linux-kernel - [PATCH] mm: page_alloc: refactor setup_per_zone_lowmem

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Message-Id: <20201129162758.115907-1-lstoakes@gmail.com>
Date:   Sun, 29 Nov 2020 16:27:58 +0000
From:   Lorenzo Stoakes <lstoakes@...il.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Baoquan He <bhe@...hat.com>, Michal Hocko <mhocko@...e.com>,
        Nicholas Piggin <npiggin@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>, Roman Gushchin <guro@...com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Lorenzo Stoakes <lstoakes@...il.com>
Subject: [PATCH] mm: page_alloc: refactor setup_per_zone_lowmem_reserve()

setup_per_zone_lowmem_reserve() iterates through each zone setting
zone->lowmem_reserve[j] = 0 (where j is the zone's index) then
iterates backwards through all proceeding zones, setting
lower_zone->lowmem_reserve[j] = sum(managed pages of higher zones) /
lowmem_reserve_ratio[idx] for each (where idx is the lower zone's
index).

If the lower zone has no managed pages or its ratio is 0 then all of its
lowmem_reserve[] entries are effectively zeroed.

As these arrays are only assigned here and all lowmem_reserve[] entries
for index < this zone's index are implicitly assumed to be 0 (as these
are specifically output in show_free_areas() and zoneinfo_show_print()
for example) there is no need to additionally zero index == this zone's
index too. This patch avoids zeroing unnecessarily.

Rather than iterating through zones and setting lowmem_reserve[j] for
each lower zone this patch reverse the process and populates each zone's
lowmem_reserve[] values in ascending order.

This clarifies what is going on especially in the case of zero managed
pages or ratio which is now explicitly shown to clear these values.

Signed-off-by: Lorenzo Stoakes <lstoakes@...il.com>
---
 mm/page_alloc.c | 35 ++++++++++++++---------------------
 1 file changed, 14 insertions(+), 21 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index f91df593bf71..39f92de1228f 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -7862,31 +7862,24 @@ static void calculate_totalreserve_pages(void)
 static void setup_per_zone_lowmem_reserve(void)
 {
 	struct pglist_data *pgdat;
-	enum zone_type j, idx;
+	enum zone_type i, j;

 	for_each_online_pgdat(pgdat) {
-		for (j = 0; j < MAX_NR_ZONES; j++) {
-			struct zone *zone = pgdat->node_zones + j;
-			unsigned long managed_pages = zone_managed_pages(zone);
-
-			zone->lowmem_reserve[j] = 0;
-
-			idx = j;
-			while (idx) {
-				struct zone *lower_zone;
-
-				idx--;
-				lower_zone = pgdat->node_zones + idx;
-
-				if (!sysctl_lowmem_reserve_ratio[idx] ||
-				    !zone_managed_pages(lower_zone)) {
-					lower_zone->lowmem_reserve[j] = 0;
-					continue;
+		for (i = 0; i < MAX_NR_ZONES - 1; i++) {
+			struct zone *zone = &pgdat->node_zones[i];
+			int ratio = sysctl_lowmem_reserve_ratio[i];
+			bool clear = !ratio || !zone_managed_pages(zone);
+			unsigned long managed_pages = 0;
+
+			for (j = i + 1; j < MAX_NR_ZONES; j++) {
+				if (clear) {
+					zone->lowmem_reserve[j] = 0;
 				} else {
-					lower_zone->lowmem_reserve[j] =
-						managed_pages / sysctl_lowmem_reserve_ratio[idx];
+					struct zone *upper_zone = &pgdat->node_zones[j];
+
+					managed_pages += zone_managed_pages(upper_zone);
+					zone->lowmem_reserve[j] = managed_pages / ratio;
 				}
-				managed_pages += zone_managed_pages(lower_zone);
 			}
 		}
 	}
-- 
2.29.2