mm: Free reclaimed pages indepdent of next reclaim

Invalidate dirty/writeback page and file/swap I/O for reclaiming
are asynchronous so that when page writeback is completed,
it will be rotated back into LRU tail for freeing in next reclaim.

But it would make unnecessary CPU overhead and more aging
with higher priority of reclaim than necessary thing.

This patch makes such pages instant release when I/O complete
without LRU movement so that we could reduce reclaim events.

This patch wakes up one waiting PG_writeback and then clear
PG_reclaim bit because the page could be released during
rotating so it makes slighly race with Readahead logic but
the chance would be small and no huge side-effect even though
that happens, I belive.

Test result is as follows,

On 1G, 12 CPU kvm guest, build kernel 5 times and result was

vanilla: records: 5 avg: 4733.80 std: 913.55(19.30%) max: 6442.00 min: 3719.00
improve: records: 5 avg: 1514.20 std: 441.69(29.17%) max: 1974.00 min: 863.00

vanilla: records: 5 avg: 873313.80 std: 40999.20(4.69%) max: 954722.00 min: 845903.00
improve: records: 5 avg: 28406.40 std: 3296.02(11.60%) max: 34552.00 min: 25047.00

Most of field in vmstat are not changed too much but things I can notice
is allocstall and pgrotated. We could save allocstall(ie, direct relcaim)
and pgrotated very much.

Signed-off-by: Minchan Kim <>
2 files changed