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Subject: Re: [PATCH v2 1/3] arm64/numa: export memory_add_physaddr_to_nid as
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To:     Dan Williams <dan.j.williams@intel.com>
Cc:     Mike Rapoport <rppt@linux.ibm.com>,
        Mike Rapoport <rppt@kernel.org>,
        Michal Hocko <mhocko@kernel.org>, Jia He <justin.he@arm.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Vishal Verma <vishal.l.verma@intel.com>,
        Dave Jiang <dave.jiang@intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Baoquan He <bhe@redhat.com>,
        Chuhong Yuan <hslester96@gmail.com>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux MM <linux-mm@kvack.org>,
        linux-nvdimm <linux-nvdimm@lists.01.org>,
        Kaly Xin <Kaly.Xin@arm.com>
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From:   David Hildenbrand <david@redhat.com>
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Date:   Wed, 8 Jul 2020 18:10:19 +0200
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On 08.07.20 17:50, Dan Williams wrote:
> On Wed, Jul 8, 2020 at 3:04 AM David Hildenbrand <david@redhat.com> wrote:
>>
>> 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 :)
> 
> I'm not quite understanding the concern, or requirement about
> "updating memblock" in the hotplug path. The routines
> memory_add_physaddr_to_nid() and phys_to_target_node() are helpers to
> interrogate platform-firmware numa info through a common abstraction.
> They place no burden on the memory hotplug code they're just used to
> see if a hot-added range lies within an existing node span when
> platform-firmware otherwise fails to communicate a node. x86 can
> continue to back those helpers with numa_meminfo, arm64 can use a
> generic memblock implementation and other archs can follow the arm64
> example if they want better numa answers for drivers.
> 

See memblock_add_node()/memblock_remove() in mm/memory_hotplug.c. I
don't want that code be reactivated for x86/s390x. That's all I am saying.

-- 
Thanks,

David / dhildenb