Re: [PATCH] Fixing soft NFS umount -f, round 5

[Emmanuel Dreyfus <manu%netbsd.org@localhost>]

On Wed, Jul 08, 2015 at 06:07:11PM +0000, Emmanuel Dreyfus wrote:
> http://ftp.espci.fr/shadow/manu/umount_f4.patch
> 
> With default values, timeout = 3 and retrans = 10, we now wait
> for ages or the unmount to completes. In the umount -f case
> it does not makes sense to wait for too long.

Here is an improved version that still use your proposed timeouts:
http://ftp.espci.fr/shadow/manu/umount_f5.patch

I introduced a NFS mount private flag NFSMNT_DISMNTFORCE that informs
the NFS subsystem that we want to forcibly unmount the filesystem, 
which can be done at the expense of cutting timeout corners.

An important point is that with the flag set, we do not attempt any
NFS server reconnect. It makes sense since dounmount() calls 
VFS_SYNC() before VFS_UNMOUNT(). NFSMNT_DISMNTFORCE being set in 
VFS_UNMOUNT() / nfs_unmount(), we already made reconnection attempts
withinin VFS_SYNC(), hence thereis benefit for doint it again.

Now with that change, in the umount -f while NFS server is gone case,
we spend 20s in VFS_SYNC(), and VFS_UNMOUNT() returns in less than a
second.

Now I think it would be nice to also cut coners in VFS_SYNC() when 
the force flag is used, but that touches filesystem-indpendent code,
in dounmount():
        if ((mp->mnt_flag & MNT_RDONLY) == 0) {
                error = VFS_SYNC(mp, MNT_WAIT, l->l_cred);
        }       
        if (error == 0 || (flags & MNT_FORCE)) {
                error = VFS_UNMOUNT(mp, flags);
        }

The first VFS_SYNC() is making us wait even if MNT_FORCE is set. This could
be solved by adding a IMNT_UMOUNTFORCE to struct mount's mnt_iflag, just
like I did for NFS in this patch. The flag would instruct underlying 
filesystem that force unmount is required and that fast return is expected.

Opinions?

-- 
Emmanuel Dreyfus
manu%netbsd.org@localhost