[PATCH V3 2/2] mm: page_alloc: enforce minimum zone size to do high atomic reserves

[Charan Teja Kalla]

Highatomic reserves are set to roughly 1% of zone for maximum and a
pageblock size for minimum.  Encountered a system with the below
configuration:
Normal free:7728kB boost:0kB min:804kB low:1004kB high:1204kB
reserved_highatomic:8192KB managed:49224kB

On such systems, even a single pageblock makes highatomic reserves are
set to ~8% of the zone memory. This high value can easily exert pressure
on the zone.

Per discussion with Michal and Mel, it is not much useful to reserve
the memory for highatomic allocations on such small systems[1]. Since
the minimum size for high atomic reserves is always going to be a
pageblock size and if 1% of zone managed pages is going to be below
pageblock size, don't reserve memory for high atomic allocations. Thanks
Michal for this suggestion[2].

Since no memory is being reserved for high atomic allocations and if
respective allocation failures are seen, this patch can be reverted.

[1] https://lore.kernel.org/linux-mm/20231117161956.d3yjdxhhm4rhl7h2@xxxxxxxxxxxxxxxxxxx/
[2] https://lore.kernel.org/linux-mm/ZVYRJMUitykepLRy@tiehlicka/

Signed-off-by: Charan Teja Kalla <quic_charante@xxxxxxxxxxx>
---
 mm/page_alloc.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index a789dfd..9f1b33e 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1885,9 +1885,12 @@ static void reserve_highatomic_pageblock(struct page *page, struct zone *zone)
 
 	/*
 	 * The number reserved as: minimum is 1 pageblock, maximum is
-	 * roughly 1% of a zone.
+	 * roughly 1% of a zone. But if 1% of a zone falls below a
+	 * pageblock size, then don't reserve any pageblocks.
 	 * Check is race-prone but harmless.
 	 */
+	if ((zone_managed_pages(zone) / 100) < pageblock_nr_pages)
+		return;
 	max_managed = ALIGN((zone_managed_pages(zone) / 100), pageblock_nr_pages);
 	if (zone->nr_reserved_highatomic >= max_managed)
 		return;
-- 
2.7.4