[RFC Patch] mm/vmscan.c: not inherit classzone_idx from previous reclaim

[Wei Yang <richardw.yang@linux.intel.com>]

Before commit e716f2eb24de ("mm, vmscan: prevent kswapd sleeping
prematurely due to mismatched classzone_idx"), classzone_idx could have
two possibilities on a new loop based on whether there is a wakeup
during reclaiming:

  * 0 if no wakeup
  * the classzone_idx request by wakeup

As described in the changelog, this commit is willing to change the
first case to (MAX_NR_ZONES - 1) to avoid some premature sleep. But it
does not achieve the goal.

There are two versions of kswapd_classzone_idx() since this change:

  * commit e716f2eb24de ("mm, vmscan: prevent kswapd sleeping
    prematurely due to mismatched classzone_idx")
  * commit dffcac2cb88e ("mm/vmscan.c: prevent useless kswapd loops")

Both of them would return the classzone_idx we passed as the 2nd
parameter when (pgdat->kswapd_classzone_idx == MAX_NR_ZONES). This
means if there is no wakeup during reclaiming, we would use
classzone_idx in previous round to sleep.

This patch fixes the logic by using (MAX_NR_ZONES - 1) for the first
case.

Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
---
 mm/vmscan.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index e7b647f70407..ea2f0abef1d4 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3879,7 +3879,7 @@ static void kswapd_try_to_sleep(pg_data_t *pgdat, int alloc_order, int reclaim_o
 static int kswapd(void *p)
 {
 	unsigned int alloc_order, reclaim_order;
-	unsigned int classzone_idx = MAX_NR_ZONES - 1;
+	unsigned int classzone_idx;
 	pg_data_t *pgdat = (pg_data_t*)p;
 	struct task_struct *tsk = current;
 	const struct cpumask *cpumask = cpumask_of_node(pgdat->node_id);
@@ -3908,7 +3908,7 @@ static int kswapd(void *p)
 		bool ret;
 
 		alloc_order = reclaim_order = pgdat->kswapd_order;
-		classzone_idx = kswapd_classzone_idx(pgdat, classzone_idx);
+		classzone_idx = kswapd_classzone_idx(pgdat, MAX_NR_ZONES - 1);
 
 kswapd_try_sleep:
 		kswapd_try_to_sleep(pgdat, alloc_order, reclaim_order,
-- 
2.17.1