From: Hugh Dickins <hugh@veritas.com>

loop_copy_bio uses one gfp_mask for bio_alloc and alloc_page calls.  The
bio_alloc obviously can't use highmem, but the alloc_page can.  Yes, the
underlying device might be unable to use highmem, and have to use one of
its bounce buffers, with an extra copy: so be it.

(Originally I did propagate the underlying device's bounce needs down to
the loop device, to avoid that possible extra copy; but let's keep this
simple, the low end doesn't have highmem and the high end can I/O it.)