On 01/31/2014 11:02 AM, James Bottomley wrote:

     3. Increase pgoff_t and the radix tree indexes to u64 for
        CONFIG_LBDAF.  This will blow out the size of struct page on 32
        bits by 4 bytes and may have other knock on effects, but at
        least it will be transparent.

I'm not sure how many acrobatics we want to go through for 32-bit, but...

Between page->mapping and page->index, we have 64 bits of space, which
*should* be plenty to uniquely identify a block.  We could easily add a
second-level lookup somewhere so that we store some cookie for the
address_space instead of a direct pointer.  How many devices would need,
practically?  8 bits worth?