mm: improve documentation of page_order

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.

[akpm@linux-foundation.org: tweaks]
[lauraa@codeaurora.org: Corrected a page_zone(page)->lock reference]
Signed-off-by: Mel Gorman <mgorman@suse.de>
Acked-by: Rafael Aquini <aquini@redhat.com>
Acked-by: Minchan Kim <minchan@kernel.org>
Cc: Laura Abbott <lauraa@codeaurora.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 9 insertions, 4 deletions

diff --git a/mm/compaction.c b/mm/compaction.c
index e0ab02d70f13..b48c5259ea33 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -523,7 +523,10 @@ 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 7e145e8cd1e6..612c14f5e0f5 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -143,9 +143,11 @@ 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 function 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 from being allocated in parallel and returning garbage as the order.
+ * If a caller does not hold page_zone(page)->lock, it must guarantee that the
+ * page cannot be allocated or merged in parallel.
  */
 static inline unsigned long page_order(struct page *page)
 {