From: Russell King Subject: Re: Boot failure with ext2 and initrds Date: Thu, 16 Nov 2006 12:34:48 +0000 Message-ID: <20061116123448.GA28311@flint.arm.linux.org.uk> References: <20061114014125.dd315fff.akpm@osdl.org> <20061114184919.GA16020@skynet.ie> <20061114113120.d4c22b02.akpm@osdl.org> <20061115214534.72e6f2e8.akpm@osdl.org> <455C0B6F.7000201@us.ibm.com> <20061115232228.afaf42f2.akpm@osdl.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Mingming Cao , Hugh Dickins , Mel Gorman , "Martin J. Bligh" , linux-kernel@vger.kernel.org, "linux-ext4@vger.kernel.org" Return-path: Received: from caramon.arm.linux.org.uk ([217.147.92.249]:30733 "EHLO caramon.arm.linux.org.uk") by vger.kernel.org with ESMTP id S1423847AbWKPMfL (ORCPT ); Thu, 16 Nov 2006 07:35:11 -0500 To: Andrew Morton Content-Disposition: inline In-Reply-To: <20061115232228.afaf42f2.akpm@osdl.org> Sender: linux-ext4-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org On Wed, Nov 15, 2006 at 11:22:28PM -0800, Andrew Morton wrote: > On Wed, 15 Nov 2006 22:55:43 -0800 > Mingming Cao 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