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

[David Hildenbrand <david@redhat.com>]

On 01.05.20 20:43, Dan Williams wrote:
> On Fri, May 1, 2020 at 11:14 AM David Hildenbrand <david@redhat.com> wrote:
>>
>> On 01.05.20 20:03, Dan Williams wrote:
>>> On Fri, May 1, 2020 at 10:51 AM David Hildenbrand <david@redhat.com> wrote:
>>>>
>>>> On 01.05.20 19:45, David Hildenbrand wrote:
>>>>> On 01.05.20 19:39, Dan Williams wrote:
>>>>>> On Fri, May 1, 2020 at 10:21 AM David Hildenbrand <david@redhat.com> wrote:
>>>>>>>
>>>>>>> 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.
>>>>>>
>>>>>> 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.
>>>
>>> ...but that's not necessarily how dax/kmem works.
>>>
>>
>> Yes, and user space could still take that memory and add it to the
>> firmware memmap if it really wants to. It knows that it is special. It
>> can figure out that it belongs to a dax device using /proc/iomem.
>>
>>>>>
>>>>
>>>> Oh, and I don't see why "System RAM (driver managed)" would hinder any
>>>> policy in user case to still do what it thinks is the right thing to do
>>>> (e.g., for dax).
>>>>
>>>> "System RAM (driver managed)" would mean: Memory is not part of the raw
>>>> firmware memmap. It was detected and added by a driver. Handle with
>>>> care, this is special.
>>>
>>> Oh, no, I was more reacting to your, "don't update
>>> /sys/firmware/memmap for the (driver managed) range" choice as being a
>>> policy decision. It otherwise feels to me "System RAM (driver
>>> managed)" adds confusion for casual users of /proc/iomem and for clued
>>> in tools they have the parent association to decide policy.
>>
>> Not sure if I understand correctly, so bear with me :).
>>
>> Adding or not adding stuff to /sys/firmware/memmap is not a policy
>> decision. If it's not part of the raw firmware-provided memmap, it has
>> nothing to do in /sys/firmware/memmap. That's what the documentation
>> from 2008 tells us.
> 
> It just occurs to me that there are valid cases for both wanting to
> start over with driver managed memory with a kexec and keeping it in
> the map.

Yes, there might be valid cases. My gut feeling is that in the general
case, you want to let the kexec kernel implement a policy/ let the user
in the new system decide.

But as I said, you can implement in kexec-tools whatever policy you
want. It has access to all information.

> Consider the case of EFI Special Purpose (SP) Memory that is
> marked EFI Conventional Memory with the SP attribute. In that case the
> firmware memory map marked it as conventional RAM, but the kernel
> optionally marks it as System RAM vs Soft Reserved. The 2008 patch
> simply does not consider that case. I'm not sure strict textualism
> works for coding decisions.

I am no expert on that matter (esp EFI). But looking at the users of
firmware_map_add_early(), the single user is in arch/x86/kernel/e820.c
. So the single source of /sys/firmware/memmap is (besides hotplug) e820.

"'e820_table_firmware': the original firmware version passed to us by
the bootloader - not modified by the kernel. ... inform the user about
the firmware's notion of memory layout via /sys/firmware/memmap"
(arch/x86/kernel/e820.c)

How is the EFI Special Purpose (SP) Memory represented in e820?

/sys/firmware/memmap is really simple: just dump in e820. No policies IIUC.

-- 
Thanks,

David / dhildenb