The patch "mm: Add support for a filesystem to activate swap files and
use direct_IO for writing swap pages" added support for using direct_IO
to write swap pages but it is insufficient for highmem pages.
To support highmem pages, this patch kmaps() the page before calling the
direct_IO() handler. As direct_IO deals with virtual addresses an
additional helper is necessary for get_kernel_pages() to lookup the
struct page for a kmap virtual address.
Signed-off-by: Mel Gorman<mgorman@xxxxxxx>