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Message-ID: <CADrL8HXLH=LL5QQq1i48U1CEbyUK2zqqHu4iRqVFBjDtwaqmDw@mail.gmail.com>
Date: Fri, 19 Apr 2024 13:34:44 -0700
From: James Houghton <jthoughton@...gle.com>
To: David Matlack <dmatlack@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Paolo Bonzini <pbonzini@...hat.com>,
Yu Zhao <yuzhao@...gle.com>, Marc Zyngier <maz@...nel.org>,
Oliver Upton <oliver.upton@...ux.dev>, Sean Christopherson <seanjc@...gle.com>,
Jonathan Corbet <corbet@....net>, James Morse <james.morse@....com>,
Suzuki K Poulose <suzuki.poulose@....com>, Zenghui Yu <yuzenghui@...wei.com>,
Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
Dave Hansen <dave.hansen@...ux.intel.com>, "H. Peter Anvin" <hpa@...or.com>,
Steven Rostedt <rostedt@...dmis.org>, Masami Hiramatsu <mhiramat@...nel.org>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Shaoqin Huang <shahuang@...hat.com>,
Gavin Shan <gshan@...hat.com>, Ricardo Koller <ricarkol@...gle.com>,
Raghavendra Rao Ananta <rananta@...gle.com>, Ryan Roberts <ryan.roberts@....com>,
David Rientjes <rientjes@...gle.com>, Axel Rasmussen <axelrasmussen@...gle.com>,
linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
kvmarm@...ts.linux.dev, kvm@...r.kernel.org, linux-mm@...ck.org,
linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/7] mm: Add a bitmap into mmu_notifier_{clear,test}_young
On Fri, Apr 12, 2024 at 11:45 AM David Matlack <dmatlack@...gle.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@...gle.com>
> > Signed-off-by: James Houghton <jthoughton@...gle.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.
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