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From:   Miklos Szeredi <miklos@szeredi.hu>
Date:   Fri, 11 Jun 2021 14:49:43 +0200
Message-ID: <CAJfpeguzPEy+UAcyT4tcpvYxeTwB+64yxRw8Sh7UBROBuafYdw@mail.gmail.com>
Subject: Re: [PATCH v6 2/7] kernfs: add a revision to identify directory node changes
To:     Ian Kent <raven@themaw.net>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        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>
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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?

You could just reduce that to a WARN_ON, or remove the conditions
altogether then.

Thanks,
Miklos