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[Xen-devel] RE: Ballooning up

To: Jeremy Fitzhardinge <jeremy@xxxxxxxx>, Xen-devel@xxxxxxxxxxxxxxxxxxx
Subject: [Xen-devel] RE: Ballooning up
From: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Date: Mon, 13 Sep 2010 14:17:35 -0700 (PDT)
Cc: Konrad Wilk <konrad.wilk@xxxxxxxxxx>, Daniel Kiper <dkiper@xxxxxxxxxxxx>, Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
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> From: Jeremy Fitzhardinge [mailto:jeremy@xxxxxxxx]
> Cc: Dan Magenheimer; Daniel Kiper; Stefano Stabellini; Konrad Rzeszutek
> Wilk
> Subject: Ballooning up
>  I finally got around to implementing "ballooning up" in the pvops
> kernels.  Now if you start a domain with "memory=X maxmem=Y", the
> domain
> will start with X MB of memory, but you can use "x[ml] mem-set" to
> expand the domain up to Y.


> As a side-effect, it also works for dom0.  If you set dom0_mem on the
> Xen command line, then nr_pages is limited to that value, but the
> kernel
> can still see the system's real E820 map, and therefore adds all the
> system's memory to its own balloon driver, potentially allowing dom0 to
> expand up to take all physical memory.
> However, this may caused bad side-effects if your system memory is much
> larger than your dom0_mem, especially if you use a 32-bit dom0.  I may
> need to add a kernel command line option to limit the max initial
> balloon size to mitigate this...

I would call this dom0 functionality a bug.  I think both Citrix
and Oracle use dom0_mem as a normal boot option for every
installation and, while I think both employ heuristics to choose
a larger dom0_mem for larger physical memory, I don't think it
grows large enough for, say, >256GB physical memory, to accommodate
the necessarily large number of page tables.

So, I'd vote for NOT allowing dom0 to balloon up to physical
memory if dom0_mem is specified, and possibly a kernel command
line option that allows it to grow beyond.  Or, possibly, no
option and never allow dom0 memory to grow beyond dom0_mem
unless (possibly) it grows with hot-plug.


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