Re: [libvirt] [PATCH 0/3] Fix memlock limit during hotplug of mdev devices
Hi,
I've thought about the issue you're fixing. Have you considered/tried
to handle the MemLockLimit upper in the call hierarchy with your new
qemuDomainAdjustMaxMemLockHostdev() function? Both
qemuDomainAttachMediatedDevice() and qemuDomainAttachHostPCIDevice()
are called from qemuDomainAttachHostDevice(). In theory, a call to
qemuDomainAdjustMaxMemLockHostdev() at the start of AttachHostDevice()
would cover all hostdev types of the function. Then you can undo the
memLock in the 'error' label there if something goes wrong.
It is possible to even argue about the be possibility of calling
qemuDomainAdjustMaxMemLock() at the start of the generic hotplug
function, qemuDomainAttachDeviceLive(), using the same principle -
temporarily add the device in the vmdef, define the new mem limit
assuming that everything will be OK, undo the adjustment if ret != 0.
This would cover the case of memory hotplug, which needs RLIMIT
changes as well and uses the same qemuDomainAdjustMaxMemLock()
function to do it.
This is more of a future work though. For now I believe it's appropriate
to fix vfio-ccw hotplug ASAP.
Thanks,
DHB
On 9/3/19 5:09 PM, Eric Farman wrote:
The routine qemuDomainGetMemLockLimitBytes() has a couple tests
to determine what the maximum limit of locked memory should be.
If I start a domain without any vfio stuff, /proc/$PID/limits says
the limit is 64KiB. If I start a guest with a vfio-ccw hostdev,
the limit is now $GUEST_MEMORY + 1GiB. It doesn't matter if I
have one hostdev or a thousand; it's always 1GiB more than the
configured amount of guest memory.
If I start a guest without any vfio devices, and hotplug that same
vfio-ccw hostdev, the limit remains at 64KiB and I start getting
I/O errors in the guest. This makes sense, since some of the I/O
chains are long enough to exceed that page limit, and the host
starts throwing errors.
There is already code that adjusts this limit during the hotplug
of vfio-pci devices, so patch 1 refactors that code so that it
can be re-used on the mdev hotplug path in patch 3.
Patch 2, meanwhile, adds some cleanup that I think is missing in
the vfio-pci path, based on my read of the mdev path.
$ grep locked /proc/83543/limits
Max locked memory 65536 65536 bytes
$ virsh attach-device guest scratch-ca8b.xml
Device attached successfully
$ grep locked /proc/83543/limits
Max locked memory 3221225472 3221225472 bytes
Eric Farman (3):
qemu: Refactor the max memlock routine
qemu: Reset the maximum locked memory on hotplug fail
qemu: Adjust max memlock on mdev hotplug
src/qemu/qemu_domain.c | 30 ++++++++++++++++++++++++++++++
src/qemu/qemu_domain.h | 2 ++
src/qemu/qemu_hotplug.c | 22 ++++++++++++----------
3 files changed, 44 insertions(+), 10 deletions(-)