[PATCH V3 1/2] mm: page_alloc: correct high atomic reserve calculations

[Charan Teja Kalla]

reserve_highatomic_pageblock() aims to reserve the 1% of the managed
pages of a zone, which is used for the high order atomic allocations.

It uses the below calculation to reserve:
static void reserve_highatomic_pageblock(struct page *page, ....) {

   .......
   max_managed = (zone_managed_pages(zone) / 100) + pageblock_nr_pages;

   if (zone->nr_reserved_highatomic >= max_managed)
       goto out;

   zone->nr_reserved_highatomic += pageblock_nr_pages;
   set_pageblock_migratetype(page, MIGRATE_HIGHATOMIC);
   move_freepages_block(zone, page, MIGRATE_HIGHATOMIC, NULL);

out:
   ....
}

Since we are always appending the 1% of zone managed pages count to
pageblock_nr_pages, the minimum it is turning into 2 pageblocks as the
nr_reserved_highatomic is incremented/decremented in pageblock sizes.

Encountered a system(actually a VM running on the Linux kernel) with the
below zone configuration:
Normal free:7728kB boost:0kB min:804kB low:1004kB high:1204kB
reserved_highatomic:8192KB managed:49224kB

The existing calculations making it to reserve the 8MB(with pageblock
size of 4MB) i.e. 16% of the zone managed memory.  Reserving such high
amount of memory can easily exert memory pressure in the system thus may
lead into unnecessary reclaims till unreserving of high atomic reserves.

Since high atomic reserves are managed in pageblock size granules, as
MIGRATE_HIGHATOMIC is set for such pageblock, fix the calculations for
high atomic reserves as,  minimum is pageblock size , maximum is
approximately 1% of the zone managed pages.

Acked-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Charan Teja Kalla <quic_charante@xxxxxxxxxxx>
---
 mm/page_alloc.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 733732e..a789dfd 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1884,10 +1884,11 @@ static void reserve_highatomic_pageblock(struct page *page, struct zone *zone)
 	unsigned long max_managed, flags;
 
 	/*
-	 * Limit the number reserved to 1 pageblock or roughly 1% of a zone.
+	 * The number reserved as: minimum is 1 pageblock, maximum is
+	 * roughly 1% of a zone.
 	 * Check is race-prone but harmless.
 	 */
-	max_managed = (zone_managed_pages(zone) / 100) + pageblock_nr_pages;
+	max_managed = ALIGN((zone_managed_pages(zone) / 100), pageblock_nr_pages);
 	if (zone->nr_reserved_highatomic >= max_managed)
 		return;
 
-- 
2.7.4