Proposal to change page allocation to zero on free in the -virtual kernel

[Philipp Kern]

Peter,

am Mon, Jan 28, 2013 at 09:45:46AM +0100 hast du folgendes geschrieben:
> I would like to know if it would be a good idea to change the page allocator
> zeroing > policy from zero on allocate (for GFP_USER) to zero on free for the
> -virtual > kernel builds.

wouldn't it be better to signal the host that the page in question can be
discarded? If the guest really needs to hand out the page and not the
mapping to the zeroed frame it keeps anyway, it seems like a disadvantage
to have a CoW breaking on the host immediately afterwards.

Live migration wouldn't need to copy such pages and instead of applying memory
scanning and merging on those pages they could simply be reassigned to another
process by the host on memory pressure.

The pages would probably need to get part of the balloon that's used to
dynamically change memory size, potentially with some additional signalling
which pages are likely to still be allocated to the guest and a way to leave
them in the free pool.

Kind regards
Philipp Kern
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