From owner-freebsd-current  Wed Feb 23 14:35:40 2000
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To: Matthew Dillon <dillon@apollo.backplane.com>
Cc: nops@maths.tcd.ie, freebsd-current@freebsd.org,
	diablo-list@list.bart.nl
Subject: Re: ffs_blkfree: freeing free block (was Re: Panic (pmap)) 
In-reply-to: Your message of "Wed, 23 Feb 2000 12:53:11 PST."
             <200002232053.MAA31746@apollo.backplane.com> 
Date: Wed, 23 Feb 2000 22:35:29 +0000
From: Ian Dowse <iedowse@maths.tcd.ie>
Message-ID:  <200002232235.aa22335@salmon.maths.tcd.ie>
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In message <200002232053.MAA31746@apollo.backplane.com>, Matthew Dillon writes:

>    There must still be a bug in there somewhere, unrelated to softupdates.
>
>    Try turning off the vfs.ffs.doreallocblks via sysctl and see if that
>    stops the crashes, that will help narrow down where to search for the
>    problem.

I think I've found it. Here's an easy way to repeat the problem to
start with:

	mount_mfs -T fd1440 none /mnt
	cd /mnt
	dd if=/dev/zero bs=4k of=test seek=1036 count=1
	dd if=/dev/zero bs=4k of=test1 count=1
	dd if=/dev/zero bs=4k of=test2
	rm test1
	dd if=/dev/zero bs=4k of=test seek=8000 count=1
	echo > test

It looks as if the problem is in ffs_balloc(), and occurs as follows:
	- ffs_balloc() is asked to allocate a doubly indirect block.
	- The first-level indirection block already exists
	- The second-level indirection block does not exist, but is
	  successfully allocated.
	- This block is linked into the first-level indirection block by
	  the line:

		bap[indirs[i - 1].in_off] = nb;

	- Allocation of the data block fails.
	- All allocated blocks are then released, but there is still
	  a link in the first-level indirection block to what is
	  now a free block.

The fix should be relatively straightforward - either the code should
avoid linking new indirection blocks until all allocations succeed,
or it should back out the changes on failure.

Ian


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