From: Hugh Dickins <hugh@veritas.com>

One of the callers of flush_dcache_page is do_generic_mapping_read, where
file is read without i_sem and without page lock: concurrent truncation may
at any moment remove page from cache, NULLing ->mapping, making
flush_dcache_page liable to oops.  Put result of page_mapping in a local
variable and apply mapping_mapped to that (if we were to check for NULL
within mapping_mapped, it's unclear whether to say yes or no).

parisc and arm do have other locking unsafety in their i_mmap(_shared)
searching, but that's a larger issue to be dealt with down the line.