Re: [PATCH v2 1/3] arm64/numa: export memory_add_physaddr_to_nid as EXPORT_SYMBOL_GPL

From: David Hildenbrand <david@redhat.com>
To: Mike Rapoport <rppt@linux.ibm.com>
Cc: Kaly Xin <Kaly.Xin@arm.com>, Jia He <justin.he@arm.com>,
	Baoquan He <bhe@redhat.com>,
	linux-nvdimm <linux-nvdimm@lists.01.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Chuhong Yuan <hslester96@gmail.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Michal Hocko <mhocko@kernel.org>, Linux MM <linux-mm@kvack.org>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	Vishal Verma <vishal.l.verma@intel.com>,
	Dave Jiang <dave.jiang@intel.com>,
	Dan Williams <dan.j.williams@intel.com>,
	Will Deacon <will@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Mike Rapoport <rppt@kernel.org>
Subject: Re: [PATCH v2 1/3] arm64/numa: export memory_add_physaddr_to_nid as EXPORT_SYMBOL_GPL
Date: Wed, 8 Jul 2020 12:04:21 +0200	[thread overview]
Message-ID: <98166184-3aaf-479e-bfb3-fc737f4ac98d@redhat.com> (raw)
In-Reply-To: <20200708094549.GA781326@linux.ibm.com>

On 08.07.20 11:45, Mike Rapoport wrote:
> On Wed, Jul 08, 2020 at 11:25:36AM +0200, David Hildenbrand wrote:
>> On 08.07.20 11:15, Mike Rapoport wrote:
>>>>>>>>
>>>>> But on more theoretical/fundmanetal level, I think we lack a generic
>>>>> abstraction similar to e.g. x86 'struct numa_meminfo' that serves as
>>>>> translaton of firmware supplied information into data that can be used
>>>>> by the generic mm without need to reimplement it for each and every
>>>>> arch.
>>>>
>>>> Right. As I expressed, I am not a friend of using memblock for that, and
>>>> the pgdat node span is tricky.
>>>>
>>>> Maybe abstracting that x86 concept is possible in some way (and we could
>>>> restrict the information to boot-time properties, so we don't have to
>>>> mess with memory hot(un)plug - just as done for numa_meminfo AFAIKS).
>>>
>>> I agree with pgdat part and disagree about memblock. It already has
>>> non-init physmap, why won't we add memblock.memory to the mix? ;-)
>>
>> Can we generalize and tweak physmap to contain node info? That's all we
>> need, no? (the special mem= parameter handling should not matter for our
>> use case, where "physmap" and "memory" would differ)
> 
> TBH, I have only random vague thoughts at the moment. This might be an
> option. But then we need to enable physmap on !s390, right?

Yes, looks like it.

>  
>>> Now, seriously, memblock already has all the necessary information about
>>> the coldplug memory for several architectures. x86 being an exception
>>> because for some reason the reserved memory is not considered memory
>>> there. The infrastructure for quiering and iterating memory regions is
>>> already there. We just need to leave out the irrelevant parts, like
>>> memblock.reserved and allocation funcions.
>>
>> I *really* don't want to mess with memblocks on memory hot(un)plug on
>> x86 and s390x (+other architectures in the future). I also thought about
>> stopping to create memblocks for hotplugged memory on arm64, by tweaking
>> pfn_valid() to query memblocks only for early sections.
>>
>> If "physmem" is not an option, can we at least introduce something like
>> ARCH_UPDTAE_MEMBLOCK_ON_HOTPLUG to avoid doing that on x86 and s390x for
>> now (and later maybe for others)?
> 
> I have to do more memory hotplug howework to answer that ;-)
> 
> My general point is that we don't have to reinvent the wheel to have
> coldplug memory representation, it's already there. We just need a way
> to use it properly.

Yes, I tend to agree. Details to be clarified :)

-- 
Thanks,

David / dhildenb

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