Re: [PATCH 1/3] ext3/ext4: Factor out disk addressability check

[Joel Becker <Joel.Becker@oracle.com>]

On Sun, Aug 15, 2010 at 12:19:36PM -0500, Eric Sandeen wrote:
> >> +	    (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)?
> 
> Argh that was my fault...  Thankfully not too many 1k-blocksize-formatted
> 16T devices out there, I guess.
> 
> I went through the math again and also came up with:
> 
> total fs pages is blocks / (blocks per page)
> total pages is blocks / (1 << PAGE_CACHE_SHIFT / 1 << blocksize_bits)
> total pages is blocks / (1 << (PAGE_CACHE_SHIFT - blocksize_bits))
> total pages is blocks * (1 >> (PAGE_CACHE_SHIFT - blocksize_bits))
> total pages is blocks >> (PAGE_CACHE_SHIFT - blocksize_bits)
> 
> too big if total pages is > (pgoff_t)(~0ULL)
> too big if blocks >> (PAGE_CACHE_SHIFT - blocksize_bits) > (pgoff_t)(~0ULL)

	Why not stop here, which is what I put in my other email?
"blocks >> SHIFT-bits" is "how many pages do I need?".

> too big if blocks > (pgoff_t)(~0ULL) << (PAGE_CACHE_SHIFT - blocksize_bits)
> and to not overflow:
> too big if blocks > (u64)(pgoff_t)(~0ULL) << (PAGE_CACHE_SHIFT - blocksize_bits)

	This still overflows.  pgoff_t is a u64 on 64bit machines,
right?  So shift that left by anything and you wrap.

Joel

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