Now that we are correctly kicking off kswapd early (before the synch
reclaim watermark), it is really doing asynchronous pageout.  This has
exposed a latent problem where allocators running at the same time will
make kswapd think it is getting into trouble, and cause too much swapping
and suboptimal behaviour.

This patch changes the kswapd scanning algorithm to use the same metrics
for measuring pageout success as the synchronous reclaim path - namely, how
much work is required to free SWAP_CLUSTER_MAX pages.

This should make things less fragile all round, and has the added benefit
that kswapd will continue running so long as memory is low and it is
managing to free pages, rather than going through the full priority loop,
then giving up.  Should result in much better behaviour all round,
especially when there are concurrent allocators.

akpm: the patch was confirmed to fix up the excessive swapout which Ray Bryant
<raybry@sgi.com> has been reporting.
Signed-off-by: Nick Piggin <nickpiggin@yahoo.com.au>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>