linux-kernel - Re: [PATCH 9/9] nfs: don't open in ->d

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Date:	Tue, 6 Mar 2012 14:12:38 +0000
From:	"Myklebust, Trond" <Trond.Myklebust@...app.com>
To:	Miklos Szeredi <miklos@...redi.hu>
CC:	"viro@...IV.linux.org.uk" <viro@...IV.linux.org.uk>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"hch@...radead.org" <hch@...radead.org>,
	"mszeredi@...e.cz" <mszeredi@...e.cz>
Subject: Re: [PATCH 9/9] nfs: don't open in ->d_revalidate

On Tue, 2012-03-06 at 13:56 +0100, Miklos Szeredi wrote:
> From: Miklos Szeredi <mszeredi@...e.cz>
> 
> NFSv4 can't do reliable opens in d_revalidate, since it cannot know whether a
> mount needs to be followed or not.  It does check d_mountpoint() on the dentry,
> which can result in a weird error if the VFS found that the mount does not in
> fact need to be followed, e.g.:
> 
>   # mount --bind /mnt/nfs /mnt/nfs-clone
>   # echo something > /mnt/nfs/tmp/bar
>   # echo x > /tmp/file
>   # mount --bind /tmp/file /mnt/nfs-clone/tmp/bar
>   # cat  /mnt/nfs/tmp/bar
>   cat: /mnt/nfs/tmp/bar: Not a directory
> 
> Which should, by any sane filesystem, result in "something" being printed.
> 
> So instead do the open in f_op->open() and in the unlikely case that the cached
> dentry turned out to be invalid, drop the dentry and return ESTALE to let the
> VFS retry.

This patch would force a complete new walk of the path in cases where
today we just do a single lookup of the last component. It really
doesn't seem worth taking that penalty just in order to make some insane
bind mount corner cases work.

-- 
Trond Myklebust
Linux NFS client maintainer

NetApp
Trond.Myklebust@...app.com
www.netapp.com