linux-kernel - Re: [PATCH v3 04/12] mm: introduce page_ref_sub

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Message-ID: <20200203132329.oj32h4ryna4gmkwh@box>
Date:   Mon, 3 Feb 2020 16:23:29 +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>,
        Matthew Wilcox <willy@...radead.org>,
        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 v3 04/12] mm: introduce page_ref_sub_return()

On Fri, Jan 31, 2020 at 07:40:21PM -0800, John Hubbard wrote:
> An upcoming patch requires subtracting a large chunk of refcounts from
> a page, and checking what the resulting refcount is. This is a little
> different than the usual "check for zero refcount" that many of the
> page ref functions already do. However, it is similar to a few other
> routines that (like this one) are generally useful for things such as
> 1-based refcounting.
> 
> Add page_ref_sub_return(), that subtracts a chunk of refcounts
> atomically, and returns an atomic snapshot of the result.
> 
> Signed-off-by: John Hubbard <jhubbard@...dia.com>
> ---
>  include/linux/page_ref.h | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/include/linux/page_ref.h b/include/linux/page_ref.h
> index 14d14beb1f7f..b9cbe553d1e7 100644
> --- a/include/linux/page_ref.h
> +++ b/include/linux/page_ref.h
> @@ -102,6 +102,16 @@ static inline void page_ref_sub(struct page *page, int nr)
>  		__page_ref_mod(page, -nr);
>  }
>  
> +static inline int page_ref_sub_return(struct page *page, int nr)
> +{
> +	int ret = atomic_sub_return(nr, &page->_refcount);
> +
> +	if (page_ref_tracepoint_active(__tracepoint_page_ref_mod))
> +		__page_ref_mod(page, -nr);

Shouldn't it be __page_ref_mod_and_return() and relevant tracepoint?

> +
> +	return ret;
> +}
> +
>  static inline void page_ref_inc(struct page *page)
>  {
>  	atomic_inc(&page->_refcount);
> -- 
> 2.25.0
> 

-- 
 Kirill A. Shutemov