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/20201129162758.115907-1-lstoakes@gmail.com
Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Roman Gushchin <guro@fb.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 53d633125e995d6cdc715ae79de328dac6796a69..c026908b5306f132de491add958c9a7d0f1aad09 100644 (file)
--- 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);
                        }
                }
        }