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

John Hubbard
2019-12-19T00:35:23Z

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