Commit 470c61d7 authored Dec 14, 2020 by Lorenzo Stoakes Committed by Linus Torvalds Dec 15, 2020

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>

parent ba8f3587

mm/page_alloc.c

+14 −21

Original line number	Diff line number	Diff line
		@@ -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);
		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;

		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;
		} 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);
		}
		}
		}

Admin message