[PATCH] drivers: memory: check for missing sections in test_pages_in_a_zone

[Andrew Banman <abanman@sgi.com>]

test_pages_in_a_zone does not account for the possibility of missing
sections in the given pfn range. pfn_valid_within always returns 1 when
CONFIG_HOLES_IN_ZONE is not set, allowing invalid pfns from missing
sections to pass the test, leading to a kernel oops.

Wrap an additional pfn loop with PAGES_PER_SECTION granularity to check
for missing sections before proceeding into the zone-check code.

This also prevents a crash from offlining memory devices with missing
sections. Despite this, it may be a good idea to keep the related patch
'[PATCH 3/3] drivers: memory: prohibit offlining of memory blocks
with missing sections' because missing sections in a memory block may lead
to other problems not covered by the scope of this fix.

---
 mm/memory_hotplug.c | 32 ++++++++++++++++++++------------
 1 file changed, 20 insertions(+), 12 deletions(-)

diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 67d488a..0c5f876 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -1375,23 +1375,30 @@ int is_mem_section_removable(unsigned long start_pfn, unsigned long nr_pages)
  */
 int test_pages_in_a_zone(unsigned long start_pfn, unsigned long end_pfn)
 {
-	unsigned long pfn;
+	unsigned long pfn, sec_end_pfn;
 	struct zone *zone = NULL;
 	struct page *page;
 	int i;
-	for (pfn = start_pfn;
+	for (pfn = start_pfn, sec_end_pfn = SECTION_ALIGN_UP(start_pfn);
 	     pfn < end_pfn;
-	     pfn += MAX_ORDER_NR_PAGES) {
-		i = 0;
-		/* This is just a CONFIG_HOLES_IN_ZONE check.*/
-		while ((i < MAX_ORDER_NR_PAGES) && !pfn_valid_within(pfn + i))
-			i++;
-		if (i == MAX_ORDER_NR_PAGES)
+	     pfn = sec_end_pfn + 1, sec_end_pfn += PAGES_PER_SECTION) {
+		/* Make sure the memory section is present first */
+		if (!present_section_nr(pfn_to_section_nr(pfn)))
 			continue;
-		page = pfn_to_page(pfn + i);
-		if (zone && page_zone(page) != zone)
-			return 0;
-		zone = page_zone(page);
+		for (; pfn < sec_end_pfn && pfn < end_pfn;
+		     pfn += MAX_ORDER_NR_PAGES) {
+			i = 0;
+			/* This is just a CONFIG_HOLES_IN_ZONE check.*/
+			while ((i < MAX_ORDER_NR_PAGES) &&
+				!pfn_valid_within(pfn + i))
+				i++;
+			if (i == MAX_ORDER_NR_PAGES)
+				continue;
+			page = pfn_to_page(pfn + i);
+			if (zone && page_zone(page) != zone)
+				return 0;
+			zone = page_zone(page);
+		}
 	}
 	return 1;
 }
-- 
1.7.12.4