linux-ext4 - Re: Boot failure with ext2 and initrds

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Message-Id: <20061115232228.afaf42f2.akpm@osdl.org>
Date:	Wed, 15 Nov 2006 23:22:28 -0800
From:	Andrew Morton <akpm@...l.org>
To:	Mingming Cao <cmm@...ibm.com>
Cc:	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, 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'.

So I think your change is correctish.  But we don't want the "+ 1", do we?

If we're right then this bug could cause the code to scan off the end of the
bitmap.  But it won't explain Hugh's bug, because of the if (next >= maxblocks).

btw, how come try_to_extend_reservation() uses spin_trylock?
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