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

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Message-ID: <20131105133959.5be7c728@notabene.brown>
Date:	Tue, 5 Nov 2013 13:39:59 +1100
From:	NeilBrown <neilb@...e.de>
To:	Jeff Layton <jlayton@...hat.com>
Cc:	viro@...IV.linux.org.uk, linux-fsdevel@...r.kernel.org,
	linux-nfs@...r.kernel.org, linux-kernel@...r.kernel.org,
	chutzpah@...too.org, liguangc@...ibm.com
Subject: Re: [PATCH] [RFC] vfs: don't revalidate dentries that serve as
 mountpoints

On Mon,  4 Nov 2013 20:16:26 -0500 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);
>  }
>  

Makes sense to me. But what about the parent or grandparent of the mountpoint?

Presumably we don't want to invalidate them either, and I don't think
d_mountpoint() returns true for those, does it?

NeilBrown

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