From: Hugh Dickins <hugh@veritas.com>

The previous patch let the ptes of file pages be located via page
->mapping->i_mmap and i_mmap_shared lists of vmas; which works well unless
the vma is VM_NONLINEAR - one in which sys_remap_file_pages has been used
to place pages in unexpected places, to avoid an explosion of distinct
unmergable vmas.  Such pages were effectively locked in memory.

page_referenced_file is already skipping nonlinear vmas, they'd just waste
its time, and age unfairly any pages in their proper positions.  Now extend
try_to_unmap_file, to persuade it to swap from nonlinears.

Ignoring the page requested, try to unmap cluster of 32 neighbouring ptes
(in worst case all empty slots) in a nonlinear vma, then move on to the
next vma; stopping when we've unmapped at least as many maps as the
requested page had (vague guide of how hard to try), or have reached the
end.  With large sparse nonlinear vmas, this could take a long time:
inserted a cond_resched while no locks are held, unusual at this level but
I think okay, shrink_list does so.

Use vm_private_data a little like the old mm->swap_address, as a cursor
recording how far we got, so we don't attack the same ptes next time around
(earlier tried inserting an empty marker vma in the list, but that got
messy).  How well this will work on real- life nonlinear vmas remains to be
seen, but should work better than locking them all in memory, or swapping
everything out all the time.

Existing users of vm_private_data have either VM_RESERVED or VM_DONTEXPAND
set, both of which are in the VM_SPECIAL category where we never try to
merge vmas: so removed the vm_private_data test from is_mergeable_vma, so
we can still merge VM_NONLINEARs.  Of course, we could instead add another
field to vm_area_struct.