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Subject: Re: [RFC] arm64: mm: update max_pfn after memory hotplug
To:     Chris Goldsworthy <quic_cgoldswo@quicinc.com>,
        Georgi Djakov <quic_c_gdjako@quicinc.com>
Cc:     Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org,
        Sudarshan Rajagopalan <quic_sudaraja@quicinc.com>
References: <cover.1632437225.git.quic_cgoldswo@quicinc.com>
 <595d09279824faf1f54961cef52b745609b05d97.1632437225.git.quic_cgoldswo@quicinc.com>
 <833493d2-d585-47ee-c258-79eae5deff36@redhat.com>
 <ed2f59dc-ecd3-ae9c-e44c-81e903c6f08f@quicinc.com>
 <20210928061016.GA28837@hu-cgoldswo-sd.qualcomm.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
Message-ID: <94d477d0-5d05-b2c3-850e-0a749c888f3b@redhat.com>
Date:   Tue, 28 Sep 2021 09:33:38 +0200
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On 28.09.21 08:12, Chris Goldsworthy wrote:
> On Tue, Sep 28, 2021 at 02:22:59AM +0300, Georgi Djakov wrote:
>> On 9/27/2021 6:51 PM, David Hildenbrand wrote:
>>> On 24.09.21 00:54, 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.
>>>>
>>>> Signed-off-by: Sudarshan Rajagopalan <quic_sudaraja@quicinc.com>
>>>> Signed-off-by: Chris Goldsworthy <quic_cgoldswo@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;
>>>> +    }
>>>> +
>>>>        return ret;
>>>
>>> Note: didn't verify if updating max_low_pfn is correct here.
>>
>> My understanding is that max_low_pfn defines the low/high memory
>> boundary and it should be also updated.
>>
>> Thanks,
>> Georgi
> 
> To build more on Georgi's response, our assumption here after an offline
> discussion is that max_low_pfn would not be equal to max_pfn only if there is
> high memory - another assumption is that arm64 kernels will not need high memory
> due to their large logical mappings. Under these two assumptions, the patch is
> correct. Perhaps Catalin can ack or critique this, as he initially set max_pfn =
> max_low_pfn in the first arm64 mm initialization code:

Makes sense to me, thanks.


-- 
Thanks,

David / dhildenb