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

[Dan Williams]

On Fri, May 1, 2020 at 10:51 AM David Hildenbrand <david@xxxxxxxxxx> 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@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.

...but that's not necessarily how dax/kmem works.

> >
>
> 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.