On 2015/2/27 3:26, Luiz Capitulino wrote:
Hello,
Reproducer:
1. Start QEMU with balloon and memory hotplug support:
# qemu [...] -m 1G,slots=2,maxmem=2G -balloon virtio
2. Check balloon size:
(qemu) info balloon
balloon: actual=1024
(qemu)
3. Hotplug some memory:
(qemu) object_add memory-backend-ram,id=mem1,size=1G
(qemu) device_add pc-dimm,id=dimm1,memdev=mem1
4. This is step is _not_ needed to reproduce the problem,
but you may need to online memory manually on Linux so
that it becomes available in the guest
5. Check balloon size again:
(qemu) info balloon
balloon: actual=1024
(qemu)
BUG: The guest now has 2GB of memory, but the balloon thinks
the guest has 1GB
One may think that the problem is that the balloon driver is
ignoring hotplugged memory. This is not what's happening. If
you do balloon your guest, there's nothing stopping the
balloon driver in the guest from ballooning hotplugged memory.
The problem is that the balloon device in QEMU needs to know
the current amount of memory available to the guest.
Before memory hotplug this information was easy to obtain: the
current amount of memory available to the guest is the memory the
guest was booted with. This value is stored in the ram_size global
variable in QEMU and this is what the balloon device emulation
code uses today. However, when memory is hotplugged ram_size is
_not_ updated and the balloon device breaks.
I see two possible solutions for this problem:
1. In addition to reading ram_size, the balloon device in QEMU
could scan pc-dimm devices to account for hotplugged memory.
This solution was already implemented by zhanghailiang:
http://lists.gnu.org/archive/html/qemu-devel/2014-11/msg02362.html
It works, except that on Linux memory hotplug is a two-step
procedure: first memory is inserted then it has to be onlined
from user-space. So, if memory is inserted but not onlined
this solution gives the opposite problem: the balloon device
will report a larger memory amount than the guest actually has.
Can we live with that? I guess not, but I'm open for discussion.
If QEMU could be notified when Linux makes memory online, then
the problem would be gone. But I guess this can't be done.
Yes, it is really a problem, balloon can't work well with memory block online/offline now.
virtio-balloon can't be notified when memory block online/offline now, actually, we can
add this capability by using the exist kernel memory hotplug/unplug notifier mechanism. (
just a simple register_memory_notifier().)
2. Modify the balloon driver in the guest to inform the balloon
device on the host about the current memory available to the
guest. This way, whenever the balloon device in QEMU needs
to know the current amount of memory in the guest, it asks
the guest. This drops any usage of ram_size in the balloon
device
I'm not completely sure this is feasible though. For example,
what happens if the guest reports a memory amount to QEMU and
right after this more memory is plugged?
Hmm, i wonder why we notify the number of pages which should be adjusted to virtio-balloon,
why not the memory 'target' size ? Is there any special reason ?
For linux guest, it can always know exactly its current real memory size, but QEMU may not, because
guest can do online/offline memory block by themselves.
If virtio-balloon in guest know the balloon's 'target' size, it can calculate the exact memory size
that should be adjuested. and also can do corresponding action (fill or leak balloon)
when there is online/offline memory block occurred.
Besides, this solution is more complex than solution 1 and
won't address older guests.
Another important detail is that, I *suspect* that a very similar
bug already exists with 32-bit guests even without memory
hotplug: what happens if you assign 6GB to a 32-bit without PAE
support? I think the same problem we're seeing with memory
hotplug will happen and solution 1 won't fix this, although
no one seems to care about 32-bit guests...
.
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