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

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Date:   Wed, 29 Sep 2021 14:09:35 +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 13:03, Will Deacon wrote:
> On Wed, Sep 29, 2021 at 12:49:58PM +0200, David Hildenbrand wrote:
>> 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, I'd missed that x86 also updates max_low_pfn. So at least we're not
> worse off in that respect.
> 
> Looking at set_max_mapnr(), I'm wondering why we need to call that at all
> on arm64 as 'max_mapnr' only seems to be used for nommu.

I think max_mapnr is only helpful without SPARSE, I can spot the most 
prominent consumer being simplistic pfn_valid() implementation.

MEMORY_HOTPLUG on arm64 implies SPARSE. ... and I recall that FLATMEM is 
no longer possible on arm64. So most probably the arm64 call of 
set_max_mapnr() can just be dropped.

-- 
Thanks,

David / dhildenb