Re: [RFC] arm64: mm: update max_pfn after memory hotplug

From: David Hildenbrand <david@redhat.com>
To: Will Deacon <will@kernel.org>
Cc: Chris Goldsworthy <quic_cgoldswo@quicinc.com>,
	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>
Subject: Re: [RFC] arm64: mm: update max_pfn after memory hotplug
Date: Wed, 29 Sep 2021 14:09:35 +0200	[thread overview]
Message-ID: <130a50d7-92fd-31fa-261e-f73dadcb4fcf@redhat.com> (raw)
In-Reply-To: <20210929110339.GA21510@willie-the-truck>

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@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.
>>>
>>> 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