linux-kernel - Re: Boot failure with ext2 and initrds

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Date:	Thu, 16 Nov 2006 12:34:48 +0000
From:	Russell King <rmk+lkml@....linux.org.uk>
To:	Andrew Morton <akpm@...l.org>
Cc:	Mingming Cao <cmm@...ibm.com>, Hugh Dickins <hugh@...itas.com>,
	Mel Gorman <mel@...net.ie>,
	"Martin J. Bligh" <mbligh@...igh.org>,
	linux-kernel@...r.kernel.org,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: Boot failure with ext2 and initrds

On Wed, Nov 15, 2006 at 11:22:28PM -0800, Andrew Morton wrote:
> On Wed, 15 Nov 2006 22:55:43 -0800
> Mingming Cao <cmm@...ibm.com> wrote:
> 
> > Hmm, maxblocks, in bitmap_search_next_usable_block(),  is the end block 
> > number of the range  to search, not the lengh of the range. maxblocks 
> > get passed to ext2_find_next_zero_bit(), where it expecting to take the 
> > _size_ of the range to search instead...
> > 
> > Something like this: (this is not a patch)
> >   @@ -524,7 +524,7 @@ bitmap_search_next_usable_block(ext2_grp
> >    	ext2_grpblk_t next;
> > 
> >    -  	next = ext2_find_next_zero_bit(bh->b_data, maxblocks, start);
> >    +  	next = ext2_find_next_zero_bit(bh->b_data, maxblocks-start + 1, start);
> > 	if (next >= maxblocks)
> >    		return -1;
> >    	return next;
> >    }
> 
> yes, the `size' arg to find_next_zero_bit() represents the number of bits
> to scan at `offset'.

Are you sure?  That's not the way it's implemented in many architectures.
find_next_*_bit() has always taken "address, maximum offset, starting offset"
and always has returned "next offset".

Just look at arch/i386/lib/bitops.c:

int find_next_zero_bit(const unsigned long *addr, int size, int offset)
{
        unsigned long * p = ((unsigned long *) addr) + (offset >> 5);
        int set = 0, bit = offset & 31, res;
...
        /*
         * No zero yet, search remaining full bytes for a zero
         */
        res = find_first_zero_bit (p, size - 32 * (p - (unsigned long *) addr));
        return (offset + set + res);
}

So for the case that "offset" is aligned to a "long" boundary, that gives us:

	res = find_first_zero_bit(addr + (offset>>5),
			size - 32 * (addr + (offset>>5) - addr));

or:

	res = find_first_zero_bit(addr + (offset>>5), size - (offset & ~31));

So, size _excludes_ offset.

Now, considering the return value, "res" above will be relative to
"addr + (offset>>5)".  However, we add "offset" on to that, so it's
relative to addr + (offset bits).

-- 
Russell King
 Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
 maintainer of:  2.6 Serial core
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