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From: Don Lewis <Don.Lewis@tsc.tdk.com>
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Date: Sun, 5 Mar 2000 22:38:37 -0800
In-Reply-To: <200003041934.LAA16343@flamingo.McKusick.COM>
References:  <200003041934.LAA16343@flamingo.McKusick.COM>
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To: Kirk McKusick <mckusick@flamingo.McKusick.COM>,
	Alfred Perlstein <bright@wintelcom.net>
Subject: Re: changing mount options still can cause damage?
Cc: Terry Lambert <tlambert@primenet.com>, fs@FreeBSD.ORG,
	jkh@FreeBSD.ORG, Bruce Evans <bde@zeta.org.au>
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On Mar 4, 11:34am, Kirk McKusick wrote:
} Subject: Re: changing mount options still can cause damage?

} 2) In reviewing my bug logs for FFS I have found the `corruption'
} case to which I believe the bug entry in the manual page was
} alluding. It is possible to get lost inodes in a filesystem
} that has been downgraded to read-only even if it never ran
} in async mode. The senario causing trouble goes as follows:
} 
} 	A) a process opens a file for reading.
} 	B) the file is unlinked
} 	C) the filesystem is downgraded to read-only
} 	D) the process referencing the now unlinked
} 	   file exits or closes the file.
} 
} In this case, the the inode cannot be freed as the filesystem
} is now in read-only mode. Corruption of this sort is not
} particularly threatening as the lost inodes will be cleaned
} up the next time `fsck -p' is run, but the resulting loss
} of space may be annoying if the filesystem is nearly full.
} The alternative is to vgone files with link counts of zero
} when doing a (forcible) downgrade just as we do with files
} that are open for writing. This would result in the inode
} being released and the process seeing a dead file (again
} just as it would for a file open for writing). This seems
} a slightly odd semantic for a file open for reading, so I
} have not done it. Does anyone have any views on whether the
} filesystem should be changed in this way on forcible
} downgrades?

This is probably OK when doing a forceable downgrade.  A non-forceable
downgrade should probably fail with EBUSY.


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