Re: [PATCH] libxc: try to find last used pfn when migrating

[David Vrabel <david.vrabel@citrix.com>]

On 27/11/15 15:11, Juergen Gross wrote:
> On 27/11/15 16:01, David Vrabel wrote:
>> On 27/11/15 14:50, Juergen Gross wrote:
>>> For migration the last used pfn of a guest is needed to size the
>>> logdirty bitmap and as an upper bound of the page loop. Unfortunately
>>> there are pv-kernels advertising a much higher maximum pfn as they
>>> are really using in order to support memory hotplug. This will lead
>>> to allocation of much more memory in Xen tools during migration as
>>> really needed.
>>>
>>> Try to find the last used guest pfn of a pv-domu by scanning the p2m
>>> tree from the last entry towards it's start and search for an entry
>>> not being invalid.
>>>
>>> Normally the mid pages of the p2m tree containing all invalid entries
>>> are being reused, so we can just scan the top page for identical
>>> entries and skip them but the first one.
>>
>> This has been fixed in Linux by "x86/xen/p2m: hint at the last populated
>> P2M entry" 98dd166ea3a3c3b57919e20d9b0d1237fcd0349d which is tagged for
>> stable.
>>
>> Do we really need a toolstack fix as well?
> 
> xl migrate will use much less resources for a domain with a 3.x kernel
> started with max_mem being much larger than mem. E.g. in case you start
> a domain on a small stand-by system and migrate it later to the large
> production system and want to balloon it up there.
> 
> Additionally there was a discussion this week on irc regarding this
> topic and concern was raised this could block dom0 responsiveness.

Ok.

David