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There is a small problem in handling page bounce.

At the moment blk_max_pfn equals max_pfn, which is in fact not maximum
possible _number_ of a page frame, but the _amount_ of page frames.  For
example for the 32bit x86 node with 4Gb RAM, max_pfn = 0x100000, but not
0xFFFF.

request_queue structure has a member q->bounce_pfn and queue needs bounce
pages for the pages _above_ this limit.  This routine is handled by
blk_queue_bounce(), where the following check is produced:

	if (q->bounce_pfn >= blk_max_pfn)
		return;

Assume, that a driver has set q->bounce_pfn to 0xFFFF, but blk_max_pfn
equals 0x10000.  In such situation the check above fails and for each bio
we always fall down for iterating over pages tied to the bio.

I want to notice, that for quite a big range of device drivers (ide, md,
...) such problem doesn't happen because they use BLK_BOUNCE_ANY for
bounce_pfn.  BLK_BOUNCE_ANY is defined as blk_max_pfn << PAGE_SHIFT, and
then the check above doesn't fail. ...