linux-kernel - [PATCH v2 1/8] mm: dump_page: print head page's refcount, for compound pages

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Date:   Tue, 28 Jan 2020 19:24:10 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
CC:     Al Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Chinner <david@...morbit.com>,
        Ira Weiny <ira.weiny@...el.com>, Jan Kara <jack@...e.cz>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Jonathan Corbet <corbet@....net>,
        Jérôme Glisse <jglisse@...hat.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Michal Hocko <mhocko@...e.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Shuah Khan <shuah@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>, <linux-doc@...r.kernel.org>,
        <linux-fsdevel@...r.kernel.org>, <linux-kselftest@...r.kernel.org>,
        <linux-rdma@...r.kernel.org>, <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        John Hubbard <jhubbard@...dia.com>
Subject: [PATCH v2 1/8] mm: dump_page: print head page's refcount, for compound pages

When debugging a problem that involves compound pages, it is extremely
helpful if dump_page() reports not only the page->_refcount, but also
the refcount of the head page of the compound page. That's because the
head page collects refcounts for the entire compound page.

Therefore, enhance dump_page() so as to print out the refcount of the
head page of a compound page.

This approach (printing information about a struct page that is not the
struct page that was passed into dump_page()) has a precedent:
compound_mapcount is already being printed.

Signed-off-by: John Hubbard <jhubbard@...dia.com>
---
 mm/debug.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/mm/debug.c b/mm/debug.c
index a90da5337c14..4cc6cad8385d 100644
--- a/mm/debug.c
+++ b/mm/debug.c
@@ -76,9 +76,11 @@ 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,
+		pr_warn("page:%px refcount:%d head refcount:%d "
+			"mapcount:%d mapping:%px index:%#lx "
+			"compound_mapcount:%d\n",
+			page, page_ref_count(page),
+			page_ref_count(compound_head(page)), mapcount,
 			page->mapping, page_to_pgoff(page),
 			compound_mapcount(page));
 	else
-- 
2.25.0