mm: page_alloc: enforce minimum zone size to do high atomic reserves

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@techsingularity.net/
[2] https://lore.kernel.org/linux-mm/ZVYRJMUitykepLRy@tiehlicka/

Link: https://lkml.kernel.org/r/c3a2a48e2cfe08176a80eaf01c110deb9e918055.1700821416.git.quic_charante@quicinc.comSigned-off-by: Charan Teja Kalla <quic_charante@quicinc.com>
Acked-by: David Rientjes <rientjes@google.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Pavankumar Kondeti <quic_pkondeti@quicinc.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/page_alloc.c

@@ -1881,9 +1881,12 @@ 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;