[PATCH v4 01/12] mm: dump_page(): better diagnostics for compound pages

[John Hubbard <jhubbard@xxxxxxxxxx>]

A compound page collects the refcount in the head page, while leaving
the refcount of each tail page at zero. Therefore, when debugging a
problem that involves compound pages, it's best to have diagnostics that
reflect that situation. However, dump_page() is oblivious to these
points.

Change dump_page() as follows:

1) For tail pages, print relevant head page information: refcount, in
   particular. But only do this if the page is not corrupted so badly
   that the pointer to the head page is all wrong.

2) Do a separate check to catch any (rare) cases of the tail page's
   refcount being non-zero, and issue a separate, clear pr_warn() if
   that ever happens.

Suggested-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Suggested-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>
---
 mm/debug.c | 35 +++++++++++++++++++++++++++++------
 1 file changed, 29 insertions(+), 6 deletions(-)

diff --git a/mm/debug.c b/mm/debug.c
index ecccd9f17801..f074077eee11 100644
--- a/mm/debug.c
+++ b/mm/debug.c
@@ -42,6 +42,33 @@ const struct trace_print_flags vmaflag_names[] = {
 	{0, NULL}
 };
 
+static void __dump_tail_page(struct page *page, int mapcount)
+{
+	struct page *head = compound_head(page);
+
+	if ((page < head) || (page >= head + MAX_ORDER_NR_PAGES)) {
+		/*
+		 * Page is hopelessly corrupted, so limit any reporting to
+		 * information about the page itself. Do not attempt to look at
+		 * the head page.
+		 */
+		pr_warn("page:%px refcount:%d mapcount:%d mapping:%px "
+			"index:%#lx (corrupted tail page case)\n",
+			page, page_ref_count(page), mapcount, page->mapping,
+			page_to_pgoff(page));
+	} else {
+		pr_warn("page:%px compound refcount:%d mapcount:%d mapping:%px "
+			"index:%#lx compound_mapcount:%d\n",
+			page, page_ref_count(head), mapcount, head->mapping,
+			page_to_pgoff(head), compound_mapcount(page));
+	}
+
+	if (page_ref_count(page) != 0) {
+		pr_warn("page:%px PROBLEM: non-zero refcount (==%d) on this "
+			"tail page\n", page, page_ref_count(page));
+	}
+}
+
 void __dump_page(struct page *page, const char *reason)
 {
 	struct address_space *mapping;
@@ -75,12 +102,8 @@ void __dump_page(struct page *page, const char *reason)
 	 */
 	mapcount = PageSlab(page) ? 0 : page_mapcount(page);
 
-	if (PageCompound(page))
-		pr_warn("page:%px refcount:%d mapcount:%d mapping:%px "
-			"index:%#lx compound_mapcount: %d\n",
-			page, page_ref_count(page), mapcount,
-			page->mapping, page_to_pgoff(page),
-			compound_mapcount(page));
+	if (PageTail(page))
+		__dump_tail_page(page, mapcount);
 	else
 		pr_warn("page:%px refcount:%d mapcount:%d mapping:%px index:%#lx\n",
 			page, page_ref_count(page), mapcount,
-- 
2.25.0