mm: vmscan: take page buffers dirty and locked state into account

mm: vmscan: take page buffers dirty and locked state into account

Page reclaim keeps track of dirty and under writeback pages and uses it to
determine if wait_iff_congested() should stall or if kswapd should begin
writing back pages.  This fails to account for buffer pages that can be
under writeback but not PageWriteback which is the case for filesystems
like ext3 ordered mode.  Furthermore, PageDirty buffer pages can have all
the buffers clean and writepage does no IO so it should not be accounted
as congested.

This patch adds an address_space operation that filesystems may optionally
use to check if a page is really dirty or really under writeback.  An
implementation is provided for for buffer_heads is added and used for
block operations and ext3 in ordered mode.  By default the page flags are
obeyed.

Credit goes to Jan Kara for identifying that the page flags alone are not
sufficient for ext3 and sanity checking a number of ideas on how the
problem could be addressed.

Signed-off-by: Mel Gorman <mgorman@suse.de>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Rik van Riel <riel@redhat.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Jiri Slaby <jslaby@suse.cz>
Cc: Valdis Kletnieks <Valdis.Kletnieks@vt.edu>
Cc: Zlatko Calusic <zcalusic@bitsync.net>
Cc: dormando <dormando@rydia.net>
Cc: Trond Myklebust <trond.myklebust@fys.uio.no>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>