umount -f implementation

[Rick Macklem]

On Tue, 30 Jun 2009, Kostik Belousov wrote:

>>
>> I think I can fix this in the experimental nfsv4 client, since it has
>> a kernel thread that can check for MNTK_UNMOUNTF being set and then
>> kill off the RPCs in progress, but that won't help the regular client.
> This solution sounds good, but see below.
>

> It may be argued by some people, me included, that umount -f shall not
> override any ownership of kernel resources. In particular, you must
> not ignore the lockref. Instead, the threads that own misc filesystem
> resources, like mount reference counter, locked vnodes etc shall be
> weed out of the syscalls. E.g., finishing stalled rpc calls with some
> error code that is propagated to return code from vops is good solution.
>
I think that the thread "fix" above would work this way. Right now,
nfs_umount() terminates RPCs in progress for the "-f" case and they
return RPC_CANTRECV, which just becomes EACCES at the moment. The
problem is that, often, the "umount -f" thread never gets as far as
nfs_umount(). All I was thinking of doing, above, is having the kernel
thread check for MNTK_UNMOUNTF and then do the same thing. (ie. The
NFS VOPs would end up returning EACCES, or whatever Exxx might be
preferred.)

> Another problem with forced unmounts is that VFS does not block new
> threads from arriving into VOPs. When finishing the inflight rpcs,
> you may either leave some new rpcs behind or loop infinitely chasing
> rpcs that arrive while you finishing old rpcs.
>
The NFS clients already handle this by returning ESTALE at the
beginning of nfs_request() without attempting the RPC, if MNTK_UNMOUNTF
is set. (Why ESTALE?? Who knows, although I suspect that just about
any Exxx will get the job done?)

> Umount -f is needed in two different situations, one is normally worked
> filesystem that shall be unmounted by administrative request, detaching
> any resources opened by application. Second is the last-resort action
> when backing storage (server in NFS case, disk for UFS) is misbehaving.
> I think we must not break first case for the second.
>
I think this is what Bruce Evans was referring to. He suggested that
there be two flags, like -f and -F, if I understood his post.

rick