linux-kernel - Re: [PATCH v2 1/3] arm64/numa: export memory_add_physaddr_to_nid as EXPORT_SYMBOL

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Date:   Wed, 8 Jul 2020 09:21:25 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Mike Rapoport <rppt@...ux.ibm.com>,
        Dan Williams <dan.j.williams@...el.com>
Cc:     Michal Hocko <mhocko@...nel.org>, Jia He <justin.he@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Baoquan He <bhe@...hat.com>,
        Chuhong Yuan <hslester96@...il.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Kaly Xin <Kaly.Xin@....com>
Subject: Re: [PATCH v2 1/3] arm64/numa: export memory_add_physaddr_to_nid as
 EXPORT_SYMBOL_GPL

On 08.07.20 07:27, Mike Rapoport wrote:
> On Tue, Jul 07, 2020 at 03:05:48PM -0700, Dan Williams wrote:
>> On Tue, Jul 7, 2020 at 11:01 AM Mike Rapoport <rppt@...ux.ibm.com> wrote:
>>>
>>> On Tue, Jul 07, 2020 at 02:26:08PM +0200, David Hildenbrand wrote:
>>>> On 07.07.20 14:13, Mike Rapoport wrote:
>>>>> On Tue, Jul 07, 2020 at 01:54:54PM +0200, Michal Hocko wrote:
>>>>>> On Tue 07-07-20 13:59:15, Jia He wrote:
>>>>>>> This exports memory_add_physaddr_to_nid() for module driver to use.
>>>>>>>
>>>>>>> memory_add_physaddr_to_nid() is a fallback option to get the nid in case
>>>>>>> NUMA_NO_NID is detected.
>>>>>>>
>>>>>>> Suggested-by: David Hildenbrand <david@...hat.com>
>>>>>>> Signed-off-by: Jia He <justin.he@....com>
>>>>>>> ---
>>>>>>>  arch/arm64/mm/numa.c | 5 +++--
>>>>>>>  1 file changed, 3 insertions(+), 2 deletions(-)
>>>>>>>
>>>>>>> diff --git a/arch/arm64/mm/numa.c b/arch/arm64/mm/numa.c
>>>>>>> index aafcee3e3f7e..7eeb31740248 100644
>>>>>>> --- a/arch/arm64/mm/numa.c
>>>>>>> +++ b/arch/arm64/mm/numa.c
>>>>>>> @@ -464,10 +464,11 @@ void __init arm64_numa_init(void)
>>>>>>>
>>>>>>>  /*
>>>>>>>   * We hope that we will be hotplugging memory on nodes we already know about,
>>>>>>> - * such that acpi_get_node() succeeds and we never fall back to this...
>>>>>>> + * such that acpi_get_node() succeeds. But when SRAT is not present, the node
>>>>>>> + * id may be probed as NUMA_NO_NODE by acpi, Here provide a fallback option.
>>>>>>>   */
>>>>>>>  int memory_add_physaddr_to_nid(u64 addr)
>>>>>>>  {
>>>>>>> - pr_warn("Unknown node for memory at 0x%llx, assuming node 0\n", addr);
>>>>>>>   return 0;
>>>>>>>  }
>>>>>>> +EXPORT_SYMBOL_GPL(memory_add_physaddr_to_nid);
>>>>>>
>>>>>> Does it make sense to export a noop function? Wouldn't make more sense
>>>>>> to simply make it static inline somewhere in a header? I haven't checked
>>>>>> whether there is an easy way to do that sanely bu this just hit my eyes.
>>>>>
>>>>> We'll need to either add a CONFIG_ option or arch specific callback to
>>>>> make both non-empty (x86, powerpc, ia64) and empty (arm64, sh)
>>>>> implementations coexist ...
>>>>
>>>> Note: I have a similar dummy (return 0) patch for s390x lying around here.
>>>
>>> Then we'll call it a tie - 3:3 ;-)
>>
>> So I'd be happy to jump on the train of people wanting to export the
>> ARM stub for this (and add a new ARM stub for phys_to_target_node()),
>> but Will did have a plausibly better idea that I have been meaning to
>> circle back to:
>>
>> http://lore.kernel.org/r/20200325111039.GA32109@willie-the-truck
>>
>> ...i.e. iterate over node data to do the lookup. This would seem to
>> work generically for multiple archs unless I am missing something?

IIRC, only memory assigned to/onlined to a ZONE is represented in the
pgdat node span. E.g., not offline memory blocks.

Esp., when hotplugging + onlining consecutive memory, there won't really
be any intersections in most cases if I am not wrong. It would not be
"intersection" but rather "closest fit".

With overlapping nodes it's even more unclear. Which one to pick?

> 
> I think it would work on arm64, power and, most propbably on s390

With only a single dummy node I guess it should work (searching when
there is only a single node does not make too much sense).

> (David?), but not on x86. x86 does not have reserved memory in pgdat,
> it's never memblock_add()'ed (see e820__memblock_setup()).

Can you enlighten me why that is relevant for the memory hotplug path?
(or is it just a general comment to make the function as accurate as
possible for all addresses?)

-- 
Thanks,

David / dhildenb