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Date: Fri, 20 Jun 2014 11:35:53 +0200
From: =?ISO-8859-1?Q?Roger_Pau_Monn=E9?= <roger.pau@citrix.com>
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To: <virtualization@FreeBSD.org>, "freebsd-xen@freebsd.org"
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Subject: FreeBSD and memory balloon drivers
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Hello,

I've been looking into the Xen balloon driver, because I've experienced
problems when ballooning memory down which AFAICT are also present in
the VirtIO balloon driver. The problem I've experienced is that when
ballooning memory down, we basically allocate a bunch of memory as
WIRED, to make sure nobody tries to swap it do disk, since it will crash
the kernel because the memory is not populated. Due to this massive
amount of memory allocated as WIRED, user-space programs that try to use
mlock will fail because we hit the limit in vm.max_wired.

I'm not sure what's the best way to deal with this limitation, should
vm.max_wired be changed from the balloon drivers when ballooning
down/up? Is there anyway to remove the pages ballooned down from the
memory accounting of wired pages?

Thanks, Roger.