[ARM] Skip memory holes in FLATMEM when reading /proc/pagetypeinfo

Ordinarily, memory holes in flatmem still have a valid memmap and is safe
to use. However, an architecture (ARM) frees up the memmap backing memory
holes on the assumption it is never used. /proc/pagetypeinfo reads the
whole range of pages in a zone believing that the memmap is valid and that
pfn_valid will return false if it is not. On ARM, freeing the memmap breaks
the page->zone linkages even though pfn_valid() returns true and the kernel
can oops shortly afterwards due to accessing a bogus struct zone *.

This patch lets architectures say when FLATMEM can have holes in the
memmap. Rather than an expensive check for valid memory, /proc/pagetypeinfo
will confirm that the page linkages are still valid by checking page->zone
is still the expected zone. The lookup of page_zone is safe as there is a
limited range of memory that is accessed when calling page_zone.  Even if
page_zone happens to return the correct zone, the impact is that the counters
in /proc/pagetypeinfo are slightly off but fragmentation monitoring is
unlikely to be relevant on an embedded system.

Reported-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Mel Gorman <mel@csn.ul.ie>
Tested-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

1 files changed, 18 insertions, 1 deletions

diff --git a/mm/vmstat.c b/mm/vmstat.c
index b0d08e667ece..d7826af2fb07 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -516,9 +516,26 @@ static void pagetypeinfo_showblockcount_print(struct seq_file *m,
                         continue;
 
                 page = pfn_to_page(pfn);
+#ifdef CONFIG_ARCH_FLATMEM_HAS_HOLES
+                /*
+                 * Ordinarily, memory holes in flatmem still have a valid
+                 * memmap for the PFN range. However, an architecture for
+                 * embedded systems (e.g. ARM) can free up the memmap backing
+                 * holes to save memory on the assumption the memmap is
+                 * never used. The page_zone linkages are then broken even
+                 * though pfn_valid() returns true. Skip the page if the
+                 * linkages are broken. Even if this test passed, the impact
+                 * is that the counters for the movable type are off but
+                 * fragmentation monitoring is likely meaningless on small
+                 * systems.
+                 */
+                if (page_zone(page) != zone)
+                        continue;
+#endif
                 mtype = get_pageblock_migratetype(page);
 
-                count[mtype]++;
+                if (mtype < MIGRATE_TYPES)
+                        count[mtype]++;
         }
 
         /* Print counts */