Re: [Qemu-devel] Memory region allocation races

[Igor Mammedov]

On Wed, 17 Feb 2016 15:22:29 +0300
Andrey Korolyov <address@hidden> wrote:

> Hello Igor, everyone,
> 
> we are seemingly running into the issue with "virtio: error trying to
> map MMIO memory" on a 'legacy' vhost-net with 64 regions on VMs with
> relatively small amount of DIMMs, less than ten of 512Mb and larger
> ones for which it could appear literally on every boot. I could
> suggest to link the problem to the memory fragmentation as busier
> hypervisors tending to halt VM more frequently on this error, though
> it is still a very rare issue and could not be reproduced
> deterministically. Also there seems to be a (very unobvious) link to
> the CVE-2015-5307, because we started seeing these stops more
> frequently after rolling in an appropriate patch. Before it we saw
> this stop for good once per 1M machine-hours and now it appears ~20
> times more frequent all across the infrastructure. Could
> 4de7255f7d2be5e51664c6ac6011ffd6e5463571 +
> 1e0994730f772580ff98754eb5595190cdf371ef
These commit are more related to supporting more than 64 regions,
so it's unlikely that it would fix the issue.

What version of QEMU are you using?
Do you have following QEMU series applied?
 [PATCH 0/6] virtio: handle non contigious s/g entries
It's probably what you are missing.


< (and the rest of the queue,
> for example, as in RHEL kernel) be a matter of interest for fixing the
> issue or there must be an another reason, since the problem was
> amplified by a patch which could add only some timing- (therefore
> racing-) conditions? Static configurations where all memory is
> populated not through DIMMs are not affected, so we are dealing only
> with the backend device memory accesses over high (> 512Mb of the
> populated base) mem:
> 
> -m 512,slots=31,maxmem=16384M -object
> memory-backend-ram,id=mem0,size=512M -device
> pc-dimm,id=dimm0,node=0,memdev=mem0 ......
> 
> Thanks!