linux-kernel - Re: [PATCH] [RFC] vfs: don't revalidate dentries that serve as mountpoints

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Date:	Mon, 4 Nov 2013 21:00:23 -0600
From:	Shirish Pargaonkar <shirishpargaonkar@...il.com>
To:	Jeff Layton <jlayton@...hat.com>
Cc:	viro@...iv.linux.org.uk,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-nfs@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
	chutzpah@...too.org, liguangc@...ibm.com
Subject: Re: [PATCH] [RFC] vfs: don't revalidate dentries that serve as mountpoints

For a similar issue in cifs vfs (samba bugzilla 8950), I was going to try
unset the bit DCACHE_OP_REVALIDATE of d_flags of the
dentry.  Would something like work for the mountpoint dentry?



On Mon, Nov 4, 2013 at 7:16 PM, Jeff Layton <jlayton@...hat.com> wrote:
> We had a couple of reports of people that are mounting NFS filesystems,
> and then bind mounting certain local files onto dentries in that nfs
> mount (sort of like a poor-man's unionmount).
>
> This all works well until the dentry serving as the mountpoint fails
> d_revalidate. The dentry will end up being invalidated which makes the
> bind mount unreachable via pathwalk.
>
> It doesn't make much sense to me to allow dentries to serve as
> mountpoints to end up invalidated, so there's no real point in
> attempting to d_revalidate them at all.
>
> Reported-by: Patrick McLean <chutzpah@...too.org>
> Reported-by: Guang Cheng Li <liguangc@...ibm.com>
> Signed-off-by: Jeff Layton <jlayton@...hat.com>
> ---
>  fs/namei.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/fs/namei.c b/fs/namei.c
> index caa2805..5b10ad0 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -585,6 +585,9 @@ drop_root_mnt:
>
>  static inline int d_revalidate(struct dentry *dentry, unsigned int flags)
>  {
> +       /* dentries that serve as mountpoints are always considered valid */
> +       if (d_mountpoint(dentry))
> +               return 1;
>         return dentry->d_op->d_revalidate(dentry, flags);
>  }
>
> --
> 1.8.3.1
>
> --
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