Message-ID: <ab5c96681e62be6ca79e7464c1141106b6d5fbce.camel@redhat.com>
Subject: Re: Why doesn't NFSv3 implement LOOKUPP?
From:   Jeff Layton <jlayton@redhat.com>
To:     Yihao Wu <wuyihao@linux.alibaba.com>, linux-nfs@vger.kernel.org
Date:   Fri, 15 Feb 2019 04:49:25 -0500
In-Reply-To: <2a2b8156-69c1-ad7f-6000-716f28690fb7@linux.alibaba.com>
References: <3dc67361-9c2f-d183-09e5-0a4d5c48d0f7@linux.alibaba.com>
         <0bd905c30a652d19404eef82b40f6b92987ca814.camel@redhat.com>
         <2a2b8156-69c1-ad7f-6000-716f28690fb7@linux.alibaba.com>
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On Fri, 2019-02-15 at 13:14 +0800, Yihao Wu wrote:
> On 2019/2/13 7:50 PM, Jeff Layton wrote:
> > On Wed, 2019-02-13 at 16:41 +0800, Yihao Wu wrote:
> > > Hi all,
> > > 
> > > When looking into "Failures: generic/467" given by xfstests, I found that NFSv3
> > > didn't implement LOOKUPP. I know that this might be by design. But LOOKUPP was
> > > meant to replace ".." in NFSv3, right?
> > > 
> > > xfstests's generic/467 test case performs the following sequence of operations.
> > > 
> > > name_to_handle -> drop_caches -> open_by_handle
> > > 
> > > Dentry becomes disconnected due to drop_caches. NFSv3 doesn't support LOOKUPP.
> > > So when it performs open_by_handle to an directory, this test case fails.
> > > 
> > > I did some small experiment by implementing LOOKUPP for NFSv3. The way I tried
> > > is to merely pass ".." to nfs3_proc_lookup. And it seems to work. At least it's
> > > a workaround for xfstests.
> > > 
> > > I'm curious whether this sort of simulation of LOOKUPP will work or make sense.
> > > 
> > > Thanks,
> > > Yihao Wu
> > 
> > v3 was mostly designed with unix-like clients in mind. For v4, the spec
> > writers cast a wider net and decided not to put special meaning on
> > lookups of "." and "..", but they still needed a way to do a lookup of
> > "..".
> > 
> > The question is why you want to implement LOOKUPP in v3. Mostly we added
> > it to the client to support reexporting NFSv4 filesystems via NFSv3. Are
> > you looking to reexport v3 filesystems for some reason?
> > 
> 
> Thanks a lot for your reply, Jeff!
> 
> I'm just managing to figure out the source of this xfstests failure, that
> open_by_handle is simply not working for NFSv3 after drop_caches. I think NFSv3
> might become able to reconnect_path too, with LOOKUP "..". However it's not,
> because nfs_get_parent fails as long as LOOKUPP is not supported.
> 
> My server & client both support NFSv3. Can I take it that re-exporting is not a
> required option in this case?

Yes, I don't think we're too interested in reexporting v3.

The open_by_handle_at call is mostly there to support userland NFS
servers (though there are some other legit use-cases). If you're not
planning on reexporting NFSv3, then those failures probably won't matter
much.

If you do have need to use open_by_handle_at on NFSv3 for some
legitimate purpose, then we could certainly look at patching that up in
some fashion.
-- 
Jeff Layton <jlayton@redhat.com>