Subject: Re: Why doesn't NFSv3 implement LOOKUPP?
To:     Jeff Layton <jlayton@redhat.com>, linux-nfs@vger.kernel.org
References: <3dc67361-9c2f-d183-09e5-0a4d5c48d0f7@linux.alibaba.com>
 <0bd905c30a652d19404eef82b40f6b92987ca814.camel@redhat.com>
From:   Yihao Wu <wuyihao@linux.alibaba.com>
Message-ID: <2a2b8156-69c1-ad7f-6000-716f28690fb7@linux.alibaba.com>
Date:   Fri, 15 Feb 2019 13:14:00 +0800
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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?

Thanks,
Yihao Wu