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5dd33c9a4c
md/raid6 passes a list of 'struct page *' to the async_tx routines, which then either DMA map them for offload, or take the page_address for CPU based calculations. For RAID6 we sometime leave 'blanks' in the list of pages. For CPU based calcs, we want to treat theses as a page of zeros. For offloaded calculations, we simply don't pass a page to the hardware. Currently the 'blanks' are encoded as a pointer to raid6_empty_zero_page. This is a 4096 byte memory region, not a 'struct page'. This is mostly handled correctly but is rather ugly. So change the code to pass and expect a NULL pointer for the blanks. When taking page_address of a page, we need to check for a NULL and in that case use raid6_empty_zero_page. Signed-off-by: NeilBrown <neilb@suse.de>