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From: John Hubbard <jhubbard@nvidia.com>
To: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: "Andrew Morton" <akpm@linux-foundation.org>,
	"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>,
	"Michal Hocko" <mhocko@suse.com>,
	"Mike Kravetz" <mike.kravetz@oracle.com>,
	"Shuah Khan" <shuah@kernel.org>,
	"Vlastimil Babka" <vbabka@suse.cz>,
	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>
Subject: Re: [PATCH v2 1/8] mm: dump_page: print head page's refcount, for compound pages
Date: Wed, 29 Jan 2020 14:26:06 -0800
Message-ID: <b74e8aa9-fcfd-0340-594c-61f185a0ae65@nvidia.com> (raw)
In-Reply-To: <20200129112510.ulims6u36ofk2qwa@box>

On 1/29/20 3:25 AM, Kirill A. Shutemov wrote:
> On Tue, Jan 28, 2020 at 07:24:10PM -0800, John Hubbard wrote:
>> 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.
> 
> refcount on a tail must always be 0. I think we should only print it when
> it is non-zero, emphasizing this fact with a standalone message.
> 

Hi Kirill,

Yes, good point, that sounds like just the right balance. And it avoids adding 
a new item to print in the common case, which is very nice. Here's what I've
changed it to for the next version (I'll also rewrite the commit description 
accordingly):


diff --git a/mm/debug.c b/mm/debug.c
index a90da5337c14..3a45e2b77de0 100644
--- a/mm/debug.c
+++ b/mm/debug.c
@@ -75,12 +75,17 @@ 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,
+	if (PageCompound(page)) {
+		pr_warn("page:%px compound refcount:%d mapcount:%d mapping:%px "
+			"index:%#lx compound_mapcount:%d\n",
+			page, page_ref_count(compound_head(page)), mapcount,
 			page->mapping, page_to_pgoff(page),
 			compound_mapcount(page));
+
+		if (page != compound_head(page) && page_ref_count(page) != 0)
+			pr_warn("page:%px PROBLEM: non-zero refcount (==%d) on "
+				"this tail page\n", page, page_ref_count(page));
+	}
 	else
 		pr_warn("page:%px refcount:%d mapcount:%d mapping:%px index:%#lx\n",
 			page, page_ref_count(page), mapcount,


thanks,
-- 
John Hubbard
NVIDIA

  reply index

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-29  3:24 [PATCH v2 0/8] mm/gup: track FOLL_PIN pages (follow on from v12) John Hubbard
2020-01-29  3:24 ` [PATCH v2 1/8] mm: dump_page: print head page's refcount, for compound pages John Hubbard
2020-01-29 11:25   ` Kirill A. Shutemov
2020-01-29 22:26     ` John Hubbard [this message]
2020-01-29 22:59       ` Matthew Wilcox
2020-01-30  6:23         ` John Hubbard
2020-01-30  6:30           ` John Hubbard
2020-01-29  3:24 ` [PATCH v2 2/8] mm/gup: split get_user_pages_remote() into two routines John Hubbard
2020-01-29  3:24 ` [PATCH v2 3/8] mm/gup: pass a flags arg to __gup_device_* functions John Hubbard
2020-01-29  3:24 ` [PATCH v2 4/8] mm/gup: track FOLL_PIN pages John Hubbard
2020-01-29 13:51   ` Kirill A. Shutemov
2020-01-30  6:44     ` John Hubbard
2020-01-30 11:31       ` Kirill A. Shutemov
2020-01-31  3:19         ` John Hubbard
2020-01-29  3:24 ` [PATCH v2 5/8] mm/gup: page->hpage_pinned_refcount: exact pin counts for huge pages John Hubbard
2020-01-29  3:24 ` [PATCH v2 6/8] mm/gup: /proc/vmstat: pin_user_pages (FOLL_PIN) reporting John Hubbard
2020-01-29  3:24 ` [PATCH v2 7/8] mm/gup_benchmark: support pin_user_pages() and related calls John Hubbard
2020-01-29  3:24 ` [PATCH v2 8/8] selftests/vm: run_vmtests: invoke gup_benchmark with basic FOLL_PIN coverage John Hubbard

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