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Subject: Re: [RFC] arm64: mm: update max_pfn after memory hotplug
To:     Will Deacon <will@kernel.org>,
        Chris Goldsworthy <quic_cgoldswo@quicinc.com>
Cc:     Catalin Marinas <catalin.marinas@arm.com>,
        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>
 <20210929101028.GB21057@willie-the-truck>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
Message-ID: <13f56b37-afc7-bf6f-d544-8d6433588bf9@redhat.com>
Date:   Wed, 29 Sep 2021 12:29:32 +0200
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On 29.09.21 12:10, Will Deacon wrote:
> On Thu, Sep 23, 2021 at 03:54:48PM -0700, 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;
>> +	}
> 
> 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?

Only the write side is protected by memory hotplug locking. The read 
side is lockless -- just like all of the other pfn_to_online_page() 
machinery.

> 
> Will
> 


-- 
Thanks,

David / dhildenb