Re: [PATCH 3/3] memory: introduce an option to force onlining of hotplug memory

["Jürgen Groß" <jgross@suse.com>]

On 23.07.20 15:59, Roger Pau Monné wrote:
> On Thu, Jul 23, 2020 at 03:22:49PM +0200, 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?
> 
> Yes, it's used by this dual purpose, which I have to admit I don't
> like that much either.
> 
> One set of pages should be clearly used for RAM memory hotplug, and
> the other to map foreign pages that are not related to memory hotplug,
> it's just that we happen to need a physical region with backing struct
> pages.
> 
>>>
>>>> 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?
> 
> I'm certainly not familiar with the Linux memory subsystem, but if
> that gets us a backing struct page and a kernel mapping then I would
> say yes.
> 
>> Then you won't have user-visible
>> memory blocks created with unclear online semantics, partially involving
>> the buddy.
> 
> Seems like a fine solution.
> 
> Juergen: would you be OK to use a separate page-list for
> alloc_xenballooned_pages on HVM/PVH using the logic described by
> David?
> 
> I guess I would leave PV as-is, since it already has this reserved
> region to map foreign pages.

I would really like a common solution, especially as it would enable
pv driver domains to use that feature, too.

And finding a region for this memory zone in PVH dom0 should be common
with PV dom0 after all. We don't want to collide with either PCI space
or hotplug memory.


Juergen