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SubjectRe: 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@us.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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