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Subject: Re: remounting hard -> soft
From:   Chuck Lever <chuck.lever@oracle.com>
In-Reply-To: <87y2y265cm.fsf@notabene.neil.brown.name>
Date:   Thu, 3 Oct 2019 09:01:48 -0400
Cc:     Trond Myklebust <trond.myklebust@hammerspace.com>,
        Neil F Brown <nfbrown@suse.com>,
        Linux NFS Mailing List <linux-nfs@vger.kernel.org>
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 <87y2y265cm.fsf@notabene.neil.brown.name>
To:     Neil Brown <neilb@suse.de>
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> On Oct 2, 2019, at 8:27 PM, NeilBrown <neilb@suse.de> wrote:
> 
> On Wed, Oct 02 2019, Chuck Lever wrote:
> 
>> Hi Trond-
>> 
>> We (Oracle) had another (fairly rare) instance of a weekend maintenance
>> window where an NFS server's IP address changed while there were mounted
>> clients. It brought up the issue again of how we (the Linux NFS community)
>> would like to deal with cases where a client administrator has to deal
>> with a moribund mount (like that alliteration :-).
> 
> What exactly is the problem that this caused?
> 
> As I understand it, a moribund mount can still be unmounted with "-l"
> and processes accessing it can still be killed

I was asking about "-o remount,soft" because I was not certain
about the outcome last time this conversation was in full swing.
The gist then is that we want "umount -l" and "umount -f" to
work reliably and as advertised?


> ... except....
> There are some waits the VFS/MM which are not TASK_KILLABLE and
> probably should be.  I think that "we" definitely want "someone" to
> track them down and fix them.

I agree... and "someone" could mean me or someone here at Oracle.


>> Does remounting with "soft" work today? That seems like the most direct
>> way to deal with this particular situation.
> 
> I don't think this does work, and it would be non-trivial (but maybe not
> impossible) to mark all the outstanding RPCs as also "soft".

The problem I've observed with umount is umount_begin does the
killall_tasks call, then the client issues some additional requests.
Those are the requests that get stuck before umount_end can finally
shutdown the RPC client. umount_end is never called because those
requests are "hard".

We have rpc_killall_tasks which loops over all of an rpc_clnt's
outstanding RPC tasks. nfs_umount_begin could do something like

- set the rpc_clnt's "soft" flag
- kill all tasks

Then any new tasks would timeout eventually. Just a thought, maybe
not a good one.

There's also using SOFTCONN for all tasks after killall is called:
if the client can't reconnect to the server, these tasks would fail
immediately.


> If we wanted to follow a path like this (and I suspect we don't), I
> would hope that we could expose the server connection (shared among
> multiple mounts) in sysfs somewhere, and could then set "soft" (or
> "dead") on that connection, rather than having to do it on every mount
> from the particular server.

I think of your use case from last time: client shutdown should be
reliable. Seems like making "umount -f" reliable would be better
for that use case, and would work for the "make client mount points
recoverable after server dies" case too.


--
Chuck Lever