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

[NeilBrown <neilb-l3A5Bk7waGM@public.gmane.org>]

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On Mon,  4 Nov 2013 20:16:26 -0500 Jeff Layton <jlayton-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> 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-aBrp7R+bbdUdnm+yROfE0A@public.gmane.org>
> Reported-by: Guang Cheng Li <liguangc-vtt25B2cwJLQT0dZR+AlfA@public.gmane.org>
> Signed-off-by: Jeff Layton <jlayton-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
> ---
>  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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