With large highmem machines and many small cached files it is possible
to encounter ZONE_NORMAL allocation failures.  This can be demonstrated
with a large number of one-byte files on a 7G machine.

All lowmem is filled with icache and all those inodes have a small
amount of highmem pagecache which makes them unfreeable.

The patch strips the pagecache from inodes as they come off the tail of
the inode_unused list.

I play tricks in there peeking at the head of the inode_unused list to
pick up the inode again after running iput().  The alternatives seemed
to involve more widespread changes.

Or running invalidate_inode_pages() under inode_lock which would be a
bad thing from a scheduling latency and lock contention point of view.