From: Nick Piggin <nickpiggin@yahoo.com.au>

nfs_writepage() refuses to write back mapped pages at all on the page
reclaim path, causing systems to get locked up when there's a lot of dirty
mmapped data around.  The patch changes NFS so that it will start I/O
against these pages.

The code as it stands is designed to defer writeout to pdflush which can do
larger, more efficient I/Os.  But there shouldn't be much traffic by this
path, and going slow is better than not going at all.

Patch originally from Trond.
Signed-off-by: Trond Myklebust <trond.myklebust@fys.uio.no>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>