[PATCH v3 1/1] powerpc/kernel: Enables memory hot-remove after reboot on pseries guests

[Oliver O'Halloran]

On Fri, Apr 3, 2020 at 6:55 AM Leonardo Bras <leonardo at linux.ibm.com> wrote:
>
> While providing guests, it's desirable to resize it's memory on demand.
>
> By now, it's possible to do so by creating a guest with a small base
> memory, hot-plugging all the rest, and using 'movable_node' kernel
> command-line parameter, which puts all hot-plugged memory in
> ZONE_MOVABLE, allowing it to be removed whenever needed.
>
> But there is an issue regarding guest reboot:
> If memory is hot-plugged, and then the guest is rebooted, all hot-plugged
> memory goes to ZONE_NORMAL, which offers no guaranteed hot-removal.
> It usually prevents this memory to be hot-removed from the guest.
>
> It's possible to use device-tree information to fix that behavior, as
> it stores flags for LMB ranges on ibm,dynamic-memory-vN.
> It involves marking each memblock with the correct flags as hotpluggable
> memory, which mm/memblock.c puts in ZONE_MOVABLE during boot if
> 'movable_node' is passed.

I don't really understand why the flag is needed at all. According to
PAPR any memory provided by dynamic reconfiguration can be hot-removed
so why aren't we treating all DR memory as hot removable? The only
memory guaranteed to be there 100% of the time is what's in the
/memory at 0 node since that's supposed to cover the real mode area.