On 2015-01-30 at 13:41, Kashyap Chamarthy wrote:
On Fri, Jan 30, 2015 at 06:15:21PM +0100, Kevin Wolf wrote:
Am 29.01.2015 um 17:25 hat Kashyap Chamarthy geschrieben:
$ qemu-system-x86_64 \
-nographic \
-nodefconfig \
-nodefaults \
-m 2048 \
-device virtio-scsi-pci,id=scsi \
-device virtio-serial-pci \
-serial stdio \
-drive file=./overlay1.qcow2,format=qcow2,if=virtio,cache=writeback
Segmentation fault (core dumped)
On the shell where `qemu-nbd` is running, I notice this
nbd.c:nbd_receive_request():L756: read failed
Haven't investigated further with GDB, thought I'd bring it up here
first.
Versions
--------
$ rpm -q qemu; uname -r
qemu-2.1.2-7.fc21.x86_64
3.17.8-300.fc21.x86_64
Copying Stefan because he's the master of AIO contexts and it is
bs->aio_context that becomes NULL. I couldn't see anything obvious.
In the meantime, could you retest on git master?
Just tested from git, and I can still reproduce it.
That's the commit I'm at:
$ git describe
v2.2.0-682-g16017c4
Run the NBD server, from git:
$ /home/kashyapc/build/qemu/qemu-nbd -f qcow2 \
-p10809 ./f21vm.qcow2 -t
Create the overlay:
$ /home/kashyapc/build/qemu/qemu-img create \
-f qcow2 -F nbd -o backing_file=nbd://localhost overlay2-of-f21vm.qcow2
Segmentation fault (core dumped)
You want to use -F raw. The file format is raw, not nbd (nbd is the
protocol over which the data is read, which is in format raw).
Anyway, -F nbd shouldn't result in a segfault. One way to prevent this
is to check whether the backing file format specified (or any format
given to qemu-img in general) is a real format or the name of a protocol
driver and then error out if it's the latter; but that would be more of
a hotfix.
Kevin, Stefan: The real problem is that block/nbd.c stores a
BDRVNBDState object in bs->opaque and passes &BDRVNBDState.client (an
NbdClientSession object) to the block/nbd-client.c functions. Those
functions then receive the BDS pointer from client->bs. If an NBD BDS is
a root BDS (as in this case), at some point a bdrv_swap() may happen
(and it does happen here) which leads to ((BDRVNBDState
*)bs->opaque)->client.bs != bs, and that's where the segfault comes from
(bdrv_get_aio_context() returns NULL).
One way to fix this real problem is to remove the BDS pointer from the
NbdClientSession and to always pass the BDS explicitly to the
block/nbd-client.c functions; the other is to always update the BDS
pointer in NbdClientSession in block/nbd.c. I'll try the former, and if
it doesn't work, will do the latter (if you don't object).
Max
Creating the overlay from the git-compiled `qemu-img` binary fails.
So, let's create the overlay using the `qemu-img` binary from the system
(RPM version noted above) and boot the overlay from the just compiled
QEMU x86_64 binary from git, still core dumps:
$ /home/kashyapc/build/qemu/x86_64-softmmu/qemu-system-x86_64 \
-nographic \
-nodefconfig \
-nodefaults \
-m 2048 \
-device virtio-scsi-pci,id=scsi \
-device virtio-serial-pci \
-serial stdio \
-drive file=./overlay2-f21vm.qcow2,format=qcow2,if=virtio,cache=writeback
Segmentation fault (core dumped)
PS: I'm traveling, so I'll be a little slow to respond here, but can
provide more debugging info from the coredump of `qemu-img` binary as I
have access to a real computer.
