Re: [Xen-devel] superpages lost after migration of HVM domU

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 20.04.17 at 18:04, <olaf@xxxxxxxxx> wrote:
> On Thu, Apr 20, Andrew Cooper wrote:
> 
>> As it currently stands, the sending side iterates from 0 to p2m_size,
>> and sends every frame on the first pass.  This means we get PAGE_DATA
>> records linearly, in batches of 1024, or two aligned 2M superpages.
> 
> Is there a way to preserve 1G pages? This 380G domU I'm looking at is
> built with 4k:461390 2M:2341 1G:365 pages.

I think we've hashed out a possible way to deal with this, by
speculatively allocating 1G pages as long as the allocation cap for
the domain allows, subsequently punching holes into those pages
if we can't allocate any new pages anymore (due to otherwise
overrunning the cap). For ballooned down guests this may add
some unnecessary overhead during migration, but I think the
post-migration benefit outweighs this. All that would be needed
is a second bitmap paralleling the one tracking populated pages,
to also track which pages we've actually seen data for (as those
obviously aren't candidates for the hole punching).

Jan


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