[Qemu-devel] Memory region allocation races

[Andrey Korolyov]

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 (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!