Re: [PATCH v2 2/3] mm/memory_hotplug: Introduce MHP_NO_FIRMWARE_MEMMAP

[David Hildenbrand <david@redhat.com>]

On 01.05.20 18:56, Dan Williams wrote:
> On Fri, May 1, 2020 at 2:34 AM David Hildenbrand <david@redhat.com> wrote:
>>
>> On 01.05.20 00:24, Andrew Morton wrote:
>>> On Thu, 30 Apr 2020 20:43:39 +0200 David Hildenbrand <david@redhat.com> wrote:
>>>
>>>>>
>>>>> Why does the firmware map support hotplug entries?
>>>>
>>>> I assume:
>>>>
>>>> The firmware memmap was added primarily for x86-64 kexec (and still, is
>>>> mostly used on x86-64 only IIRC). There, we had ACPI hotplug. When DIMMs
>>>> get hotplugged on real HW, they get added to e820. Same applies to
>>>> memory added via HyperV balloon (unless memory is unplugged via
>>>> ballooning and you reboot ... the the e820 is changed as well). I assume
>>>> we wanted to be able to reflect that, to make kexec look like a real reboot.
>>>>
>>>> This worked for a while. Then came dax/kmem. Now comes virtio-mem.
>>>>
>>>>
>>>> But I assume only Andrew can enlighten us.
>>>>
>>>> @Andrew, any guidance here? Should we really add all memory to the
>>>> firmware memmap, even if this contradicts with the existing
>>>> documentation? (especially, if the actual firmware memmap will *not*
>>>> contain that memory after a reboot)
>>>
>>> For some reason that patch is misattributed - it was authored by
>>> Shaohui Zheng <shaohui.zheng@intel.com>, who hasn't been heard from in
>>> a decade.  I looked through the email discussion from that time and I'm
>>> not seeing anything useful.  But I wasn't able to locate Dave Hansen's
>>> review comments.
>>
>> Okay, thanks for checking. I think the documentation from 2008 is pretty
>> clear what has to be done here. I will add some of these details to the
>> patch description.
>>
>> Also, now that I know that esp. kexec-tools already don't consider
>> dax/kmem memory properly (memory will not get dumped via kdump) and
>> won't really suffer from a name change in /proc/iomem, I will go back to
>> the MHP_DRIVER_MANAGED approach and
>> 1. Don't create firmware memmap entries
>> 2. Name the resource "System RAM (driver managed)"
>> 3. Flag the resource via something like IORESOURCE_MEM_DRIVER_MANAGED.
>>
>> This way, kernel users and user space can figure out that this memory
>> has different semantics and handle it accordingly - I think that was
>> what Eric was asking for.
>>
>> Of course, open for suggestions.
> 
> I'm still more of a fan of this being communicated by "System RAM"

I was mentioning somewhere in this thread that "System RAM" inside a
hierarchy (like dax/kmem) will already be basically ignored by
kexec-tools. So, placing it inside a hierarchy already makes it look
special already.

But after all, as we have to change kexec-tools either way, we can
directly go ahead and flag it properly as special (in case there will
ever be other cases where we could no longer distinguish it).

> being parented especially because that tells you something about how
> the memory is driver-managed and which mechanism might be in play.

The could be communicated to some degree via the resource hierarchy.

E.g.,

            [root@localhost ~]# cat /proc/iomem
            ...
            140000000-33fffffff : Persistent Memory
              140000000-1481fffff : namespace0.0
              150000000-33fffffff : dax0.0
                150000000-33fffffff : System RAM (driver managed)

vs.

           :/# cat /proc/iomem
            [...]
            140000000-333ffffff : virtio-mem (virtio0)
              140000000-147ffffff : System RAM (driver managed)
              148000000-14fffffff : System RAM (driver managed)
              150000000-157ffffff : System RAM (driver managed)

Good enough for my taste.

> What about adding an optional /sys/firmware/memmap/X/parent attribute.

I really don't want any firmware memmap entries for something that is
not part of the firmware provided memmap. In addition,
/sys/firmware/memmap/ is still a fairly x86_64 specific thing. Only mips
and two arm configs enable it at all.

So, IMHO, /sys/firmware/memmap/ is definitely not the way to go.

-- 
Thanks,

David / dhildenb