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

[Eric Sandeen]

Joel Becker wrote:
> 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?".

yeah, ok.  Was going for pointless symmetry w/ the other test...

>> 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.

Er, yeah.  I had 32 bits in my head since that's the case we're
checking for... whoops.

So I guess your

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

is right :)  (my feeble brain has a hard time reading that, though, TBH)

-Eric

> Joel
>