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

[Dan Williams]

On Fri, May 1, 2020 at 10:21 AM David Hildenbrand <david at redhat.com> wrote:
>
> On 01.05.20 18:56, Dan Williams wrote:
> > On Fri, May 1, 2020 at 2:34 AM David Hildenbrand <david at redhat.com> wrote:
> >>
> >> On 01.05.20 00:24, Andrew Morton wrote:
> >>> On Thu, 30 Apr 2020 20:43:39 +0200 David Hildenbrand <david at 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 at 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 at 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.