[PATCH v6 05/11] mm: Add fast_only bool to test_young and clear_young MMU notifiers

[David Hildenbrand]

On 02.08.24 01:13, James Houghton wrote:
> On Thu, Aug 1, 2024 at 2:36 AM David Hildenbrand <david at redhat.com> wrote:
>>
>> On 24.07.24 03:10, James Houghton wrote:
>>> For implementers, the fast_only bool indicates that the age information
>>> needs to be harvested such that we do not slow down other MMU operations,
>>> and ideally that we are not ourselves slowed down by other MMU
>>> operations.  Usually this means that the implementation should be
>>> lockless.
>>
>> But what are the semantics if "fast_only" cannot be achieved by the
>> implementer?
>>
>> Can we add some documentation to the new functions that explain what
>> this mysterious "fast_only" is and what the expected semantics are?
>> Please? :)
> 
> Thanks for pointing out the missing documentation. How's this?
> 
> diff --git a/include/linux/mmu_notifier.h b/include/linux/mmu_notifier.h
> index 45c5995ebd84..c21992036dd3 100644
> --- a/include/linux/mmu_notifier.h
> +++ b/include/linux/mmu_notifier.h
> @@ -106,6 +106,18 @@ struct mmu_notifier_ops {
>           * clear_young is a lightweight version of clear_flush_young. Like the
>           * latter, it is supposed to test-and-clear the young/accessed bitflag
>           * in the secondary pte, but it may omit flushing the secondary tlb.
> +        *

Probably makes sense to highlight the parameters like @fast_only

> +        * The fast_only parameter indicates that this call should not block,
> +        * and this function should not cause other MMU notifier calls to
> +        * block. Usually this means that the implementation should be
> +        * lockless.
> +        *
> +        * When called with fast_only, this notifier will be a no-op unless
> +        * has_fast_aging is set on the struct mmu_notifier.

"... and will return 0 (NOT young)." ?

> +        *
> +        * When fast_only is true, if the implementer cannot determine that a
> +        * range is young without blocking, it should return 0 (i.e.,
> +        * that the range is NOT young).
>           */
>          int (*clear_young)(struct mmu_notifier *subscription,
>                             struct mm_struct *mm,
> @@ -118,6 +130,8 @@ struct mmu_notifier_ops {
>           * the secondary pte. This is used to know if the page is
>           * frequently used without actually clearing the flag or tearing
>           * down the secondary mapping on the page.
> +        *
> +        * The fast_only parameter has the same meaning as with clear_young.
>           */
>          int (*test_young)(struct mmu_notifier *subscription,
>                            struct mm_struct *mm,
> 
> I've also moved the commit that follows this one (the one that adds
> has_fast_aging) to be before this one so that the comment makes sense.


Makes sense, thanks!

-- 
Cheers,

David / dhildenb