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Subject: Re: [PATCH 3/3] memory: introduce an option to force onlining of
 hotplug memory
To:     David Hildenbrand <david@redhat.com>,
        =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= <roger.pau@citrix.com>
Cc:     linux-kernel@vger.kernel.org,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Stefano Stabellini <sstabellini@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        xen-devel@lists.xenproject.org, linux-mm@kvack.org
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 <429c2889-93c2-23b3-ba1e-da56e3a76ba4@redhat.com>
From:   =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= <jgross@suse.com>
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Date:   Thu, 23 Jul 2020 15:53:09 +0200
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On 23.07.20 15:47, David Hildenbrand wrote:
> On 23.07.20 15:22, David Hildenbrand wrote:
>> On 23.07.20 14:23, Roger Pau Monné wrote:
>>> On Thu, Jul 23, 2020 at 01:37:03PM +0200, David Hildenbrand wrote:
>>>> On 23.07.20 10:45, Roger Pau Monne wrote:
>>>>> Add an extra option to add_memory_resource that overrides the memory
>>>>> hotplug online behavior in order to force onlining of memory from
>>>>> add_memory_resource unconditionally.
>>>>>
>>>>> This is required for the Xen balloon driver, that must run the
>>>>> online page callback in order to correctly process the newly added
>>>>> memory region, note this is an unpopulated region that is used by Linux
>>>>> to either hotplug RAM or to map foreign pages from other domains, and
>>>>> hence memory hotplug when running on Xen can be used even without the
>>>>> user explicitly requesting it, as part of the normal operations of the
>>>>> OS when attempting to map memory from a different domain.
>>>>>
>>>>> Setting a different default value of memhp_default_online_type when
>>>>> attaching the balloon driver is not a robust solution, as the user (or
>>>>> distro init scripts) could still change it and thus break the Xen
>>>>> balloon driver.
>>>>
>>>> I think we discussed this a couple of times before (even triggered by my
>>>> request), and this is responsibility of user space to configure. Usually
>>>> distros have udev rules to online memory automatically. Especially, user
>>>> space should eb able to configure *how* to online memory.
>>>
>>> Note (as per the commit message) that in the specific case I'm
>>> referring to the memory hotplugged by the Xen balloon driver will be
>>> an unpopulated range to be used internally by certain Xen subsystems,
>>> like the xen-blkback or the privcmd drivers. The addition of such
>>> blocks of (unpopulated) memory can happen without the user explicitly
>>> requesting it, and hence not even aware such hotplug process is taking
>>> place. To be clear: no actual RAM will be added to the system.
>>
>> Okay, but there is also the case where XEN will actually hotplug memory
>> using this same handler IIRC (at least I've read papers about it). Both
>> are using the same handler, correct?
>>
>>>
>>>> It's the admin/distro responsibility to configure this properly. In case
>>>> this doesn't happen (or as you say, users change it), bad luck.
>>>>
>>>> E.g., virtio-mem takes care to not add more memory in case it is not
>>>> getting onlined. I remember hyper-v has similar code to at least wait a
>>>> bit for memory to get onlined.
>>>
>>> I don't think VirtIO or Hyper-V use the hotplug system in the same way
>>> as Xen, as said this is done to add unpopulated memory regions that
>>> will be used to map foreign memory (from other domains) by Xen drivers
>>> on the system.
>>
>> Indeed, if the memory is never exposed to the buddy (and all you need is
>> struct pages +  a kernel virtual mapping), I wonder if
>> memremap/ZONE_DEVICE is what you want? Then you won't have user-visible
>> memory blocks created with unclear online semantics, partially involving
>> the buddy.
> 
> And just a note that there is also DCSS on s390x / z/VM which allows to
> map segments into the VM physical address space (e.g., you can share
> segments between VMs). They don't need any memmap (struct page) for that
> memory, though. All they do is create the identity mapping in the kernel
> virtual address space manually. Not sure what the exact requirements on
> the XEN side are. I assume you need a memmap for this memory.

We need to be able to do I/O with that memory via normal drivers and we
need to be able to map it, both from user land and from the kernel.


Juergen