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Subject: Re: [PATCH] arm64: mm: update max_pfn after memory hotplug
To:     Anshuman Khandual <anshuman.khandual@arm.com>,
        Chris Goldsworthy <quic_cgoldswo@quicinc.com>,
        Andrew Morton <akpm@linux-foundation.org>
Cc:     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>
 <f0ea994c-b3a0-1577-85e3-0620ac5a0de7@arm.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
Message-ID: <c3d5b781-c02e-1126-01d5-6c54db320233@redhat.com>
Date:   Fri, 1 Oct 2021 10:39:48 +0200
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On 01.10.21 08:38, Anshuman Khandual wrote:
> 
> 
> 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.

s390x sets them to the possible maximum (based on the direct mapping 
size) in init code.

Other archs like x86-64 have to update other parameters. So I guess it 
just made sense to let the archs handle updating these 2 variables.

-- 
Thanks,

David / dhildenb