Marcelo noticed that the BUG_ON in __set_page_dirty_nobuffers doesn't make
much sense: it lost its way in 2.6.7, amidst so many page_mappings!

It's supposed to be checking that, although page->mapping may suddenly go NULL
from truncation, and although tmpfs swizzles page_mapping(page) between tmpfs
inode address_space and swapper_space, there's sufficient stabilization while
here in __set_page_dirty_nobuffers that the mapping after we locked
mapping->tree_lock is the same as the mapping before we locked
mapping->tree_lock i.e.  the lock we hold is the right one.
Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>