This patch stops reclaim/compaction entering sync reclaim as this was only
intended for lumpy reclaim and an oversight. Page migration has its own
logic for stalling on writeback pages if necessary and memory compaction
is already using it.
Waiting on page writeback is bad for a number of reasons but the primary
one is that waiting on writeback to a slow device like USB can take a
considerable length of time. Page reclaim instead uses wait_iff_congested()
to throttle if too many dirty pages are being scanned.
Signed-off-by: Mel Gorman<mgorman@xxxxxxx>