Re: [PATCH v3 1/7] mm: Add a bitmap into mmu_notifier_{clear,test}_young

[David Hildenbrand <david@redhat.com>]

On 09.04.24 20:31, James Houghton wrote:
> Ah, I didn't see this in my inbox, sorry David!

No worries :)

> 
> On Thu, Apr 4, 2024 at 11:52 AM David Hildenbrand <david@redhat.com> wrote:
>>
>> On 02.04.24 01:29, James Houghton wrote:
>>> diff --git a/include/linux/mmu_notifier.h b/include/linux/mmu_notifier.h
>>> index f349e08a9dfe..daaa9db625d3 100644
>>> --- a/include/linux/mmu_notifier.h
>>> +++ b/include/linux/mmu_notifier.h
>>> @@ -61,6 +61,10 @@ enum mmu_notifier_event {
>>>
>>>    #define MMU_NOTIFIER_RANGE_BLOCKABLE (1 << 0)
>>>
>>> +#define MMU_NOTIFIER_YOUNG                   (1 << 0)
>>> +#define MMU_NOTIFIER_YOUNG_BITMAP_UNRELIABLE (1 << 1)
>>
>> Especially this one really deserves some documentation :)
> 
> Yes, will do. Something like
> 
>      MMU_NOTIFIER_YOUNG_BITMAP_UNRELIABLE indicates that the passed-in
> bitmap either (1) does not accurately represent the age of the pages
> (in the case of test_young), or (2) was not able to be used to
> completely clear the age/access bit (in the case of clear_young).

Make sense. I do wonder what the expected reaction from the caller is :)

> 
>>
>>> +#define MMU_NOTIFIER_YOUNG_FAST                      (1 << 2)
>>
>> And that one as well.
> 
> Something like
> 
>     Indicates that (1) passing a bitmap ({test,clear}_young_bitmap)
> would have been supported for this address range.
> 
> The name MMU_NOTIFIER_YOUNG_FAST really comes from the fact that KVM
> is able to harvest the access bit "fast" (so for x86, locklessly, and
> for arm64, with the KVM MMU read lock), "fast" enough that using a
> bitmap to do look-around is probably a good idea.

Is that really the right way to communicate that ("would have been 
supported") -- wouldn't we want to sense support differently?

> 
>>
>> Likely best to briefly document all of them, and how they are
>> supposed to be used (return value for X).
> 
> Right. Will do.
> 
>>
>>> +
>>>    struct mmu_notifier_ops {
>>>        /*
>>>         * Called either by mmu_notifier_unregister or when the mm is
>>> @@ -106,21 +110,36 @@ 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.
>>> +      *
>>> +      * If @bitmap is given but is not supported, return
>>> +      * MMU_NOTIFIER_YOUNG_BITMAP_UNRELIABLE.
>>> +      *
>>> +      * If the walk is done "quickly" and there were young PTEs,
>>> +      * MMU_NOTIFIER_YOUNG_FAST is returned.
>>>         */
>>>        int (*clear_young)(struct mmu_notifier *subscription,
>>>                           struct mm_struct *mm,
>>>                           unsigned long start,
>>> -                        unsigned long end);
>>> +                        unsigned long end,
>>> +                        unsigned long *bitmap);
>>>
>>>        /*
>>>         * test_young is called to check the young/accessed bitflag in
>>>         * 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.
>>> +      *
>>> +      * If @bitmap is given but is not supported, return
>>> +      * MMU_NOTIFIER_YOUNG_BITMAP_UNRELIABLE.
>>> +      *
>>> +      * If the walk is done "quickly" and there were young PTEs,
>>> +      * MMU_NOTIFIER_YOUNG_FAST is returned.
>>>         */
>>>        int (*test_young)(struct mmu_notifier *subscription,
>>>                          struct mm_struct *mm,
>>> -                       unsigned long address);
>>> +                       unsigned long start,
>>> +                       unsigned long end,
>>> +                       unsigned long *bitmap);
>>
>> What does "quickly" mean (why not use "fast")? What are the semantics, I
>> don't find any existing usage of that in this file.
> 
> "fast" means fast enough such that using a bitmap to scan adjacent
> pages (e.g. with MGLRU) is likely to be beneficial. I'll write more in
> this comment. Perhaps I should just rename it to
> MMU_NOTIFIER_YOUNG_BITMAP_SUPPORTED and drop the whole "likely to be
> beneficial" thing -- that's for MGLRU/etc. to decide really.

Yes!

> 
>>
>> Further, what is MMU_NOTIFIER_YOUNG you introduce used for?
> 
> MMU_NOTIFIER_YOUNG is the return value when the page was young, but we
> (1) didn't use a bitmap, and (2) the "fast" access bit harvesting
> wasn't possible. In this case we simply return 1, which is
> MMU_NOTIFIER_YOUNG. I'll make kvm_mmu_notifier_test_clear_young()
> properly return MMU_NOTIFIER_YOUNG instead of relying on the fact that
> it will be 1.

Yes, that will clarify it!

-- 
Cheers,

David / dhildenb