Re: [Xen-devel] Xen 4.0.1 "xc_map_foreign_batch: mmap failed: Cannot allocate memory"

[Keir Fraser <keir@xxxxxxx>]

On 16/12/2010 20:44, "Charles Arnold" <carnold@xxxxxxxxxx> wrote:

>>> On 12/16/2010 at 01:33 PM, in message <C9302813.2966F%keir@xxxxxxx>, Keir
> Fraser <keir@xxxxxxx> wrote:
>> On 16/12/2010 19:23, "Charles Arnold" <carnold@xxxxxxxxxx> wrote:
>> 
>>> The bug is that qemu-dm seems to make the assumption that it can mmap from
>>> dom0 all the memory with which the guest has been defined instead of the
>>> memory
>>> that is actually available on the host.
>> 
>> 32-bit dom0? Hm, I thought the qemu mapcache was supposed to limit the total
>> amount of guest memory mapped at one time, for a 32-bit qemu. For 64-bit
>> qemu I wouldn't expect to find a limit as low as 3.25G.
> 
> Sorry, I should have specified that it is a 64 bit dom0 / hypervisor.

Okay, well I'm not sure what limit qemu-dm is hitting then. Mapping 3.25G of
guest memory will only require a few megabytes of pagetables for the qemu
process in dom0. Perhaps there is a ulimit or something set on the qemu
process?

If we can work out and detect this limit, perhaps 64-bit qemu-dm could have
a mapping cache similar to 32-bit qemu-dm, limited to some fraction of the
detected mapping limit. And/or, on mapping failure, we could reclaim
resources by simply zapping the existing cached mappings. Seems there's a
few options. I don't really maintain qemu-dm myself -- you might get some
help from Ian Jackson, Stefano, or Anthony Perard if you need more advice.

 -- Keir

> - Charles
> 
> 
> 
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