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Subject: Re: [PATCH v6 2/7] kernfs: add a revision to identify directory
 node changes
From:   Ian Kent <raven@themaw.net>
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     Miklos Szeredi <miklos@szeredi.hu>, Tejun Heo <tj@kernel.org>,
        Eric Sandeen <sandeen@sandeen.net>,
        Fox Chen <foxhlchen@gmail.com>,
        Brice Goglin <brice.goglin@gmail.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Rick Lindsley <ricklind@linux.vnet.ibm.com>,
        David Howells <dhowells@redhat.com>,
        Marcelo Tosatti <mtosatti@redhat.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Carlos Maiolino <cmaiolino@redhat.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Kernel Mailing List <linux-kernel@vger.kernel.org>
Date:   Fri, 11 Jun 2021 21:31:36 +0800
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On Fri, 2021-06-11 at 15:11 +0200, Greg Kroah-Hartman wrote:
> On Fri, Jun 11, 2021 at 08:56:18PM +0800, Ian Kent wrote:
> > On Fri, 2021-06-11 at 14:49 +0200, Miklos Szeredi wrote:
> > > On Wed, 9 Jun 2021 at 10:50, Ian Kent <raven@themaw.net> wrote:
> > > > 
> > > > Add a revision counter to kernfs directory nodes so it can be
> > > > used
> > > > to detect if a directory node has changed during negative
> > > > dentry
> > > > revalidation.
> > > > 
> > > > There's an assumption that sizeof(unsigned long) <=
> > > > sizeof(pointer)
> > > > on all architectures and as far as I know that assumption
> > > > holds.
> > > > 
> > > > So adding a revision counter to the struct kernfs_elem_dir
> > > > variant
> > > > of
> > > > the kernfs_node type union won't increase the size of the
> > > > kernfs_node
> > > > struct. This is because struct kernfs_elem_dir is at least
> > > > sizeof(pointer) smaller than the largest union variant. It's
> > > > tempting
> > > > to make the revision counter a u64 but that would increase the
> > > > size
> > > > of
> > > > kernfs_node on archs where sizeof(pointer) is smaller than the
> > > > revision
> > > > counter.
> > > > 
> > > > Signed-off-by: Ian Kent <raven@themaw.net>
> > > > ---
> > > >  fs/kernfs/dir.c             |    2 ++
> > > >  fs/kernfs/kernfs-internal.h |   23 +++++++++++++++++++++++
> > > >  include/linux/kernfs.h      |    5 +++++
> > > >  3 files changed, 30 insertions(+)
> > > > 
> > > > diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
> > > > index 33166ec90a112..b3d1bc0f317d0 100644
> > > > --- a/fs/kernfs/dir.c
> > > > +++ b/fs/kernfs/dir.c
> > > > @@ -372,6 +372,7 @@ static int kernfs_link_sibling(struct
> > > > kernfs_node *kn)
> > > >         /* successfully added, account subdir number */
> > > >         if (kernfs_type(kn) == KERNFS_DIR)
> > > >                 kn->parent->dir.subdirs++;
> > > > +       kernfs_inc_rev(kn->parent);
> > > > 
> > > >         return 0;
> > > >  }
> > > > @@ -394,6 +395,7 @@ static bool kernfs_unlink_sibling(struct
> > > > kernfs_node *kn)
> > > > 
> > > >         if (kernfs_type(kn) == KERNFS_DIR)
> > > >                 kn->parent->dir.subdirs--;
> > > > +       kernfs_inc_rev(kn->parent);
> > > > 
> > > >         rb_erase(&kn->rb, &kn->parent->dir.children);
> > > >         RB_CLEAR_NODE(&kn->rb);
> > > > diff --git a/fs/kernfs/kernfs-internal.h b/fs/kernfs/kernfs-
> > > > internal.h
> > > > index ccc3b44f6306f..b4e7579e04799 100644
> > > > --- a/fs/kernfs/kernfs-internal.h
> > > > +++ b/fs/kernfs/kernfs-internal.h
> > > > @@ -81,6 +81,29 @@ static inline struct kernfs_node
> > > > *kernfs_dentry_node(struct dentry *dentry)
> > > >         return d_inode(dentry)->i_private;
> > > >  }
> > > > 
> > > > +static inline void kernfs_set_rev(struct kernfs_node *kn,
> > > > +                                 struct dentry *dentry)
> > > > +{
> > > > +       if (kernfs_type(kn) == KERNFS_DIR)
> > > > +               dentry->d_time = kn->dir.rev;
> > > > +}
> > > > +
> > > > +static inline void kernfs_inc_rev(struct kernfs_node *kn)
> > > > +{
> > > > +       if (kernfs_type(kn) == KERNFS_DIR)
> > > > +               kn->dir.rev++;
> > > > +}
> > > > +
> > > > +static inline bool kernfs_dir_changed(struct kernfs_node *kn,
> > > > +                                     struct dentry *dentry)
> > > > +{
> > > > +       if (kernfs_type(kn) == KERNFS_DIR) {
> > > 
> > > Aren't these always be called on a KERNFS_DIR node?
> > 
> > Yes they are.
> > 
> > > 
> > > You could just reduce that to a WARN_ON, or remove the conditions
> > > altogether then.
> > 
> > I was tempted to not use the check, a WARN_ON sounds better than
> > removing the check, I'll do that in a v7.
> 
> No, WARN_ON is not ok, as systems will crash if panic-on-warn is set.

Thanks Greg, understood.

> 
> If these are impossible to hit, great, let's not check this and we
> can
> just drop the code.  If they can be hit, then the above code is
> correct
> and it should stay.

It's a programming mistake to call these on a non-directory node.

I can remove the check but do you think there's any value in passing
the node and updating it's parent to avoid possible misuse?

Ian