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

From: Dan Williams <dan.j.williams@intel.com>
To: David Hildenbrand <david@redhat.com>
Cc: Mike Rapoport <rppt@linux.ibm.com>,
	Michal Hocko <mhocko@kernel.org>, Jia He <justin.he@arm.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	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>
Subject: Re: [PATCH v2 1/3] arm64/numa: export memory_add_physaddr_to_nid as EXPORT_SYMBOL_GPL
Date: Wed, 8 Jul 2020 00:50:50 -0700	[thread overview]
Message-ID: <CAPcyv4jyk_tkDRewTVvRAv0g4LwemEyKYQyuJBXkF4VuYrBdrw@mail.gmail.com> (raw)
In-Reply-To: <9a009cf6-6c30-91ca-a1a5-9aa090c66631@redhat.com>

On Wed, Jul 8, 2020 at 12:22 AM David Hildenbrand <david@redhat.com> wrote:
>
> 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@linux.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@redhat.com>
> >>>>>>> Signed-off-by: Jia He <justin.he@arm.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.

So this dovetails somewhat with Will's idea. What if we populated
node_data for "offline" ranges? I started there, but then saw
ARCH_KEEP_MEMBLOCK and thought it would be safer to just teach
phys_to_target_node() to use that rather than update other code paths
to expect node_data might not always reflect online data.

> 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?

In the overlap case you get what you get. Some signal is better than
the noise of a dummy function. The consequences of picking the wrong
node might be that the kernel can't properly associate a memory range
to its performance data tables in firmware, but then again firmware
messed up with an overlapping node definition in the first instance.
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