[PATCH] kswapd throttling fixes

[PATCH] kswapd throttling fixes

The logic in balance_pgdat() is all bollixed up.

- the incoming arg `nr_pages' should be used to determine if we're being
  asked to free a specific number of pages, not `to_free'.

- local variable `to_free' is not appropriate for the determination of
  whether we failed to bring all zones to appropriate free pages levels.

  Fix this by correctly calculating `all_zones_ok' and then use
  all_zones_ok to determine whether we need to throttle kswapd.

So the logic now is:

	for (increasing priority) {

		all_zones_ok = 1;

		for (all zones) {
			to_reclaim = number of pages to try to reclaim
				     from this zone;
			max_scan = number of pages to scan in this pass
				   (gets larger as `priority' decreases)
			/*
			 * set `reclaimed' to the number of pages which were
			 * actually freed up
			 */
			reclaimed = scan(max_scan pages);
			reclaimed += shrink_slab();

			to_free -= reclaimed;	/* for the `nr_pages>0' case */

			/*
			 * If this scan failed to reclaim `to_reclaim' or more
			 * pages, we're getting into trouble.  Need to scan
			 * some more, and throttle kswapd.   Note that this
			 * zone may now have sufficient free pages due to
			 * freeing activity by some other process.   That's
			 * OK - we'll pick that info up on the next pass
			 * through the loop.
			 */
			if (reclaimed < to_reclaim)
				all_zones_ok = 0;
		}
		if (to_free > 0)
			continue;	/* swsusp: need to do more work */
		if (all_zones_ok)
			break;		/* kswapd is done */
		/*
		 * OK, kswapd is getting into trouble.  Take a nap, then take
		 * another pass across the zones.
		 */
		blk_congestion_wait();
	}