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

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Message-ID: <c9f4bb05-457d-a7ea-f449-dfb399facb3c@nvidia.com>
Date:   Mon, 3 Feb 2020 12:03:49 -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>,
        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 2/3/20 5:23 AM, Kirill A. Shutemov wrote:
> 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?


Why yes, it should. I didn't even notice that that more precise function existed,
thanks for catching that. I've changed it to this for the next version of the
patchset:

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_and_return(page, -nr, ret);
	return ret;
}



thanks,
-- 
John Hubbard
NVIDIA

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