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Date: Fri, 12 Apr 2024 11:45:33 -0700
From: David Matlack <dmatlack@google.com>
To: James Houghton <jthoughton@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Paolo Bonzini <pbonzini@redhat.com>, Yu Zhao <yuzhao@google.com>,
	Marc Zyngier <maz@kernel.org>,
	Oliver Upton <oliver.upton@linux.dev>,
	Sean Christopherson <seanjc@google.com>,
	Jonathan Corbet <corbet@lwn.net>, James Morse <james.morse@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Zenghui Yu <yuzenghui@huawei.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>, Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Shaoqin Huang <shahuang@redhat.com>, Gavin Shan <gshan@redhat.com>,
	Ricardo Koller <ricarkol@google.com>,
	Raghavendra Rao Ananta <rananta@google.com>,
	Ryan Roberts <ryan.roberts@arm.com>,
	David Rientjes <rientjes@google.com>,
	Axel Rasmussen <axelrasmussen@google.com>,
	linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
	kvmarm@lists.linux.dev, kvm@vger.kernel.org, linux-mm@kvack.org,
	linux-trace-kernel@vger.kernel.org
Subject: Re: [PATCH v3 1/7] mm: Add a bitmap into
 mmu_notifier_{clear,test}_young
Message-ID: <ZhmBTUIhypg-Kxbx@google.com>
References: <20240401232946.1837665-1-jthoughton@google.com>
 <20240401232946.1837665-2-jthoughton@google.com>
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In-Reply-To: <20240401232946.1837665-2-jthoughton@google.com>

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.

> +#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.

> 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.