Mount related panic with FreeBSD 6?

[Václav Haisman]

Doug White wrote:
> On Sun, 20 Nov 2005, [UTF-8] Václav Haisman wrote:
> 
> 
>>Hi,
>>I got this panic on freshly installed FreeBSD 6. I did this df -h and
>>noticed that /mnt/oldroot/home is somewhat mangled. The /mnt/oldroot is
>>root of FreeBSD 4.11 system. I successfully copied some settings and all
>>user accounts from that /mnt/oldroot/home earlier today. This is what I
>>did before the panic:
> 
> 
> [...]
> 
> I discovered this by accident with a CDROM the other day. In 6.0 you can
> overlay read-only mounts (i.e., mount the same R/O FS on top of itself)
> but unmounting it will cause GEOM to tear down the underlying device while
> leaving the first mount behind. Next access to the mountpoint will panic
> the system.
> 
> You can't mount a read/write mount on top of itself, or a r/o mount on a
> r/w mount -- you get an error. A quick discussion with phk points to a
> faulty or missing access check in GEOM. I'm not familiar with the VFS
> operations required to mount a filesystem, though, so I'm not sure where
> to look to put in the fix.
> 
> In the interim, be careful not to mount a read-only FS multiple times.
> 

Thanks, I will remember this. Though it really was not my intention to
mount it twice. I did so because I got confused by the mangled
`/mnt/oldroot/hom' name in the df output and thought it was not mounted.

Vaclav Haisman

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