linux-ext4 - Re: [Ocfs2-devel] [PATCH 1/3] ext3/ext4: Factor out disk addressability check

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Date:	Fri, 13 Aug 2010 15:52:46 -0700
From:	Joel Becker <Joel.Becker@...cle.com>
To:	Jan Kara <jack@...e.cz>, Andreas Dilger <adilger@...ger.ca>,
	"Ted Ts'o" <tytso@....edu>, ocfs2-devel@....oracle.com,
	linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
	"linux-kernel@...r.kernel.org Patrick J. LoPresti" 
	<lopresti@...il.com>
Subject: Re: [Ocfs2-devel] [PATCH 1/3] ext3/ext4: Factor out disk
 addressability check

On Fri, Aug 13, 2010 at 01:47:01PM -0700, Joel Becker wrote:
> 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**@...cksize_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.

	Wouldn't it rather be:

	    ... ||
	    ((last_fs_block >> (PAGE_CACHE_SHIFT - blocksize_bits)) >
	     (pgoff_t)(!0ULL))) {

Joel

-- 

"What no boss of a programmer can ever understand is that a programmer
 is working when he's staring out of the window"
	- With apologies to Burton Rascoe

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