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

Dan Williams
Fri, 01 May 2020 20:12:06 -0000

On Fri, May 1, 2020 at 12:18 PM David Hildenbrand <david at redhat.com>
wrote:
>
> On 01.05.20 20:43, Dan Williams wrote:
> > On Fri, May 1, 2020 at 11:14 AM David Hildenbrand <david at
redhat.com> wrote:
> >>
> >> On 01.05.20 20:03, Dan Williams wrote:
> >>> On Fri, May 1, 2020 at 10:51 AM David Hildenbrand <david at
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 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.
> >>>>>
> >>>>> 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.

Right, so why is a new type needed if all the information is there by
other means?

> > 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.

e820 now has a Soft Reserved translation for this which means "try to
reserve, but treat as System RAM is ok too". It seems generically
useful to me that the toggle for determining whether Soft Reserved or
System RAM shows up /sys/firmware/memmap is a determination that
policy can make. The kernel need not preemptively block it.