linux-kernel - Re: [PATCH v1 2/3] mm/memory_hotplug: Introduce MHP_DRIVER

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Date:   Thu, 30 Apr 2020 01:34:00 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>, virtio-dev@...ts.oasis-open.org,
        virtualization@...ts.linux-foundation.org,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Linux ACPI <linux-acpi@...r.kernel.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        linux-hyperv@...r.kernel.org,
        linux-s390 <linux-s390@...r.kernel.org>,
        xen-devel <xen-devel@...ts.xenproject.org>,
        Michal Hocko <mhocko@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        Michal Hocko <mhocko@...e.com>,
        Pankaj Gupta <pankaj.gupta.linux@...il.com>,
        Wei Yang <richard.weiyang@...il.com>,
        Baoquan He <bhe@...hat.com>,
        Eric Biederman <ebiederm@...ssion.com>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH v1 2/3] mm/memory_hotplug: Introduce MHP_DRIVER_MANAGED

On Thu, Apr 30, 2020 at 1:21 AM David Hildenbrand <david@...hat.com> wrote:
> >> Just because we decided to use some DAX memory in the current kernel as
> >> system ram, doesn't mean we should make that decision for the kexec
> >> kernel (e.g., using it as initial memory, placing kexec binaries onto
> >> it, etc.). This is also not what we would observe during a real reboot.
> >
> > Agree.
> >
> >> I can see that the "System RAM" resource will show up as child resource
> >> under the device e.g., in /proc/iomem.
> >>
> >> However, entries in /sys/firmware/memmap/ are created as "System RAM".
> >
> > True. Do you think this rename should just be limited to what type
> > /sys/firmware/memmap/ emits? I have the concern, but no proof
>
> We could split this patch into
>
> MHP_NO_FIRMWARE_MEMMAP (create firmware memmap entries)
>
> and
>
> MHP_DRIVER_MANAGED (name of the resource)
>
> See below, the latter might not be needed.
>
> > currently, that there are /proc/iomem walkers that explicitly look for
> > "System RAM", but might be thrown off by "System RAM (driver
> > managed)". I was not aware of /sys/firmware/memmap until about 5
> > minutes ago.
>
> The only two users of /proc/iomem I am aware of are kexec-tools and some
> s390x tools.
>
> kexec-tools on x86-64 uses /sys/firmware/memmap to craft the initial
> memmap, but uses /proc/iomem to
> a) Find places for kexec images
> b) Detect memory regions to dump via kdump
>
> I am not yet sure if we really need the "System RAM (driver managed)"
> part. If we can teach kexec-tools to
> a) Don't place kexec images on "System RAM" that has a parent resource
> (most likely requires kexec-tools changes)
> b) Consider for kdump "System RAM" that has a parent resource
> we might be able to avoid renaming that. (I assume that's already done)
>
> E.g., regarding virtio-mem (patch #3) I am currently also looking into
> creating a parent resource instead, like dax/kmem to avoid the rename:
>
> :/# cat /proc/iomem
> 00000000-00000fff : Reserved
> [...]
> 100000000-13fffffff : System RAM
> 140000000-33fffffff : virtio0
>   140000000-147ffffff : System RAM
>   148000000-14fffffff : System RAM
>   150000000-157ffffff : System RAM
> 340000000-303fffffff : virtio1
>   340000000-347ffffff : System RAM
> 3280000000-32ffffffff : PCI Bus 0000:00

Looks good to me if it flies with kexec-tools.