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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>,
	"Christoph Hellwig" <hch@infradead.org>,
	"Dan Williams" <dan.j.williams@intel.com>,
	"Dave Chinner" <david@fromorbit.com>,
	"Ira Weiny" <ira.weiny@intel.com>, "Jan Kara" <jack@suse.cz>,
	"Jason Gunthorpe" <jgg@ziepe.ca>,
	"Jonathan Corbet" <corbet@lwn.net>,
	"Jérôme Glisse" <jglisse@redhat.com>,
	"Kirill A . Shutemov" <kirill@shutemov.name>,
	"Michal Hocko" <mhocko@suse.com>,
	"Mike Kravetz" <mike.kravetz@oracle.com>,
	"Shuah Khan" <shuah@kernel.org>,
	"Vlastimil Babka" <vbabka@suse.cz>,
	"Matthew Wilcox" <willy@infradead.org>,
	linux-doc@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-kselftest@vger.kernel.org, linux-rdma@vger.kernel.org,
	linux-mm@kvack.org, LKML <linux-kernel@vger.kernel.org>,
	"John Hubbard" <jhubbard@nvidia.com>,
	"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
Subject: [PATCH v5 01/12] mm: dump_page(): better diagnostics for compound pages
Date: Thu, 6 Feb 2020 19:37:24 -0800
Message-ID: <20200207033735.308000-2-jhubbard@nvidia.com> (raw)
In-Reply-To: <20200207033735.308000-1-jhubbard@nvidia.com>

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>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
 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


  reply index

Thread overview: 20+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-07  3:37 [PATCH v5 00/12] mm/gup: track FOLL_PIN pages John Hubbard
2020-02-07  3:37 ` John Hubbard [this message]
2020-02-07 17:27   ` [PATCH v5 01/12] mm: dump_page(): better diagnostics for compound pages Matthew Wilcox
2020-02-07 21:05     ` John Hubbard
2020-02-07 21:14       ` John Hubbard
2020-02-07  3:37 ` [PATCH v5 02/12] mm/gup: split get_user_pages_remote() into two routines John Hubbard
2020-02-07  3:37 ` [PATCH v5 03/12] mm/gup: pass a flags arg to __gup_device_* functions John Hubbard
2020-02-07  3:37 ` [PATCH v5 04/12] mm: introduce page_ref_sub_return() John Hubbard
2020-02-07 13:18   ` Kirill A. Shutemov
2020-02-07  3:37 ` [PATCH v5 05/12] mm/gup: pass gup flags to two more routines John Hubbard
2020-02-07  3:37 ` [PATCH v5 06/12] mm/gup: require FOLL_GET for get_user_pages_fast() John Hubbard
2020-02-07  3:37 ` [PATCH v5 07/12] mm/gup: track FOLL_PIN pages John Hubbard
2020-02-07  3:37 ` [PATCH v5 08/12] mm/gup: page->hpage_pinned_refcount: exact pin counts for huge pages John Hubbard
2020-02-07  3:37 ` [PATCH v5 09/12] mm: dump_page(): better diagnostics for huge pinned pages John Hubbard
2020-02-07  3:37 ` [PATCH v5 10/12] mm/gup: /proc/vmstat: pin_user_pages (FOLL_PIN) reporting John Hubbard
2020-02-07 13:19   ` Kirill A. Shutemov
2020-02-10 10:16   ` Jan Kara
2020-02-10 17:07     ` John Hubbard
2020-02-07  3:37 ` [PATCH v5 11/12] mm/gup_benchmark: support pin_user_pages() and related calls John Hubbard
2020-02-07  3:37 ` [PATCH v5 12/12] selftests/vm: run_vmtests: invoke gup_benchmark with basic FOLL_PIN coverage John Hubbard

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