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Date:   Mon, 9 Oct 2023 10:15:25 -0700
From:   Krister Johansen <kjlx@templeofstupid.com>
To:     Bernd Schubert <bernd.schubert@fastmail.fm>
Cc:     Krister Johansen <kjlx@templeofstupid.com>,
        Miklos Szeredi <miklos@szeredi.hu>,
        linux-fsdevel@vger.kernel.org,
        Miklos Szeredi <mszeredi@redhat.com>,
        linux-kernel@vger.kernel.org,
        German Maglione <gmaglione@redhat.com>,
        Greg Kurz <groug@kaod.org>, Max Reitz <mreitz@redhat.com>
Subject: Re: [resend PATCH v2 2/2] fuse: ensure that submounts lookup their
 parent
Message-ID: <20231009171525.GA1973@templeofstupid.com>
References: <cover.1696043833.git.kjlx@templeofstupid.com>
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On Mon, Oct 09, 2023 at 02:52:42PM +0200, Bernd Schubert wrote:
> 
> 
> On 10/7/23 02:41, Krister Johansen wrote:
> > On Fri, Oct 06, 2023 at 07:13:06PM +0200, Bernd Schubert wrote:
> > > 
> > > 
> > > On 10/2/23 17:24, Krister Johansen wrote:
> > > > The submount code uses the parent nodeid passed into the function in
> > > > order to create the root dentry for the new submount.  This nodeid does
> > > > not get its remote reference count incremented by a lookup option.
> > > > 
> > > > If the parent inode is evicted from its superblock, due to memory
> > > > pressure for example, it can result in a forget opertation being sent to
> > > > the server.  Should this nodeid be forgotten while it is still in use in
> > > > a submount, users of the submount get an error from the server on any
> > > > subsequent access.  In the author's case, this was an EBADF on all
> > > > subsequent operations that needed to reference the root.
> > > > 
> > > > Debugging the problem revealed that the dentry shrinker triggered a forget
> > > > after killing the dentry with the last reference, despite the root
> > > > dentry in another superblock still using the nodeid.
> > > > 
> > > > As a result, a container that was also using this submount failed to
> > > > access its filesystem because it had borrowed the reference instead of
> > > > taking its own when setting up its superblock for the submount.
> > > > 
> > > > This commit fixes the problem by having the new submount trigger a
> > > > lookup for the parent as part of creating a new root dentry for the
> > > > virtiofsd submount superblock.  This allows each superblock to have its
> > > > inodes removed by the shrinker when unreferenced, while keeping the
> > > > nodeid reference count accurate and active with the server.
> > > > 
> > > > Signed-off-by: Krister Johansen <kjlx@templeofstupid.com>
> > > > ---
> > > >    fs/fuse/dir.c    | 10 +++++-----
> > > >    fs/fuse/fuse_i.h |  6 ++++++
> > > >    fs/fuse/inode.c  | 43 +++++++++++++++++++++++++++++++++++++------
> > > >    3 files changed, 48 insertions(+), 11 deletions(-)
> > > > 
> > > > diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
> > > > index 5e01946d7531..333730c74619 100644
> > > > --- a/fs/fuse/dir.c
> > > > +++ b/fs/fuse/dir.c
> > > > @@ -183,11 +183,11 @@ static void fuse_lookup_init(struct fuse_conn *fc, struct fuse_args *args,
> > > >    	args->out_args[0].value = outarg;
> > > >    }
> > > > -static int fuse_dentry_revalidate_lookup(struct fuse_mount *fm,
> > > > -					 struct dentry *entry,
> > > > -					 struct inode *inode,
> > > > -					 struct fuse_entry_out *outarg,
> > > > -					 bool *lookedup)
> > > > +int fuse_dentry_revalidate_lookup(struct fuse_mount *fm,
> > > > +				  struct dentry *entry,
> > > > +				  struct inode *inode,
> > > > +				  struct fuse_entry_out *outarg,
> > > > +				  bool *lookedup)
> > > >    {
> > > >    	struct dentry *parent;
> > > >    	struct fuse_forget_link *forget;
> > > > diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
> > > > index 405252bb51f2..a66fcf50a4cc 100644
> > > > --- a/fs/fuse/fuse_i.h
> > > > +++ b/fs/fuse/fuse_i.h
> > > > @@ -1325,6 +1325,12 @@ void fuse_dax_dontcache(struct inode *inode, unsigned int flags);
> > > >    bool fuse_dax_check_alignment(struct fuse_conn *fc, unsigned int map_alignment);
> > > >    void fuse_dax_cancel_work(struct fuse_conn *fc);
> > > > +/* dir.c */
> > > > +int fuse_dentry_revalidate_lookup(struct fuse_mount *fm, struct dentry *entry,
> > > > +				  struct inode *inode,
> > > > +				  struct fuse_entry_out *outarg,
> > > > +				  bool *lookedup);
> > > > +
> > > >    /* ioctl.c */
> > > >    long fuse_file_ioctl(struct file *file, unsigned int cmd, unsigned long arg);
> > > >    long fuse_file_compat_ioctl(struct file *file, unsigned int cmd,
> > > > diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
> > > > index 444418e240c8..79a31cb55512 100644
> > > > --- a/fs/fuse/inode.c
> > > > +++ b/fs/fuse/inode.c
> > > > @@ -1464,7 +1464,13 @@ static int fuse_fill_super_submount(struct super_block *sb,
> > > >    	struct fuse_mount *fm = get_fuse_mount_super(sb);
> > > >    	struct super_block *parent_sb = parent_fi->inode.i_sb;
> > > >    	struct fuse_attr root_attr;
> > > > +	struct fuse_inode *fi;
> > > >    	struct inode *root;
> > > > +	struct inode *parent;
> > > > +	struct dentry *pdent;
> > > > +	struct fuse_entry_out outarg;
> > > > +	bool lookedup = false;
> > > > +	int ret;
> > > >    	fuse_sb_defaults(sb);
> > > >    	fm->sb = sb;
> > > > @@ -1480,14 +1486,39 @@ static int fuse_fill_super_submount(struct super_block *sb,
> > > >    	if (parent_sb->s_subtype && !sb->s_subtype)
> > > >    		return -ENOMEM;
> > > > -	fuse_fill_attr_from_inode(&root_attr, parent_fi);
> > > > -	root = fuse_iget(sb, parent_fi->nodeid, 0, &root_attr, 0, 0);
> > > >    	/*
> > > > -	 * This inode is just a duplicate, so it is not looked up and
> > > > -	 * its nlookup should not be incremented.  fuse_iget() does
> > > > -	 * that, though, so undo it here.
> > > > +	 * It is necessary to lookup the parent_if->nodeid in case the dentry
> > > > +	 * that triggered the automount of the submount is later evicted.
> > > > +	 * If this dentry is evicted without the lookup count getting increased
> > > > +	 * on the submount root, then the server can subsequently forget this
> > > > +	 * nodeid which leads to errors when trying to access the root of the
> > > > +	 * submount.
> > > >    	 */
> > > > -	get_fuse_inode(root)->nlookup--;
> > > > +	parent = &parent_fi->inode;
> > > > +	pdent = d_find_alias(parent);
> > > > +	if (!pdent)
> > > > +		return -EINVAL;
> > > > +
> > > > +	ret = fuse_dentry_revalidate_lookup(fm, pdent, parent, &outarg,
> > > > +	    &lookedup);
> > > > +	dput(pdent);
> > > > +	/*
> > > > +	 * The new root owns this nlookup on success, and it is incremented by
> > > > +	 * fuse_iget().  In the case the lookup succeeded but revalidate fails,
> > > > +	 * ensure that the lookup count is tracked by the parent.
> > > > +	 */
> > > > +	if (ret <= 0) {
> > > > +		if (lookedup) {
> > > > +			fi = get_fuse_inode(parent);
> > > > +			spin_lock(&fi->lock);
> > > > +			fi->nlookup++;
> > > > +			spin_unlock(&fi->lock);
> > > > +		}
> > > 
> > > I might be wrong, but doesn't that mean that
> > > "get_fuse_inode(root)->nlookup--" needs to be called?
> > 
> > In the case where ret > 0, the nlookup on get_fuse_inode(root) is set to
> > 1 by fuse_iget().  That ensures that the root is forgotten when later
> > unmounted.  The code that handles the forget uses the count of nlookup
> > to tell the server-side how many references to forget.  (That's in
> > fuse_evict_inode()).
> > 
> > However, if the fuse_dentry_revalidate_lookup() call performs a valid
> > lookup but returns an error, this function will return before it fills
> > out s_root in the superblock or calls fuse_iget().  If the superblock
> > doesn't have a s_root set, then the code in generic_kill_super() won't
> > dput() the root dentry and trigger the forget.
> > 
> > The intention of this code was to handle the case where the lookup had
> > succeeded, but the code determined it was still necessary to return an
> > error.  In that situation, the reference taken by the lookup has to be
> > accounted somewhere, and the parent seemed like a plausible candidate.
> 
> Yeah sorry, I had just missed that fuse_iget() also moved and then thought
> it would have increased fi->nlookup already.

No worries; I'd much rather get feedback if something doesn't look
right, even if it turns out okay in the end.

> > However, after writing up this response, I can see that there's still a
> > problem here if d_make_root(root) returns NULL, because we'll also lose
> > track of the nlookup in that case.
> > 
> > If you agree that charging this to the parent on error makes sense, I'll
> > re-work the error handling here so that the right thing happens when
> > either fuse_dentry_revalidate_lookup() or d_make_root() encounter an
> > error.
> 
> Oh yeah, I also missed that. Although, iput() calls iput_final, which then
> calls evict and sends the fuse forget - isn't that the right action already?

Thanks, I had forgotten that d_make_root() would call iput() for me if
d_alloc_anon() fails.  Let me restate this to suggest that I account the
nlookup to the parent if fuse_dentry_revalidate_lookup() or fuse_iget()
fail instead.  Does that sound right?

> > Thanks for the feedback.
> 
> Well, false alarm from my side, sorry again!

No apology necessary; I appreciate you spending the time to look and ask
questions.

-K