vmscan: fix zone shrinking exit when scan work is done

Commit 3e7d3449 ("mm: vmscan: reclaim order-0 and use compaction
instead of lumpy reclaim") introduced an indefinite loop in
shrink_zone().

It meant to break out of this loop when no pages had been reclaimed and
not a single page was even scanned.  The way it would detect the latter
is by taking a snapshot of sc->nr_scanned at the beginning of the
function and comparing it against the new sc->nr_scanned after the scan
loop.  But it would re-iterate without updating that snapshot, looping
forever if sc->nr_scanned changed at least once since shrink_zone() was
invoked.

This is not the sole condition that would exit that loop, but it
requires other processes to change the zone state, as the reclaimer that
is stuck obviously can not anymore.

This is only happening for higher-order allocations, where reclaim is
run back to back with compaction.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Reported-by: Michal Hocko <mhocko@suse.cz>
Tested-by: Kent <Overstreet&lt;kent.overstreet@gmail.com>
Reported-by: Kent Overstreet <kent.overstreet@gmail.com>
Acked-by: Mel Gorman <mel@csn.ul.ie>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Rik van Riel <riel@redhat.com>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>