linux-kernel - [PATCH 1/5] mm/page_alloc.c: only tune sysctl_lowmem_reserve

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Date:   Tue, 24 Mar 2020 22:22:25 +0800
From:   Baoquan He <bhe@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     linux-mm@...ck.org, akpm@...ux-foundation.org,
        iamjoonsoo.kim@....com, hannes@...xchg.org, mhocko@...nel.org,
        vbabka@...e.cz, bhe@...hat.com
Subject: [PATCH 1/5] mm/page_alloc.c: only tune sysctl_lowmem_reserve_ratio value once when changing it

When people write to /proc/sys/vm/lowmem_reserve_ratio to change
sysctl_lowmem_reserve_ratio[], setup_per_zone_lowmem_reserve()
is called to recalculate all ->lowmem_reserve[] for each zone of all
nodes as below:

static void setup_per_zone_lowmem_reserve(void)
{
...
	for_each_online_pgdat(pgdat) {
		for (j = 0; j < MAX_NR_ZONES; j++) {
			...
			while (idx) {
				...
				if (sysctl_lowmem_reserve_ratio[idx] < 1) {
					sysctl_lowmem_reserve_ratio[idx] = 0;
					lower_zone->lowmem_reserve[j] = 0;
                                } else {
				...
			}
		}
	}
}

Meanwhile, here, sysctl_lowmem_reserve_ratio[idx] will be tuned if its
value is smaller than '1'. As we know, sysctl_lowmem_reserve_ratio[] is
set for zone without regarding to which node it belongs to. That means
the tuning will be done on all nodes, even though it has been done in the
first node.

And the tuning will be done too even when init_per_zone_wmark_min()
calls setup_per_zone_lowmem_reserve(), where actually nobody tries to
change sysctl_lowmem_reserve_ratio[].

So now move the tuning into lowmem_reserve_ratio_sysctl_handler(), to
make code logic more reasonable.

Signed-off-by: Baoquan He <bhe@...hat.com>
---
 mm/page_alloc.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index ca1453204e66..c0c788798d8b 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -7840,8 +7840,7 @@ static void setup_per_zone_lowmem_reserve(void)
 				idx--;
 				lower_zone = pgdat->node_zones + idx;
 
-				if (sysctl_lowmem_reserve_ratio[idx] < 1) {
-					sysctl_lowmem_reserve_ratio[idx] = 0;
+				if (!sysctl_lowmem_reserve_ratio[idx]) {
 					lower_zone->lowmem_reserve[j] = 0;
 				} else {
 					lower_zone->lowmem_reserve[j] =
@@ -8106,7 +8105,15 @@ int sysctl_min_slab_ratio_sysctl_handler(struct ctl_table *table, int write,
 int lowmem_reserve_ratio_sysctl_handler(struct ctl_table *table, int write,
 	void __user *buffer, size_t *length, loff_t *ppos)
 {
+	int i;
+
 	proc_dointvec_minmax(table, write, buffer, length, ppos);
+
+	for (i = 0; i < MAX_NR_ZONES; i++) {
+		if (sysctl_lowmem_reserve_ratio[i] < 1)
+			sysctl_lowmem_reserve_ratio[i] = 0;
+	}
+
 	setup_per_zone_lowmem_reserve();
 	return 0;
 }
-- 
2.17.2