[PATCH] mm: Improve documentation of page_order

[Mel Gorman <mgorman@suse.de>]

Developers occasionally try and optimise PFN scanners by using page_order
but miss that in general it requires zone->lock. This has happened twice for
compaction.c and rejected both times.  This patch clarifies the documentation
of page_order and adds a note to compaction.c why page_order is not used.

Signed-off-by: Mel Gorman <mgorman@suse.de>
---
 mm/compaction.c | 5 ++++-
 mm/internal.h   | 8 +++++---
 2 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index f58bcd0..f91d26b 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -522,7 +522,10 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
 		if (!isolation_suitable(cc, page))
 			goto next_pageblock;
 
-		/* Skip if free */
+		/*
+		 * Skip if free. page_order cannot be used without zone->lock
+		 * as nothing prevents parallel allocations or buddy merging.
+		 */
 		if (PageBuddy(page))
 			continue;
 
diff --git a/mm/internal.h b/mm/internal.h
index 684f7aa..09cd8be 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -144,9 +144,11 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
 #endif
 
 /*
- * function for dealing with page's order in buddy system.
- * zone->lock is already acquired when we use these.
- * So, we don't need atomic page->flags operations here.
+ * This functions returns the order of a free page in the buddy system.
+ * In general, page_zone(page)->lock must be held by the caller to prevent
+ * the page being allocated in parallel and returning garbage as the order.
+ * If the caller does not hold page_zone(page), they must guarantee that
+ * the page cannot be allocated or merged in parallel.
  */
 static inline unsigned long page_order(struct page *page)
 {

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