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 17:10:03 +0200 |
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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
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