Re: [Qemu-devel] [RFC PATCH v0] spapr: Disable memory hotplug when HTAB size is insufficient

[Bharata B Rao <bharata@linux.vnet.ibm.com>]

On Mon, Aug 24, 2015 at 09:01:51AM +0530, Bharata B Rao wrote:
> The hash table size allocated to guest depends on the maxmem size.
> If the host isn't able to allocate the required hash table size but
> instead allocates less than the optimal requested size, then it will
> not be possible to grow the RAM until maxmem via memory hotplug.
> Attempts to hotplug memory till maxmem could fail and this failure
> isn't being currently handled gracefully by the guest kernel thereby
> causing guest kernel oops.
> 
> This should eventually get fixed when we move to completely in-kernel
> memory hotplug instead of the current method where userspace tool drmgr
> drives the hotplug. Until the in-kernel memory hotplug is available
> for PowerKVM, disable memory hotplug when requested hash table size
> isn't allocated.

David - Do you have any views on how to go about this ? Due to the way
we do hotplug currently using drmgr, it appears that it is very difficult
to have a graceful recovery within the guest kernel when memory hotplug
request can't be fulfilled due to insufficient HTAB size. (Anshuman can
elaborate on this with the exact description on why it is so hard to
recover).

Do you think disabling memory hotplug upfront is a reasonable workaround
for this problem ?

Nathan - When you enable in-kernel memory hotplug for PowerKVM, will you
be exporting something for the userspace (capability ?) to check and
determine the presense of in-kernel memory hotplug feature so that we
can depend on graceful recovery instead of upfront disablement of
memory hotplug from QEMU ?

Regards,
Bharata.