From: Hugh Dickins <hugh@veritas.com>

First of batch of six patches which introduce Rajesh Venkatasubramanian's
implementation of a radix priority search tree of vmas, to handle object-based
reverse mapping corner cases well.

rmap 14 i_shared_lock fixes

Start the sequence with a couple of outstanding i_shared_lock fixes.

Since i_shared_sem became i_shared_lock, we've had to shift and then
temporarily remove mremap move's protection of concurrent truncation - if
mremap moves ptes while unmap_mapping_range_list is making its way through the
vmas, there's a danger we'd move a pte from an area yet to be cleaned back
into an area already cleared.

Now site the i_shared_lock with the page_table_lock in move_one_page.  Replace
page_table_present by get_one_pte_map, so we know when it's necessary to
allocate a new page table: in which case have to drop i_shared_lock, trylock
and perhaps reorder locks on the way back.  Yet another fix: must check for
NULL dst before pte_unmap(dst).

And over in rmap.c, try_to_unmap_file's cond_resched amidst its lengthy
nonlinear swapping was now causing might_sleep warnings: moved to a rather
unsatisfactory and less frequent cond_resched_lock on i_shared_lock when we
reach the end of the list; and one before starting on the nonlinears too: the
"cursor" may become out-of-date if we do schedule, but I doubt it's worth
bothering about.