[PATCH v3 06/11] mm/memory_hotplug: Fix crashes in shrink_zone_span()

[David Hildenbrand]

We can currently crash in shrink_zone_span() in case we access an
uninitialized memmap (via page_to_nid()). Root issue is that we cannot
always identify which memmap was actually initialized.

Let's improve the situation by looking only at online PFNs for
!ZONE_DEVICE memory. This is now very reliable - similar to
set_zone_contiguous(). (Side note: set_zone_contiguous() will never
succeed on ZONE_DEVICE memory right now as we have no online PFNs ...).

For ZONE_DEVICE memory, make sure we don't crash by special-casing
poisoned pages and always checking that the NID has a sane value. We
might still read garbage and get false positives, but it certainly
improves the situation.

Note: Especially subsections make it very hard to detect which parts of
a ZONE_DEVICE memmap were actually initialized - otherwise we could just
have reused SECTION_IS_ONLINE. This needs more thought.

Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Oscar Salvador <osalvador@xxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>
Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
Cc: Wei Yang <richardw.yang@xxxxxxxxxxxxxxx>
Reported-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxx>
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
---
 mm/memory_hotplug.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 663853bf97ed..65b3fdf7f838 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -334,6 +334,17 @@ static unsigned long find_smallest_section_pfn(int nid, struct zone *zone,
 		if (unlikely(!pfn_valid(start_pfn)))
 			continue;
 
+		/*
+		 * TODO: There is no way we can identify whether the memmap
+		 * of ZONE_DEVICE memory was initialized. We might get
+		 * false positives when reading garbage.
+		 */
+		if (zone_idx(zone) == ZONE_DEVICE) {
+			if (PagePoisoned(pfn_to_page(start_pfn)))
+				continue;
+		} else if (!pfn_to_online_page(start_pfn))
+			continue;
+
 		if (unlikely(pfn_to_nid(start_pfn) != nid))
 			continue;
 
@@ -359,6 +370,17 @@ static unsigned long find_biggest_section_pfn(int nid, struct zone *zone,
 		if (unlikely(!pfn_valid(pfn)))
 			continue;
 
+		/*
+		 * TODO: There is no way we can identify whether the memmap
+		 * of ZONE_DEVICE memory was initialized. We might get
+		 * false positives when reading garbage.
+		 */
+		if (zone_idx(zone) == ZONE_DEVICE) {
+			if (PagePoisoned(pfn_to_page(pfn)))
+				continue;
+		} else if (!pfn_to_online_page(pfn))
+			continue;
+
 		if (unlikely(pfn_to_nid(pfn) != nid))
 			continue;
 
-- 
2.21.0