Re: [PATCH v3 04/12] mm: introduce page_ref_sub_return()

[John Hubbard <jhubbard@nvidia.com>]

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@nvidia.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
>>
>