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

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Message-ID: <b0717829-c8dd-c7c0-47bb-5392ed22f700@redhat.com>
Date:   Wed, 29 Sep 2021 12:49:58 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Will Deacon <will@...nel.org>
Cc:     Chris Goldsworthy <quic_cgoldswo@...cinc.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        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 29.09.21 12:42, Will Deacon wrote:
> On Wed, Sep 29, 2021 at 12:29:32PM +0200, David Hildenbrand wrote:
>> 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@...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?
>>
>> 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.
> 
> Hmm. So the readers can see one of the variables updated but the other one
> stale?

Yes, just like it has been on x86-64 for a long time:

arch/x86/mm/init_64.c:update_end_of_memory_vars()

Not sure if anyone really cares about slightly delayed updates while 
memory is getting hotplugged. The users that I am aware of don't care.

-- 
Thanks,

David / dhildenb