linux-kernel - Re: [PATCH v2 2/4] mm: rmap: Allow platforms without mm

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Date:   Mon, 11 Jul 2022 21:35:04 +0800
From:   Kefeng Wang <wangkefeng.wang@...wei.com>
To:     Barry Song <21cnbao@...il.com>, <akpm@...ux-foundation.org>,
        <linux-mm@...ck.org>, <linux-arm-kernel@...ts.infradead.org>,
        <x86@...nel.org>, <catalin.marinas@....com>, <will@...nel.org>,
        <linux-doc@...r.kernel.org>
CC:     <corbet@....net>, <arnd@...db.de>, <linux-kernel@...r.kernel.org>,
        <darren@...amperecomputing.com>, <yangyicong@...ilicon.com>,
        <huzhanyuan@...o.com>, <lipeifeng@...o.com>,
        <zhangshiming@...o.com>, <guojian@...o.com>, <realmz6@...il.com>,
        <linux-mips@...r.kernel.org>, <openrisc@...ts.librecores.org>,
        <linuxppc-dev@...ts.ozlabs.org>, <linux-riscv@...ts.infradead.org>,
        <linux-s390@...r.kernel.org>, Barry Song <v-songbaohua@...o.com>
Subject: Re: [PATCH v2 2/4] mm: rmap: Allow platforms without mm_cpumask to
 defer TLB flush

Hi Barry，

On 2022/7/11 11:46, Barry Song wrote:
> From: Barry Song <v-songbaohua@...o.com>
>
> Platforms like ARM64 have hareware TLB shootdown broadcast. They
> don't maintain mm_cpumask but just send tlbi and related sync
> instructions for TLB flush. task's mm_cpumask is normally empty
> in this case. We also allow deferred TLB flush on this kind of
> platforms.
>
> Signed-off-by: Barry Song <v-songbaohua@...o.com>>
> ---
...
> diff --git a/mm/Kconfig b/mm/Kconfig
> index 169e64192e48..7bf54f57ca01 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -951,6 +951,9 @@ config ARCH_HAS_CURRENT_STACK_POINTER
>   	  register alias named "current_stack_pointer", this config can be
>   	  selected.
>   
> +config ARCH_HAS_MM_CPUMASK
> +	bool
> +
>   config ARCH_HAS_VM_GET_PAGE_PROT
>   	bool
>   
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 5bcb334cd6f2..13d4f9a1d4f1 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -692,6 +692,10 @@ static bool should_defer_flush(struct mm_struct *mm, enum ttu_flags flags)
>   	if (!(flags & TTU_BATCH_FLUSH))
>   		return false;
>   
> +#ifndef CONFIG_ARCH_HAS_MM_CPUMASK
> +	return true;
> +#endif
> +

Here is another option to enable arch's tlbbatch defer

[1] 
https://patchwork.ozlabs.org/project/linuxppc-dev/patch/20171101101735.2318-2-khandual@linux.vnet.ibm.com/

>   	/* If remote CPUs need to be flushed then defer batch the flush */
>   	if (cpumask_any_but(mm_cpumask(mm), get_cpu()) < nr_cpu_ids)
>   		should_defer = true;