fix for per-mount i/o counting in ffs

[Bruce Evans]

On Wed, 18 May 2016, Bruce Evans wrote:

> On Tue, 17 May 2016, Konstantin Belousov wrote:
>> ...
>> For the accounting patch, don't we want to account for all io, including
>> the mount-time metadata reads and initial superblock update ?
>> 
>> diff --git a/sys/ufs/ffs/ffs_vfsops.c b/sys/ufs/ffs/ffs_vfsops.c
>> index 9776554..712fc21 100644
>> --- a/sys/ufs/ffs/ffs_vfsops.c
>> +++ b/sys/ufs/ffs/ffs_vfsops.c
>> @@ -780,6 +780,8 @@ ffs_mountfs(devvp, mp, td)
>> 		mp->mnt_iosize_max = MAXPHYS;
>> 
>> 	devvp->v_bufobj.bo_ops = &ffs_ops;
>> +	if (devvp->v_type == VCHR)
>> +		devvp->v_rdev->si_mountpt = mp;
>> 
>> 	fs = NULL;
>> 	sblockloc = 0;
>> @@ -1049,8 +1051,6 @@ ffs_mountfs(devvp, mp, td)
>> 			ffs_flushfiles(mp, FORCECLOSE, td);
>> 			goto out;
>> 		}
>> -		if (devvp->v_type == VCHR && devvp->v_rdev != NULL)
>> -			devvp->v_rdev->si_mountpt = mp;
>> 		if (fs->fs_snapinum[0] != 0)
>> 			ffs_snapshot_mount(mp);
>> 		fs->fs_fmod = 1;
>> @@ -1083,6 +1083,8 @@ ffs_mountfs(devvp, mp, td)
>> out:
>> 	if (bp)
>> 		brelse(bp);
>> +	if (devvp->v_type == VCHR && devvp->v_rdev != NULL)
>> +		devvp->v_rdev->si_mountpt = NULL;
>> 	if (cp != NULL) {
>> 		DROP_GIANT();
>> 		g_topology_lock();
>
> Yes, that looks better.

Further cleanups:
- the null pointer check is bogus since we already dereferenced
   devvp->v_rdev.  We also assigned devvp->v_rdev to the variable
   dev but spelled out devvp->v_rdev in a couple of other places.
- the VCHR check is bogus since we only work for VCHR and have
   already checked for VCHR in vn_isdisk().

Similarly in ffs_umount() except there is no dev variable there.

Similarly in msdosfs.

NOT similarly in ext2fs.  I was looking at the wrong tree again.
Only 1 of my trees has the patch to do this in ext2fs.  The patch
for ffs applies almost verbatim.

Bruce