From: Dan Williams <dan.j.williams@intel.com>
To: David Hildenbrand <david@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
"Eric W. Biederman" <ebiederm@xmission.com>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Linux MM <linux-mm@kvack.org>,
virtio-dev@lists.oasis-open.org,
virtualization@lists.linux-foundation.org,
linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
Linux ACPI <linux-acpi@vger.kernel.org>,
linux-nvdimm <linux-nvdimm@lists.01.org>,
linux-hyperv@vger.kernel.org,
linux-s390 <linux-s390@vger.kernel.org>,
xen-devel <xen-devel@lists.xenproject.org>,
Michal Hocko <mhocko@kernel.org>,
"Michael S . Tsirkin" <mst@redhat.com>,
Michal Hocko <mhocko@suse.com>,
Pankaj Gupta <pankaj.gupta.linux@gmail.com>,
Wei Yang <richard.weiyang@gmail.com>, Baoquan He <bhe@redhat.com>
Subject: Re: [PATCH v2 2/3] mm/memory_hotplug: Introduce MHP_NO_FIRMWARE_MEMMAP
Date: Fri, 1 May 2020 10:39:28 -0700 [thread overview]
Message-ID: <CAPcyv4iOqS0Wbfa2KPfE1axQFGXoRB4mmPRP__Lmqpw6Qpr_ig@mail.gmail.com> (raw)
In-Reply-To: <2d019c11-a478-9d70-abd5-4fd2ebf4bc1d@redhat.com>
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.
next prev parent reply other threads:[~2020-05-01 17:39 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-04-30 10:29 [PATCH v2 0/3] mm/memory_hotplug: Allow to not create firmware memmap entries David Hildenbrand
2020-04-30 10:29 ` [PATCH v2 1/3] mm/memory_hotplug: Prepare passing flags to add_memory() and friends David Hildenbrand
2020-04-30 10:29 ` [PATCH v2 2/3] mm/memory_hotplug: Introduce MHP_NO_FIRMWARE_MEMMAP David Hildenbrand
2020-04-30 15:38 ` Eric W. Biederman
2020-04-30 15:52 ` David Hildenbrand
2020-04-30 16:04 ` Dave Hansen
2020-04-30 16:33 ` Eric W. Biederman
2020-04-30 16:49 ` David Hildenbrand
2020-04-30 18:06 ` Eric W. Biederman
2020-04-30 18:43 ` David Hildenbrand
2020-04-30 18:58 ` Dan Williams
2020-04-30 22:24 ` Andrew Morton
2020-05-01 9:34 ` David Hildenbrand
2020-05-01 16:56 ` Dan Williams
2020-05-01 17:21 ` David Hildenbrand
2020-05-01 17:39 ` Dan Williams [this message]
2020-05-01 17:45 ` David Hildenbrand
2020-05-01 17:51 ` David Hildenbrand
2020-05-01 18:03 ` Dan Williams
2020-05-01 18:14 ` David Hildenbrand
2020-05-01 18:43 ` Dan Williams
2020-05-01 19:17 ` David Hildenbrand
2020-05-01 20:12 ` Dan Williams
2020-05-01 21:10 ` David Hildenbrand
2020-05-01 21:52 ` Dan Williams
2020-05-02 9:26 ` David Hildenbrand
2020-05-02 18:03 ` Dan Williams
2020-04-30 10:29 ` [PATCH v2 3/3] device-dax: Add system ram (add_memory()) with MHP_NO_FIRMWARE_MEMMAP David Hildenbrand
2020-04-30 11:23 ` Dave Hansen
2020-04-30 15:28 ` David Hildenbrand
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