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



On Thu, 2014-01-30 at 18:10 -0500, Mikulas Patocka wrote:
> 
> 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.

Um, that's the index into the mapping, not the device; a device can have
multiple mappings and each mapping has a radix tree of pages.  For most
filesystems a mapping is equivalent to a file, so we can have large
filesystems, but they can't have files over actually 4GB on 32 bits
otherwise mmap fails.

Are we running into a problems with struct address_space where we've
assumed the inode belongs to the file and lvm is doing something where
it's the whole device?

> > > 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).

I don't think the people who did the large block device work expected to
gain only 3 bits for all their pain.

James




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