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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Mon, 26 Sep 2011 15:24:59 -0700
Message-ID: <CA+55aFzaScGiKmsSrgXivoUPw4xHjKobsB4VquuEu1fG=7R+wA@mail.gmail.com>
Subject: Re: [PATCH] VFS: Suppress automount on [l]stat, [l]getxattr, etc.
To: Trond Myklebust <Trond.Myklebust@netapp.com>
Cc: Ian Kent <raven@themaw.net>, Jeff Layton <jlayton@redhat.com>,
        Miklos Szeredi <miklos@szeredi.hu>,
        David Howells <dhowells@redhat.com>, viro@zeniv.linux.org.uk,
        gregkh@suse.de, linux-nfs@vger.kernel.org, leonardo.lists@gmail.com
Content-Type: text/plain; charset=ISO-8859-1
Sender: linux-nfs-owner@vger.kernel.org
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On Mon, Sep 26, 2011 at 2:31 PM, Trond Myklebust
<Trond.Myklebust@netapp.com> wrote:
>
> A bind mount doesn't actually want us to set up OPEN state on the NFSv4
> server and return a fully initialised 'struct file' in the struct
> nameidata. If it did, we would already have set the LOOKUP_OPEN flag
> together with the accompanying open intent structure in the struct
> nameidata.
>
> Ditto for the quotactl().

Umm. As far as I can see, adding LOOKUP_OPEN doesn't actually even
*do* what you claim.

The whole "allocate struct file in struct nameidata" happens in
path_openat(), it doesn't happen in the normal path creation
(do_path_lookup() and friends). The only thing NFS does with
LOOKUP_OPEN is to check whether it needs to do a revalidate
regardless.

But even more importantly than your apparent confusion about filling
in the 'nameidata.intent' fields, even *if* we were to do unnecessary
filp allocations etc (which we don't do, as mentioned), from a user
standpoint, what would be the actual downside?

It would be extra work for a (very rare) operation, but compared to
the known downside that is LOOKUP_FOLLOW and auto-mounting when we
shouldn't, even that hypothetical downside seems pretty benign,
doesn't it?

In other words, what I don't understand is how you seem to be totally
happy ignoring the known problems with LOOKUP_FOLLOW, but LOOKUP_OPEN
is some monster thing that you don't want to deal with.

WHY?

                        Linus

PS. That said, looking at the *cifs* code, it seems to do some nasty
things on LOOKUP_OPEN, and assumes that it always has a 'file'
pointer. Sad. That might actually argue for another flag, or
alternatively for just fixing CIFS.