Re: [PATCH v11 04/25] mm: devmap: refactor 1-based refcounting for ZONE_DEVICE pages

[John Hubbard <jhubbard@nvidia.com>]

On 12/18/19 8:04 AM, Kirill A. Shutemov wrote:
> On Mon, Dec 16, 2019 at 02:25:16PM -0800, John Hubbard wrote:
>> An upcoming patch changes and complicates the refcounting and
>> especially the "put page" aspects of it. In order to keep
>> everything clean, refactor the devmap page release routines:
>>
>> * Rename put_devmap_managed_page() to page_is_devmap_managed(),
>>    and limit the functionality to "read only": return a bool,
>>    with no side effects.
>>
>> * Add a new routine, put_devmap_managed_page(), to handle checking
>>    what kind of page it is, and what kind of refcount handling it
>>    requires.
>>
>> * Rename __put_devmap_managed_page() to free_devmap_managed_page(),
>>    and limit the functionality to unconditionally freeing a devmap
>>    page.
> 
> What the reason to separate put_devmap_managed_page() from
> free_devmap_managed_page() if free_devmap_managed_page() has exacly one
> caller? Is it preparation for the next patches?


Yes. A later patch, #23, adds another caller: __unpin_devmap_managed_user_page().

...
>> @@ -971,7 +971,14 @@ static inline bool put_devmap_managed_page(struct page *page)
>>   	return false;
>>   }
>>   
>> +bool put_devmap_managed_page(struct page *page);
>> +
>>   #else /* CONFIG_DEV_PAGEMAP_OPS */
>> +static inline bool page_is_devmap_managed(struct page *page)
>> +{
>> +	return false;
>> +}
>> +
>>   static inline bool put_devmap_managed_page(struct page *page)
>>   {
>>   	return false;
>> @@ -1028,8 +1035,10 @@ static inline void put_page(struct page *page)
>>   	 * need to inform the device driver through callback. See
>>   	 * include/linux/memremap.h and HMM for details.
>>   	 */
>> -	if (put_devmap_managed_page(page))
>> +	if (page_is_devmap_managed(page)) {
>> +		put_devmap_managed_page(page);
> 
> put_devmap_managed_page() has yet another page_is_devmap_managed() check
> inside. It looks strange.
> 

Good point, it's an extra unnecessary check. So to clean it up, I'll note
that the "if" check is required here in put_page(), in order to stay out of
non-inlined function calls in the hot path (put_page()). So I'll do the
following:

* Leave the above code as it is here

* Simplify put_devmap_managed_page(), it was trying to do two separate things,
   and those two things have different requirements. So change it to a void
   function, with a WARN_ON_ONCE to assert that page_is_devmap_managed() is true,

* And change the other caller (release_pages()) to do that check.

...
>> @@ -1102,3 +1102,27 @@ void __init swap_setup(void)
>>   	 * _really_ don't want to cluster much more
>>   	 */
>>   }
>> +
>> +#ifdef CONFIG_DEV_PAGEMAP_OPS
>> +bool put_devmap_managed_page(struct page *page)
>> +{
>> +	bool is_devmap = page_is_devmap_managed(page);
>> +
>> +	if (is_devmap) {
> 
> Reversing the condition would save you an indentation level.

Yes. Done.

I'll also git-reply with an updated patch so you can see what it looks like.


thanks,
-- 
John Hubbard
NVIDIA