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From:   guoren@kernel.org
To:     jszhang@kernel.org
Cc:     aou@eecs.berkeley.edu, linux-kernel@vger.kernel.org,
        linux-riscv@lists.infradead.org, palmer@dabbelt.com,
        paul.walmsley@sifive.com, surenb@google.com, chenhuacai@kernel.org,
        Guo Ren <guoren@kernel.org>
Subject: Re: [PATCH] riscv: mm: try VMA lock-based page fault handling first
Date:   Wed, 24 May 2023 01:02:59 -0400
Message-Id: <20230524050259.104358-1-guoren@kernel.org>
In-Reply-To: <20230523165942.2630-1-jszhang@kernel.org>
References: <20230523165942.2630-1-jszhang@kernel.org>
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> Attempt VMA lock-based page fault handling first, and fall back to the
> existing mmap_lock-based handling if that fails.
> 
> A simple running the ebizzy benchmark on Lichee Pi 4A shows that
> PER_VMA_LOCK can improve the ebizzy benchmark by about 32.68%. In
Good improvement, I think VMA lock is worth to support in riscv.

Please give more details about ebizzy, Is it 
https://github.com/linux-test-project/ltp/blob/master/utils/benchmark/ebizzy-0.3/ebizzy.c
?

> theory, the more CPUs, the bigger improvement, but I don't have any
> HW platform which has more than 4 CPUs.
> 
> This is the riscv variant of "x86/mm: try VMA lock-based page fault
> handling first".
>

How about add Link tag here:
Link: https://lwn.net/Articles/906852/

> Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
> ---
> Any performance numbers are welcome! Especially the numbers on HW
> platforms with 8 or more CPUs.
> 
>  arch/riscv/Kconfig    |  1 +
>  arch/riscv/mm/fault.c | 33 +++++++++++++++++++++++++++++++++
>  2 files changed, 34 insertions(+)
> 
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index 62e84fee2cfd..b958f67f9a12 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -42,6 +42,7 @@ config RISCV
>  	select ARCH_SUPPORTS_DEBUG_PAGEALLOC if MMU
>  	select ARCH_SUPPORTS_HUGETLBFS if MMU
>  	select ARCH_SUPPORTS_PAGE_TABLE_CHECK if MMU
> +	select ARCH_SUPPORTS_PER_VMA_LOCK if MMU
>  	select ARCH_USE_MEMTEST
>  	select ARCH_USE_QUEUED_RWLOCKS
>  	select ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT if MMU
> diff --git a/arch/riscv/mm/fault.c b/arch/riscv/mm/fault.c
> index 8685f85a7474..eccdddf26f4b 100644
> --- a/arch/riscv/mm/fault.c
> +++ b/arch/riscv/mm/fault.c
> @@ -286,6 +286,36 @@ void handle_page_fault(struct pt_regs *regs)
>  		flags |= FAULT_FLAG_WRITE;
>  	else if (cause == EXC_INST_PAGE_FAULT)
>  		flags |= FAULT_FLAG_INSTRUCTION;
> +#ifdef CONFIG_PER_VMA_LOCK
> +	if (!(flags & FAULT_FLAG_USER))
> +		goto lock_mmap;
> +
> +	vma = lock_vma_under_rcu(mm, addr);
> +	if (!vma)
> +		goto lock_mmap;
> +
> +	if (unlikely(access_error(cause, vma))) {
> +		vma_end_read(vma);
> +		goto lock_mmap;
> +	}
> +
> +	fault = handle_mm_fault(vma, addr, flags | FAULT_FLAG_VMA_LOCK, regs);
> +	vma_end_read(vma);
> +
> +	if (!(fault & VM_FAULT_RETRY)) {
> +		count_vm_vma_lock_event(VMA_LOCK_SUCCESS);
> +		goto done;
> +	}
> +	count_vm_vma_lock_event(VMA_LOCK_RETRY);
> +
> +	if (fault_signal_pending(fault, regs)) {
> +		if (!user_mode(regs))
> +			no_context(regs, addr);
> +		return;
> +	}
> +lock_mmap:
> +#endif /* CONFIG_PER_VMA_LOCK */
> +
>  retry:
>  	mmap_read_lock(mm);
>  	vma = find_vma(mm, addr);
> @@ -355,6 +385,9 @@ void handle_page_fault(struct pt_regs *regs)
>  
>  	mmap_read_unlock(mm);
>  
> +#ifdef CONFIG_PER_VMA_LOCK
> +done:
> +#endif
It's very close to cd7f176aea5f ("arm64/mm: try VMA lock-based page fault 
handling first"), and I didn't find any problem. So:

Reviewed-by: Guo Ren <guoren@kernel.org>

F.Y.I Huacai Chen, maybe he also would be interesting this new feature.


>  	if (unlikely(fault & VM_FAULT_ERROR)) {
>  		tsk->thread.bad_cause = cause;
>  		mm_fault_error(regs, addr, fault);
> -- 
> 2.40.1