Messages in this thread | | | Subject | Re: [PATCH 3/3] memory: introduce an option to force onlining of hotplug memory | From | Jürgen Groß <> | 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
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