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From:   John Hubbard <jhubbard@nvidia.com>
To:     Andrew Morton <akpm@linux-foundation.org>
CC:     Al Viro <viro@zeniv.linux.org.uk>,
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Subject: [PATCH v3 01/12] mm: dump_page(): better diagnostics for compound pages
Date:   Fri, 31 Jan 2020 19:40:18 -0800
Message-ID: <20200201034029.4063170-2-jhubbard@nvidia.com>
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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@infradead.org>
Suggested-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
 mm/debug.c | 34 ++++++++++++++++++++++++++++------
 1 file changed, 28 insertions(+), 6 deletions(-)

diff --git a/mm/debug.c b/mm/debug.c
index ecccd9f17801..beb1c59d784b 100644
--- a/mm/debug.c
+++ b/mm/debug.c
@@ -42,6 +42,32 @@ const struct trace_print_flags vmaflag_names[] =3D {
 	{0, NULL}
 };
=20
+static void __dump_tail_page(struct page *page, int mapcount)
+{
+	struct page *head =3D compound_head(page);
+
+	if ((page < head) || (page >=3D 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) !=3D 0)
+		pr_warn("page:%px PROBLEM: non-zero refcount (=3D=3D%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 +101,8 @@ void __dump_page(struct page *page, const char *reason)
 	 */
 	mapcount =3D PageSlab(page) ? 0 : page_mapcount(page);
=20
-	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,
--=20
2.25.0