Re: [Xen-devel] [PATCH RFC] mm/memory_hotplug: Introduce memory block types

[Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>]

> How should a policy in user space look like when new memory gets added
> - on s390x? Not onlining paravirtualized memory is very wrong.

Because we're going to balloon it away in a moment anyway?

We have auto-onlining.  Why isn't that being used on s390?


> So the type of memory is very important here to have in user space.
> Relying on checks like "isS390()", "isKVMGuest()" or "isHyperVGuest()"
> to decide whether to online memory and how to online memory is wrong.
> Only some specific memory types (which I call "normal") are to be
> handled by user space.
> 
> For the other ones, we exactly know what to do:
> - standby? don't online

I think you're horribly conflating the software desire for what the stae
should be and the hardware itself.

>> As for the OOM issues, that sounds like something we need to fix by
>> refusing to do (or delaying) hot-add operations once we consume too much
>> ZONE_NORMAL from memmap[]s rather than trying to indirectly tell
>> userspace to hurry thing along.
> 
> That is a moving target and doing that automatically is basically
> impossible.

Nah.  We know how much metadata we've allocated.  We know how much
ZONE_NORMAL we are eating.  We can *easily* add something to
add_memory() that just sleeps until the ratio is not out-of-whack.

> You can add a lot of memory to the movable zone and
> everything is fine. Suddenly a lot of processes are started - boom.
> MOVABLE should only every be used if you expect an unplug. And for
> paravirtualized devices, a "typical" unplug does not exist.

No, it's more complicated than that.  People use MOVABLE, for instance,
to allow more consistent huge page allocations.  It's certainly not just
hot-remove.

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