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

David Hildenbrand
2024-04-04T18:52:24Z

On 02.04.24 01:29, James Houghton wrote:
> The bitmap is provided for secondary MMUs to use if they support it. For
> test_young(), after it returns, the bitmap represents the pages that
> were young in the interval [start, end). For clear_young, it represents
> the pages that we wish the secondary MMU to clear the accessed/young bit
> for.
>
> If a bitmap is not provided, the mmu_notifier_{test,clear}_young() API
> should be unchanged except that if young PTEs are found and the
> architecture supports passing in a bitmap, instead of returning 1,
> MMU_NOTIFIER_YOUNG_FAST is returned.
>
> This allows MGLRU's look-around logic to work faster, resulting in a 4%
> improvement in real workloads[1]. Also introduce MMU_NOTIFIER_YOUNG_FAST
> to indicate to main mm that doing look-around is likely to be
> beneficial.
>
> If the secondary MMU doesn't support the bitmap, it must return
> an int that contains MMU_NOTIFIER_YOUNG_BITMAP_UNRELIABLE.
>
> [1]: https://lore.kernel.org/all/20230609005935.42390-1-yuzhao@google.com/
>
> Suggested-by: Yu Zhao <yuzhao@google.com>
> Signed-off-by: James Houghton <jthoughton@google.com>
> ---
> include/linux/mmu_notifier.h | 93 +++++++++++++++++++++++++++++++++---
> include/trace/events/kvm.h | 13 +++--
> mm/mmu_notifier.c | 20 +++++---
> virt/kvm/kvm_main.c | 19 ++++++--
> 4 files changed, 123 insertions(+), 22 deletions(-)
>
> 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 :)

> +#define MMU_NOTIFIER_YOUNG_FAST (1 << 2)

And that one as well.

Likely best to briefly document all of them, and how they are
supposed to be used (return value for X).

> +
> 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.

Further, what is MMU_NOTIFIER_YOUNG you introduce used for?

--
Cheers,

David / dhildenb