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

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Message-ID: <b74e8aa9-fcfd-0340-594c-61f185a0ae65@nvidia.com>
Date:   Wed, 29 Jan 2020 14:26:06 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        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>,
        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>
Subject: Re: [PATCH v2 1/8] mm: dump_page: print head page's refcount, for
 compound pages

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