From: Hugh Dickins <hugh@veritas.com>

Some arches refer to page->mapping for their dcache flushing: use
page_mapping(page) for safety, to avoid confusion on anon pages, which will
store a different pointer there - though in most cases flush_dcache_page is
being applied to pagecache pages.

arm has a useful mapping_mapped macro: move that to generic, and add
mapping_writably_mapped, to avoid explicit list_empty checks on i_mmap and
i_mmap_shared in several places.

Very tempted to add page_mapped(page) tests, perhaps along with the
mapping_writably_mapped tests in do_generic_mapping_read and
do_shmem_file_read, to cut down on wasted flush_dcache effort; but the
serialization is not obvious, too unsafe to do in a hurry.