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

James Houghton
2024-04-19T20:35:22Z

On Fri, Apr 12, 2024 at 11:45 AM David Matlack <dmatlack@google.com> wrote:
>
> On 2024-04-01 11:29 PM, 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)
>
> MMU_NOTIFIER_YOUNG_BITMAP_UNRELIABLE appears to be unused by all callers
> of test/clear_young(). I would vote to remove it.

Works for me.

>
> > +#define MMU_NOTIFIER_YOUNG_FAST (1 << 2)
>
> Instead of MMU_NOTIFIER_YOUNG_FAST, how about
> MMU_NOTIFIER_YOUNG_LOOK_AROUND? i.e. The secondary MMU is returning
> saying it recommends doing a look-around and passing in a bitmap?
>
> That would avoid the whole "what does FAST really mean" confusion.

I think MMU_NOTIFIER_YOUNG_LOOK_AROUND is fine.

>
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index fb49c2a60200..ca4b1ef9dfc2 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -917,10 +917,15 @@ static int kvm_mmu_notifier_clear_flush_young(struct mmu_notifier *mn,
> > static int kvm_mmu_notifier_clear_young(struct mmu_notifier *mn,
> > struct mm_struct *mm,
> > unsigned long start,
> > - unsigned long end)
> > + unsigned long end,
> > + unsigned long *bitmap)
> > {
> > trace_kvm_age_hva(start, end);
> >
> > + /* We don't support bitmaps. Don't test or clear anything. */
> > + if (bitmap)
> > + return MMU_NOTIFIER_YOUNG_BITMAP_UNRELIABLE;
>
> Wouldn't it be a bug to get a bitmap here? The main MM is only suppost
> to pass in a bitmap if the secondary MMU returns
> MMU_NOTIFIER_YOUNG_FAST, which KVM does not do at this point.
>
> Put another way, this check seems unneccessary.
>
> > +
> > /*
> > * Even though we do not flush TLB, this will still adversely
> > * affect performance on pre-Haswell Intel EPT, where there is
> > @@ -939,11 +944,17 @@ static int kvm_mmu_notifier_clear_young(struct mmu_notifier *mn,
> >
> > static int kvm_mmu_notifier_test_young(struct mmu_notifier *mn,
> > struct mm_struct *mm,
> > - unsigned long address)
> > + unsigned long start,
> > + unsigned long end,
> > + unsigned long *bitmap)
> > {
> > - trace_kvm_test_age_hva(address);
> > + trace_kvm_test_age_hva(start, end);
> > +
> > + /* We don't support bitmaps. Don't test or clear anything. */
> > + if (bitmap)
> > + return MMU_NOTIFIER_YOUNG_BITMAP_UNRELIABLE;
>
> Same thing here.

I will remove them, they are indeed unnecessary, as it is just dead code.