The overhead is going be negligeble, the overhead of highmem itself is
much worse. Not to mention that today some dma_addr_t's might not be
packed properly in data structure hence they ending up taking 8 bytes
anyway.
What kind of packing makes a 32-bit value take 8-bytes on any
currently supported archicture? The worst-case I can think of is
7-bytes in the case of misaligned by 3 (e.g. __attribute__((packed))
struct blah { char foo[3]; long bar }; sort of thing).
--cw
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