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

[Anshuman Khandual <khandual@linux.vnet.ibm.com>]

On 08/24/2015 09:01 AM, 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.

Even when the in-kernel memory hotplug will be available on PKVM,
it still makes sense to disable memory hotplug when the hash table
size received from host is not sufficient for the permissible/
requested maximum memory size of the guest. Whats the point of
enabling memory hotplug when we know it cannot fulfill all the
memory hotplug request.

IIUC, the hash table size received from the host some times can
be greater than what is required for the current memory size and
less than max hot pluggable memory on the guest. With this patch
in that case we will disable memory hotplug but then why use hash
table size which is bigger than required for the current memory
size. We will not be doing *any* memory hotplug at all afterwards,
so lets shrink the hash page size to what is just required for
the current memory requested by the guest and save some RAM
on the system.