linux-kernel - More fun with MinixFS V3

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Message-ID: <20090704182940.GA19185@nortel.com>
Date:	Sat, 4 Jul 2009 14:29:40 -0400
From:	"Doug Graham" <dgraham@...tel.com>
To:	linux-kernel@...r.kernel.org
Subject: More fun with MinixFS V3

I was just looking over an old thread about MinixFs V3 support and
starting to get worried that my tweaks to support blocksizes of 512
bytes (for a small filesystem with many small files) might be broken.
This is the thread here: http://lkml.org/lkml/2006/5/6/50.
The nblocks() routine in fs/minix/itree_common.c now looks like this:

static inline unsigned nblocks(loff_t size, struct super_block *sb)
{
        int k = sb->s_blocksize_bits - 10;
        unsigned blocks, res, direct = DIRECT, i = DEPTH;
        blocks = (size + sb->s_blocksize - 1) >> (BLOCK_SIZE_BITS + k);
        ...

and what had be worred at first is that sb->s_blocksize_bits will be 9
with a blocksize of 512, which makes k negative.  But all may not be lost,
since k is only used once after this, after adding BLOCK_SIZE_BITS (10)
to it.  At least that makes it nonnegative again, but then the question
is: what exactly is being accomplished here?  First we subtract 10,
then add it back?  The result is effectively this:

  blocks = (size + sb->s_blocksize - 1) >> sb->s_blocksize_bits

which looks entirely correct to me.  Is there any reason the code couldn't
just be written this way to begin with?

--Doug
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