misc/118160: unable to mount / rw while booting 7.0-BETA3

[Yuri]

The following reply was made to PR bin/118160; it has been noted by GNATS.

From: Yuri <yuri at rawbw.com>
To: Bruce Evans <brde at optusnet.com.au>
Cc: Yuri <yuri at tsoft.com>, freebsd-gnats-submit at freebsd.org,
        freebsd-bugs at freebsd.org
Subject: Re: misc/118160: unable to mount / rw while booting 7.0-BETA3
Date: Tue, 27 Nov 2007 22:36:28 -0800

 Quoting Bruce Evans <brde at optusnet.com.au>:
 
 > Using the 'c' partition is normally an error.  Here it is a very large
 > error.  ad12s1b overlaps ad12s1c, so if swap on ad12s1b is actually
 > used then it will normally clobber the file system on ad12s1c.  It
 > might be possible to make the clobbering harmless, e.g., by putting
 > ad12s1b at the end of ad12s1c and not using all of ad12s1c for the
 > file system, but this is harder to configure than a normal configuration
 > and is supposed to be disallowed.
 >
 Having root mounted on ad12s1c is probably a result of my typo.
 
 > 
 > This misbehaviour seems to be a result of the disallowment not being
 > complete.  Mounted file systems normally have exclusive write access
 > to their device.  However, for early root mounts, while the mount is
 > only r/o, exclusive write access is given up so that fsck can work.
 > There seems to be no exclusivity at all, so swapon on a different but
 > overlapping device is granted write access.  Then while swapon is on,
 > it holds exclusive write access and r/w mounts of root are denied
 > (correctly except for the wrong errno and other misleading things in
 > the error message).  Also, after the root mount becomes r/w, swapon
 > fails (correctly except for the wrong errno).
 > 
 
 Should this situation generate system log message, not only EPERM?
 If it would have said: 'Attempt to mount already exclusively locked
 partition' it would have been clear what the problem is right away.
 
 So I would suggest such message to be added, otherwise this PR can be closed.
 
 Yuri