[PATCH v3 05/11] mm/memory_hotplug: Optimize zone shrinking code when checking for holes

David Hildenbrand
2019-08-29T07:00:54Z

... and clarify why this is needed at all right now. It all boils down
to false positives. We will try to remove the false positives for
!ZONE_DEVICE memory, soon, however, for ZONE_DEVICE memory we won't be
able to easily get rid of false positives.

Don't only detect "all holes" but try to shrink using the existing
functions we have.

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: David Hildenbrand <david@redhat.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Pavel Tatashin <pasha.tatashin@soleen.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Wei Yang <richardw.yang@linux.intel.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
mm/memory_hotplug.c | 45 +++++++++++++++++++++++----------------------
1 file changed, 23 insertions(+), 22 deletions(-)

diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index d3c34bbeb36d..663853bf97ed 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -411,32 +411,33 @@ static void shrink_zone_span(struct zone *zone, unsigned long start_pfn,
}
}

- /*
- * The section is not biggest or smallest mem_section in the zone, it
- * only creates a hole in the zone. So in this case, we need not
- * change the zone. But perhaps, the zone has only hole data. Thus
- * it check the zone has only hole or not.
- */
- for (pfn = zone->zone_start_pfn;
- pfn < zone_end_pfn(zone); pfn += PAGES_PER_SUBSECTION) {
- if (unlikely(!pfn_valid(pfn)))
- continue;
-
- if (page_zone(pfn_to_page(pfn)) != zone)
- continue;
-
- /* Skip range to be removed */
- if (pfn >= start_pfn && pfn < end_pfn)
- continue;
-
- /* If we find valid section, we have nothing to do */
+ if (!zone->spanned_pages) {
zone_span_writeunlock(zone);
return;
}

- /* The zone has no valid section */
- zone->zone_start_pfn = 0;
- zone->spanned_pages = 0;
+ /*
+ * Due to false positives in previous skrink attempts, it can happen
+ * that we can shrink the zones further (possibly to zero). Once we
+ * can reliably detect which PFNs actually belong to a zone
+ * (especially for ZONE_DEVICE memory where we don't have online
+ * sections), this can go.
+ */
+ pfn = find_smallest_section_pfn(nid, zone, zone->zone_start_pfn,
+ zone_end_pfn(zone));
+ if (pfn) {
+ zone->spanned_pages = zone_end_pfn(zone) - pfn;
+ zone->zone_start_pfn = pfn;
+
+ pfn = find_biggest_section_pfn(nid, zone, zone->zone_start_pfn,
+ zone_end_pfn(zone));
+ if (pfn)
+ zone->spanned_pages = pfn - zone->zone_start_pfn + 1;
+ }
+ if (!pfn) {
+ zone->zone_start_pfn = 0;
+ zone->spanned_pages = 0;
+ }
zone_span_writeunlock(zone);
}

--
2.21.0