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

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Message-ID: <20200129112510.ulims6u36ofk2qwa@box>
Date:   Wed, 29 Jan 2020 14:25:10 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     John Hubbard <jhubbard@...dia.com>
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 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.

-- 
 Kirill A. Shutemov