From: Joel Becker <Joel.Becker@oracle.com>
Subject: Re: [Ocfs2-devel] [PATCH 1/3] ext3/ext4: Factor out disk
 addressability check
Date: Fri, 13 Aug 2010 13:47:01 -0700
Message-ID: <20100813204701.GB19568@mail.oracle.com>
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Cc: Andreas Dilger <adilger@dilger.ca>, "Ted Ts'o" <tytso@mit.edu>,
	ocfs2-devel@oss.oracle.com, linux-fsdevel@vger.kernel.org,
	linux-ext4@vger.kernel.org,
	"linux-kernel@vger.kernel.org Patrick J. LoPresti"
	<lopresti@gmail.com>
To: Jan Kara <jack@suse.cz>
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On Fri, Aug 13, 2010 at 06:30:07PM +0200, Jan Kara wrote:
> On Thu 12-08-10 15:29:49, Joel Becker wrote:
> > +/**
> > + * generic_check_addressable - Check addressability of file system
> > + * @blocksize_bits:	log of file system block size
> > + * @num_blocks:		number of blocks in file system
> > + *
> > + * Determine whether a file system with @num_blocks blocks (and a
> > + * block size of 2**@blocksize_bits) is addressable by the sector_t
> > + * and page cache of the system.  Return 0 if so and -EFBIG otherwise.
> > + */
> > +int generic_check_addressable(unsigned blocksize_bits, u64 num_blocks)
> > +{
> > +	u64 last_fs_block = num_blocks - 1;
> > +
> > +	if (unlikely(num_blocks == 0))
> > +		return 0;
> > +
> > +	if ((blocksize_bits < 9) || (blocksize_bits > PAGE_CACHE_SHIFT))
> > +		return -EINVAL;
> > +
> > +	if ((last_fs_block >
> > +	     (sector_t)(~0ULL) >> (blocksize_bits - 9)) ||
> > +	    (last_fs_block >
> > +	     (pgoff_t)(~0ULL) >> (PAGE_CACHE_SHIFT - blocksize_bits))) {
>             ^^^ I don't get the pgoff_t check. Shouldn't it rather be
> (u64)(pgoff_t)(~0ULL) << (PAGE_CACHE_SHIFT - blocksize_bits)?
> Because on 32-bit arch we are able to address 16TB device, which is for 1KB
> blocksize 1<<34 blocks. But your math gives 1<<30 blocks...

	This code is directly lifted from ext4.  But that said, I am
starting to think you're right.  1 page == 4 x 1K blocks, rather than 4
pages == 1 1K block.

Joel

-- 

"I always thought the hardest questions were those I could not answer.
 Now I know they are the ones I can never ask."
			- Charlie Watkins

Joel Becker
Consulting Software Developer
Oracle
E-mail: joel.becker@oracle.com
Phone: (650) 506-8127