[PATCH] vmscan: Fix temp_priority race

The temp_priority field in zone is racy, as we can walk through a reclaim
path, and just before we copy it into prev_priority, it can be overwritten
(say with DEF_PRIORITY) by another reclaimer.

The same bug is contained in both try_to_free_pages and balance_pgdat, but
it is fixed slightly differently.  In balance_pgdat, we keep a separate
priority record per zone in a local array.  In try_to_free_pages there is
no need to do this, as the priority level is the same for all zones that we
reclaim from.

Impact of this bug is that temp_priority is copied into prev_priority, and
setting this artificially high causes reclaimers to set distress
artificially low.  They then fail to reclaim mapped pages, when they are,
in fact, under severe memory pressure (their priority may be as low as 0).
This causes the OOM killer to fire incorrectly.

From: Andrew Morton <akpm@osdl.org>

__zone_reclaim() isn't modifying zone->prev_priority.  But zone->prev_priority
is used in the decision whether or not to bring mapped pages onto the inactive
list.  Hence there's a risk here that __zone_reclaim() will fail because
zone->prev_priority ir large (ie: low urgency) and lots of mapped pages end up
stuck on the active list.

Fix that up by decreasing (ie making more urgent) zone->prev_priority as
__zone_reclaim() scans the zone's pages.

This bug perhaps explains why ZONE_RECLAIM_PRIORITY was created.  It should be
possible to remove that now, and to just start out at DEF_PRIORITY?

Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: Christoph Lameter <clameter@engr.sgi.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

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