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

[David Hildenbrand]

On 01.05.20 19:39, Dan Williams wrote:
> On Fri, May 1, 2020 at 10:21 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
>>
>> On 01.05.20 18:56, Dan Williams wrote:
>>> On Fri, May 1, 2020 at 2:34 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
>>>>
>>>> On 01.05.20 00:24, Andrew Morton wrote:
>>>>> On Thu, 30 Apr 2020 20:43:39 +0200 David Hildenbrand <david@xxxxxxxxxx> 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@xxxxxxxxx>, 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.
> 
> I think that's a policy decision and policy decisions do not belong in
> the kernel. Give the tooling the opportunity to decide whether System
> RAM stays that way over a kexec. The parenthetical reference otherwise
> looks out of place to me in the /proc/iomem output. What makes it
> "driver managed" is how the kernel handles it, not how the kernel
> names it.

At least, virtio-mem is different. It really *has to be handled* by the
driver. This is not a policy. It's how it works.

-- 
Thanks,

David / dhildenb