On Wed, Sep 02, 2015 at 08:58:54AM +0530, Bharata B Rao wrote: > 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 ?
So, I kind of dislike magically disabling requested options - it can
make debugging problems really confusing.
In theory, what I'd prefer is to just not start the guest if we don't
get a big enough hash table to cover maxram. Unfortunately we don't
discover this until reset time at which point it is not
straightforward to bail out cleanly :/
--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson
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