Updating the mm lock ordering documentation drew attention to the fact that
we were wrong to blithely add down_read(&mm->mmap_sem) to swapoff's
unuse_process, while it holds swapcache page lock: not very likely, but it
could deadlock against, say, mlock faulting a page back in from swap.

But it looks like these days it's safe to drop and reacquire page lock if
down_read_trylock fails: the page lock is held to stop try_to_unmap
unmapping the page's ptes as fast as try_to_unuse is mapping them back in;
but the recent fix for get_user_pages works to prevent that too.
Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>