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Subject: Re: [PATCH] arm64: mm: update max_pfn after memory hotplug
To:     Chris Goldsworthy <quic_cgoldswo@quicinc.com>,
        Andrew Morton <akpm@linux-foundation.org>
Cc:     David Hildenbrand <david@redhat.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Sudarshan Rajagopalan <quic_sudaraja@quicinc.com>,
        Georgi Djakov <quic_c_gdjako@quicinc.com>,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, Florian Fainelli <f.fainelli@gmail.com>
References: <cover.1632853776.git.quic_cgoldswo@quicinc.com>
 <a51a27ee7be66024b5ce626310d673f24107bcb8.1632853776.git.quic_cgoldswo@quicinc.com>
From:   Anshuman Khandual <anshuman.khandual@arm.com>
Message-ID: <f0ea994c-b3a0-1577-85e3-0620ac5a0de7@arm.com>
Date:   Fri, 1 Oct 2021 12:08:05 +0530
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On 9/29/21 12:21 AM, Chris Goldsworthy wrote:
> From: Sudarshan Rajagopalan <quic_sudaraja@quicinc.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.
> Without this patch, debug-related functions that use max_pfn such as
> get_max_dump_pfn() or read_page_owner() will not work with any page in
> memory that is hot-added after boot. 
> 
> Fixes: 4ab215061554 ("arm64: Add memory hotplug support")
> Signed-off-by: Sudarshan Rajagopalan <quic_sudaraja@quicinc.com>
> Signed-off-by: Chris Goldsworthy <quic_cgoldswo@quicinc.com>
> Acked-by: David Hildenbrand <david@redhat.com>
> Cc: Florian Fainelli <f.fainelli@gmail.com>
> Cc: Georgi Djakov <quic_c_gdjako@quicinc.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;
> +	}

Do these variables get updated on *all* platforms that support memory
hotplug via an arch_add_memory() ? If not, dont they all face similar
issues as well ? Why not just update these in generic hotplug instead
, after looking into arch_add_memory() return code.

> +
>  	return ret;
>  }
>  
>