[136/200] mm: page_alloc: refactor setup_per_zone_lowmem_reserve()

Commit Message

Andrew Morton Dec. 15, 2020, 3:11 a.m. UTC

From: Lorenzo Stoakes <[e-mail]>
Subject: 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 preceding 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.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Lorenzo Stoakes <[e-mail]>
Cc: Baoquan He <[e-mail]>
Cc: Michal Hocko <[e-mail]>
Cc: Nicholas Piggin <[e-mail]>
Cc: Vlastimil Babka <[e-mail]>
Cc: Roman Gushchin <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
---

 mm/page_alloc.c |   33 +++++++++++++--------------------
 1 file changed, 13 insertions(+), 20 deletions(-)

Patch

--- a/mm/page_alloc.c~mm-page_alloc-refactor-setup_per_zone_lowmem_reserve
+++ a/mm/page_alloc.c
@@ -7862,31 +7862,24 @@  static void calculate_totalreserve_pages
 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);
+		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;
 
-			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 (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);
 			}
 		}
 	}