considering was a bit earlier where we set
DCACHE_PAR_LOOKUP in new->d_flags but then I've decided to put this under
parent's d_lock so that parent->d_flags cannot change under us. But in
principle that race is harmless so do you prefer to move this setting out
of parent->d_lock critical section?

								Honza
-- 
Jan Kara <[email-masked]>
SUSE Labs, CR

---

From: Al Viro <[email-masked]>
Date: Thu, 2 Oct 2025 17:36:49 +0100

---

From: Jan Kara <[email-masked]>
Date: Thu, 2 Oct 2025 17:55:07 +0200

When user calls open_by_handle_at() on some inode that is not cached, we
will create disconnected dentry for it. If such dentry is a directory,
exportfs_decode_fh_raw() will then try to connect this dentry to the
dentry tree through reconnect_path(). It may happen for various reasons
(such as corrupted fs or race with rename) that the call to
lookup_one_unlocked() in reconnect_one() will fail to find the dentry we
are trying to reconnect and instead create a new dentry under the
parent. Now this dentry will not be marked as disconnected although the
parent still may well be disconnected (at least in case this
inconsistency happened because the fs is corrupted and .. doesn't point
to the real parent directory). This creates inconsistency in
disconnected flags but AFAICS it was mostly harmless. At least until
commit f1ee616214cb ("VFS: don't keep disconnected dentries on d_anon")
which removed adding of most disconnected dentries to sb->s_anon list.
Thus after this commit cleanup of disconnected dentries implicitely
relies on the fact that dput() will immediately reclaim such dentries.
However when some leaf dentry isn't marked as disconnected, as in the
scenario described above, the reclaim doesn't happen and the dentries
are "leaked". Memory reclaim can eventually reclaim them but otherwise
they stay in memory and if umount comes first, we hit infamous "Busy
inodes after unmount" bug. Make sure all dentries created under a
disconnected parent are marked as disconnected as well.

Reported-by: [email-masked]
Fixes: f1ee616214cb ("VFS: don't keep disconnected dentries on d_anon")
CC: [email-masked]
Signed-off-by: Jan Kara <[email-masked]>
---
 fs/dcache.c | 2 ++
 1 file changed, 2 insertions(+)

 retry:
-- 
2.51.0