When a page is freed with the PG_mlocked set, it is considered an

unexpected but recoverable situation.  A counter records how often this
event happens but it is easy to miss that this event has occured at
all.  This patch warns once when PG_mlocked is set to prompt debuggers
to check the counter to see how often it is happening.
Signed-off-by: Mel Gorman <mel@csn.ul.ie>
Reviewed-by: Christoph Lameter <cl@linux-foundation.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/page_alloc.c

@@ -495,6 +495,11 @@ static inline void __free_one_page(struct page *page,
  */
 static inline void free_page_mlock(struct page *page)
 {
+	WARN_ONCE(1, KERN_WARNING
+		"Page flag mlocked set for process %s at pfn:%05lx\n"
+		"page:%p flags:%#lx\n",
+		current->comm, page_to_pfn(page),
+		page, page->flags|__PG_MLOCKED);
 	__dec_zone_page_state(page, NR_MLOCK);
 	__count_vm_event(UNEVICTABLE_MLOCKFREED);
 }