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Date:   Mon, 3 Oct 2022 15:24:55 +0200
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Subject: Re: nfs4.1+: workaround for defunct clientaddr?
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To:     Jeff Layton <jlayton@kernel.org>, linux-nfs@vger.kernel.org
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 <c163b299164cd9abe21325a947c9efe908a04331.camel@kernel.org>
From:   Manfred Schwarb <manfred99@gmx.ch>
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Am 03.10.22 um 14:26 schrieb Jeff Layton:
> On Mon, 2022-10-03 at 13:55 +0200, Manfred Schwarb wrote:
>> Am 03.10.22 um 13:39 schrieb Jeff Layton:
>>> On Sun, 2022-10-02 at 14:35 +0200, Manfred Schwarb wrote:
>>>> Hi,
>>>>
>>>> I have 2 boxes connected with 2 network cards each, one
>>>> crossover connection and one connection via LAN.
>>>> I want to use the crossover connection for backup,
>>>> so I want to be able to select exactly this wire when
>>>> doing my NFS backup transfers. Everything interconnected via NFS4.1
>>>> and automount.
>>>>
>>>> Now the thing is, if there is an already existing connection
>>>> via LAN, I am not able to select the crossover connection,
>>>> there is some session reuse against my will.
>>>>
>>>> automount config:
>>>> /net/192.168.99.1  -fstype=3Dnfs4,nfsvers=3D4,minorversion=3D1,client=
addr=3D192.168.99.100   /  192.168.99.1:/
>>>> /net2/192.168.98.1 -fstype=3Dnfs4,nfsvers=3D4,minorversion=3D1,client=
addr=3D192.168.98.100   /  192.168.98.1:/
>>>>
>>>> mount -l:
>>>> 192.168.99.1:/data on /net/192.168.99.1/data type nfs4 (...,clientadd=
r=3D192.168.99.100,addr=3D192.168.99.1)
>>>> 192.168.99.1:/data on /net2/192.168.98.1/data type nfs4 (...,clientad=
dr=3D192.168.99.100,addr=3D192.168.99.1)
>>>>
>>>> As you see, both connections are on "192.168.99.1:/data", and the bac=
kup runs
>>>> over the same wire as all user communication, which is not desired.
>>>> This even happens if I explicitly set some clientaddr=3D option.
>>>>
>>>> Now I found two workarounds:
>>>> - downgrade to NFS 4.0, clientaddr seems to work with it
>>>> - choose different NFS versions, i.e. one connection with
>>>>   minorversion=3D1 and the other with minorversion=3D2
>>>>
>>>> Both possibilities seem a bit lame to me.
>>>> Are there some other (recommended) variants which do what I want?
>>>>
>>>> It seems different minor versions result in different "nfs4_unique_id=
" values,
>>>> and therefore no session sharing occurs. But why do different network
>>>> interfaces (via explicitly set clientaddr=3D by user) not result in d=
ifferent
>>>> "nfs4_unique_id" values?
>>>>
>>>> Thanks for any comments and advice,
>>>> Manfred
>>>
>>> That sounds like a bug. We probably need to compare the clientaddr
>>> values in nfs_compare_super or nfs_compare_mount_options so that it
>>> doesn't match if the clientaddrs are different.
>>>
>
>
> Actually, I take it back, clientaddr is specifically advertised as being
> for NFSv4.0 only. The workaround for you is "nosharecache", which will
> force the mount under /net2 to get a new superblock altogether.

But clientaddr is silently accepted on NFS4.1+, and seemingly silently doe=
s nothing.

The point is, RFC5661 explicitely tells
"NFS minor version 1 is deemed superior to NFS minor version 0 with no los=
s of functionality".

So this behavior comes as a surprise.

>
>>> As a workaround, you can probably mount the second mount with
>>> -o nosharecache and get what you want.
>>
>> Indeed, nosharecache works. But the man page has some scary words for i=
t:
>>   "This is considered a data risk".
>>
>
> Yeah, it does sound scary but it's not a huge issue unless you're doing
> I/O to the same files at the same time via both mounts. With
> "sharecache" (the default) you get better cache coherency in that
> situation since the inode and its pagecache are the same.
>

So I guess this is equivalent to the minorversion=3D1/minorversion=3D2 tri=
ck
cache coherency wise then?


> With "nosharecache" you need to be more careful to flush caches, etc. if
> you are doing reads and writes to the same files via different paths. If
> you need careful coordination there, then you probably want to use file
> locking.

Thanks for these explanations, it is appreciated!
Manfred

> --
> Jeff Layton <jlayton@kernel.org>