[PATCH 03/11] mm: mmzone: introduce zone_pfn_same_memmap()

[Marek Szyprowski]

From: Michal Nazarewicz <mina86 at mina86.com>

This commit introduces zone_pfn_same_memmap() function which checkes
whether two PFNs within the same zone have struct pages within the
same memmap.  This check is needed because in general pointer
arithmetic on struct pages may lead to invalid pointers.

On memory models that are not affected, zone_pfn_same_memmap() is
defined as always returning true so the call should be optimised
at compile time.

Signed-off-by: Michal Nazarewicz <mina86 at mina86.com>
Signed-off-by: Marek Szyprowski <m.szyprowski at samsung.com>
---
 include/linux/mmzone.h |   16 ++++++++++++++++
 mm/compaction.c        |    5 ++++-
 2 files changed, 20 insertions(+), 1 deletions(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 188cb2f..84e07d0 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -1166,6 +1166,22 @@ static inline int memmap_valid_within(unsigned long pfn,
 }
 #endif /* CONFIG_ARCH_HAS_HOLES_MEMORYMODEL */
 
+#if defined(CONFIG_SPARSEMEM) && !defined(CONFIG_SPARSEMEM_VMEMMAP)
+/*
+ * Both PFNs must be from the same zone!  If this function returns
+ * true, pfn_to_page(pfn1) + (pfn2 - pfn1) == pfn_to_page(pfn2).
+ */
+static inline bool zone_pfn_same_memmap(unsigned long pfn1, unsigned long pfn2)
+{
+	return pfn_to_section_nr(pfn1) == pfn_to_section_nr(pfn2);
+}
+
+#else
+
+#define zone_pfn_same_memmap(pfn1, pfn2) (true)
+
+#endif
+
 #endif /* !__GENERATING_BOUNDS.H */
 #endif /* !__ASSEMBLY__ */
 #endif /* _LINUX_MMZONE_H */
diff --git a/mm/compaction.c b/mm/compaction.c
index 6afae0e..09c9702 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -111,7 +111,10 @@ skip:
 
 next:
 		pfn += isolated;
-		page += isolated;
+		if (zone_pfn_same_memmap(pfn - isolated, pfn))
+			page += isolated;
+		else
+			page = pfn_to_page(pfn);
 	}
 
 	trace_mm_compaction_isolate_freepages(nr_scanned, total_isolated);
-- 
1.7.1.569.g6f426