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

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Fri, 2015-11-27 at 16:11 +0100, 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.

We certainly want the toolstack domain to be more resilient in the face of
ludicrous guest-supplied numbers than it is today.

Ian.


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