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Re: [dm-devel] [PATCH] block devices: validate block device capacity




On Thu, 30 Jan 2014, James Bottomley wrote:

> Why is this?  the whole reason for CONFIG_LBDAF is supposed to be to
> allow 64 bit offsets for block devices on 32 bit.  It sounds like
> there's somewhere not using sector_t ... or using it wrongly which needs
> fixing.

The page cache uses unsigned long as a page index. Therefore, if unsigned 
long is 32-bit, the block device may have at most 2^32-1 pages.

> > On 32-bit architectures, we must limit block device size to
> > PAGE_SIZE*(2^32-1).
> 
> So you're saying CONFIG_LBDAF can never work, why?
> 
> James

CONFIG_LBDAF works, but it doesn't allow unlimited capacity: on x86, 
without CONFIG_LBDAF, the limit is 2TiB. With CONFIG_LBDAF, the limit is 
16TiB (4096*2^32).

Mikulas


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