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 10:45:17 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Mike Rapoport <rppt@...ux.ibm.com>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        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 10:39, Mike Rapoport wrote:
> On Wed, Jul 08, 2020 at 10:26:41AM +0200, David Hildenbrand wrote:
>> On 08.07.20 09:50, Dan Williams wrote:
>>> On Wed, Jul 8, 2020 at 12:22 AM David Hildenbrand <david@...hat.com> 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.
> 
>> I'd be curious if what we are trying to optimize here is actually worth
>> optimizing. IOW, is there a well-known scenario where the dummy value on
>> arm64 would be problematic and is worth the effort?
> 
> Well, it started with Michal's comment above that EXPORT_SYMBOL_GPL()
> for a stub might be an overkill.
> 
> I think Jia's suggestion [1] with addition of a comment that explains
> why and when the stub will be used, can work for both
> memory_add_physaddr_to_nid() and phys_to_target_node().

Agreed.

> 
> 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).

-- 
Thanks,

David / dhildenb