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-rw-r--r--mm/compaction.c5
-rw-r--r--mm/internal.h8
2 files changed, 9 insertions, 4 deletions
diff --git a/mm/compaction.c b/mm/compaction.c
index e0ab02d..b48c525 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 7e145e8..612c14f 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)
{
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