linux-kernel - Re: [RFC] arm64: mm: update max

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Message-ID: <20210929101028.GB21057@willie-the-truck>
Date:   Wed, 29 Sep 2021 11:10:29 +0100
From:   Will Deacon <will@...nel.org>
To:     Chris Goldsworthy <quic_cgoldswo@...cinc.com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org,
        Sudarshan Rajagopalan <quic_sudaraja@...cinc.com>
Subject: Re: [RFC] arm64: mm: update max_pfn after memory hotplug

On Thu, Sep 23, 2021 at 03:54:48PM -0700, Chris Goldsworthy wrote:
> From: Sudarshan Rajagopalan <quic_sudaraja@...cinc.com>
> 
> After new memory blocks have been hotplugged, max_pfn and max_low_pfn
> needs updating to reflect on new PFNs being hot added to system.
> 
> Signed-off-by: Sudarshan Rajagopalan <quic_sudaraja@...cinc.com>
> Signed-off-by: Chris Goldsworthy <quic_cgoldswo@...cinc.com>
> ---
>  arch/arm64/mm/mmu.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index cfd9deb..fd85b51 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -1499,6 +1499,11 @@ int arch_add_memory(int nid, u64 start, u64 size,
>  	if (ret)
>  		__remove_pgd_mapping(swapper_pg_dir,
>  				     __phys_to_virt(start), size);
> +	else {
> +		max_pfn = PFN_UP(start + size);
> +		max_low_pfn = max_pfn;
> +	}

We use 'max_pfn' as part of the argument to set_max_mapnr(). Does that need
updating as well?

Do we have sufficient locking to ensure nobody is looking at max_pfn or
max_low_pfn while we update them?

Will