mm/debug: improve memcg debugging

The memcg_data is only valid on the head page, not the tail pages.  Change
the format and location of the printout within the dump to match the other
parts of struct page better.

Link: https://lkml.kernel.org/r/20210114190200.1894484-1-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/debug.c

@@ -110,6 +110,11 @@ void __dump_page(struct page *page, const char *reason)
 					head_compound_mapcount(head));
 		}
 	}
+
+#ifdef CONFIG_MEMCG
+	if (head->memcg_data)
+		pr_warn("memcg:%lx\n", head->memcg_data);
+#endif
 	if (PageKsm(page))
 		type = "ksm ";
 	else if (PageAnon(page))
@@ -180,11 +185,6 @@ void __dump_page(struct page *page, const char *reason)
 
 	if (reason)
 		pr_warn("page dumped because: %s\n", reason);
-
-#ifdef CONFIG_MEMCG
-	if (!page_poisoned && page->memcg_data)
-		pr_warn("pages's memcg:%lx\n", page->memcg_data);
-#endif
 }
 
 void dump_page(struct page *page, const char *reason)