mm/page-writeback: introduce tracepoint for wait_on_page_writeback()

Recently there have been some hung tasks on our server due to
wait_on_page_writeback(), and we want to know the details of this
PG_writeback, i.e.  this page is writing back to which device.  But it is
not so convenient to get the details.

I think it would be better to introduce a tracepoint for diagnosing the
writeback details.

Link: http://lkml.kernel.org/r/1556274402-19018-1-git-send-email-laoar.shao@gmail.comSigned-off-by: Yafang Shao <laoar.shao@gmail.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/pagemap.h

@@ -540,15 +540,7 @@ static inline int wait_on_page_locked_killable(struct page *page)
 
 extern void put_and_wait_on_page_locked(struct page *page);
 
-/* 
- * Wait for a page to complete writeback
- */
-static inline void wait_on_page_writeback(struct page *page)
-{
-	if (PageWriteback(page))
-		wait_on_page_bit(page, PG_writeback);
-}
-
+void wait_on_page_writeback(struct page *page);
 extern void end_page_writeback(struct page *page);
 void wait_for_stable_page(struct page *page);
 

include/trace/events/writeback.h

@@ -53,7 +53,7 @@ WB_WORK_REASON
 
 struct wb_writeback_work;
 
-TRACE_EVENT(writeback_dirty_page,
+DECLARE_EVENT_CLASS(writeback_page_template,
 
 	TP_PROTO(struct page *page, struct address_space *mapping),
 
@@ -79,6 +79,20 @@ TRACE_EVENT(writeback_dirty_page,
 	)
 );
 
+DEFINE_EVENT(writeback_page_template, writeback_dirty_page,
+
+	TP_PROTO(struct page *page, struct address_space *mapping),
+
+	TP_ARGS(page, mapping)
+);
+
+DEFINE_EVENT(writeback_page_template, wait_on_page_writeback,
+
+	TP_PROTO(struct page *page, struct address_space *mapping),
+
+	TP_ARGS(page, mapping)
+);
+
 DECLARE_EVENT_CLASS(writeback_dirty_inode_template,
 
 	TP_PROTO(struct inode *inode, int flags),

mm/page-writeback.c

@@ -2808,6 +2808,18 @@ int __test_set_page_writeback(struct page *page, bool keep_write)
 }
 EXPORT_SYMBOL(__test_set_page_writeback);
 
+/*
+ * Wait for a page to complete writeback
+ */
+void wait_on_page_writeback(struct page *page)
+{
+	if (PageWriteback(page)) {
+		trace_wait_on_page_writeback(page, page_mapping(page));
+		wait_on_page_bit(page, PG_writeback);
+	}
+}
+EXPORT_SYMBOL_GPL(wait_on_page_writeback);
+
 /**
  * wait_for_stable_page() - wait for writeback to finish, if necessary.
  * @page:	The page to wait on.