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

[Jürgen Groß <jgross@xxxxxxxx>]

On 23.07.20 15:08, Roger Pau Monné wrote:
> On Thu, Jul 23, 2020 at 02:28:13PM +0200, Jürgen Groß 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.
> > >
> > > Failure to online such blocks using the Xen specific online handler
> > > (which does not handle back the memory to the allocator in any way)
> > > will result in the system getting stuck and malfunctioning.
> > >
> > > > 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.
> > >
> > > Maybe this should somehow use a different mechanism to hotplug such
> > > empty memory blocks? I don't mind doing this differently, but I would
> > > need some pointers. Allowing user-space to change a (seemingly
> > > unrelated) parameter and as a result produce failures on Xen drivers
> > > is not an acceptable solution IMO.
> > Maybe we can use the same approach as Xen PV-domains: pre-allocate a
> > region in the memory map to be used for mapping foreign pages. For the
> > kernel it will look like pre-ballooned memory, so it will create struct
> > page for the region (which is what we are after), but it won't give the
> > memory to the allocator.
> IMO using something similar to memory hotplug would give us more
> flexibility, and TBH the logic is already there in the balloon driver.
> It seems quite wasteful to allocate such region(s) beforehand for all
> domains, even when most of them won't end up using foreign mappings at
> all.
We can do it for dom0 only per default, and add a boot parameter e.g.
for driver domains.

And the logic is already there (just pv-only right now).

> Anyway, I'm going to take a look at how to do that, I guess it's going
> to involve playing with the memory map and reserving some space.
Look at arch/x86/xen/setup.c (xen_add_extra_mem() and its usage).

> I suggest we should remove the Xen balloon hotplug logic, as it's not
> working properly and we don't have a plan to fix it.
I have used memory hotplug successfully not very long ago.


Juergen