[PATCH] mm: consider zone which is not fully populated to have holes

[Michal Hocko]

__pageblock_pfn_to_page has two users currently, set_zone_contiguous
which checks whether the given zone contains holes and
pageblock_pfn_to_page which then carefully returns a first valid
page from the given pfn range for the given zone. This doesn't handle
zones which are not fully populated though. Memory pageblocks can be
offlined or might not have been onlined yet. In such a case the zone
should be considered to have holes otherwise pfn walkers can touch
and play with offline pages.

Current callers of pageblock_pfn_to_page in compaction seem to work
properly right now because they only isolate PageBuddy
(isolate_freepages_block) or PageLRU resp. __PageMovable
(isolate_migratepages_block) which will be always false for these pages.
It would be safer to skip these pages altogether, though. In order
to do that let's add pfn_to_online_page helper which checks PageReserved
because offline pages are reserved until they are onlined. There might
be other users of the PageReserved flag but they are rare and even if we
hit into those pages we should skip them in pfn walkers anyway. So this
is not harmful.

Use the new helper in __pageblock_pfn_to_page and skip the whole page
block in such a case. Vlastimil has noted that we might skip over
the page block even when there is a single reserved page but that
shouldn't lead to major issues because reserved pages are used very
seldom.

Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
---
 include/linux/memory_hotplug.h | 28 ++++++++++++++++++++++++++++
 mm/page_alloc.c                |  4 +++-
 2 files changed, 31 insertions(+), 1 deletion(-)

diff --git a/include/linux/memory_hotplug.h b/include/linux/memory_hotplug.h
index 3c8cf86201c3..736fe73e65af 100644
--- a/include/linux/memory_hotplug.h
+++ b/include/linux/memory_hotplug.h
@@ -14,6 +14,26 @@ struct memory_block;
 struct resource;
 
 #ifdef CONFIG_MEMORY_HOTPLUG
+/*
+ * Return page for the valid pfn only if the page is online.
+ * Offline pages are marked reserved. There are other users of PageReserved
+ * but pfn walkers should avoid them in general so such a false positive
+ * is not harmful.
+ *
+ * It would be great if this was a static inline but dependency hell doesn't
+ * allow that for now.
+ */
+#define pfn_to_online_page(pfn)				\
+({							\
+	struct page *___page = NULL;			\
+							\
+	if (pfn_valid(pfn)) {				\
+		___page = pfn_to_page(pfn);		\
+		if (unlikely(PageReserved(___page))) 	\
+			___page = NULL;			\
+	}						\
+	___page;					\
+})
 
 /*
  * Types for free bootmem stored in page->lru.next. These have to be in
@@ -203,6 +223,14 @@ extern void set_zone_contiguous(struct zone *zone);
 extern void clear_zone_contiguous(struct zone *zone);
 
 #else /* ! CONFIG_MEMORY_HOTPLUG */
+#define pfn_to_online_page(pfn)			\
+({						\
+	struct page *___page = NULL;		\
+	if (pfn_valid(pfn))			\
+		___page = pfn_to_page(pfn);	\
+	___page;				\
+ })
+
 /*
  * Stub functions for when hotplug is off
  */
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 5d72d29a6ece..9dd814f4e7f5 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1353,7 +1353,9 @@ struct page *__pageblock_pfn_to_page(unsigned long start_pfn,
 	if (!pfn_valid(start_pfn) || !pfn_valid(end_pfn))
 		return NULL;
 
-	start_page = pfn_to_page(start_pfn);
+	start_page = pfn_to_online_page(start_pfn);
+	if (!start_page)
+		return NULL;
 
 	if (page_zone(start_page) != zone)
 		return NULL;
-- 
2.11.0

-- 
Michal Hocko
SUSE Labs