Normally, file I/O for reclaiming is asynchronous so that
when page writeback is completed, reclaimed page will be
rotated into LRU tail for fast reclaiming in next turn.
But it makes unnecessary CPU overhead and more iteration with higher
priority of reclaim could reclaim too many pages than needed
pages.
This patch frees reclaimed pages by paging out instantly without
rotating back them into LRU's tail when the I/O is completed so
that we can get out of reclaim loop as soon as poosbile and avoid
unnecessary CPU overhead for moving them.
Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>