[libvirt] [PATCH 0/3] Fix memlock limit during hotplug of mdev devices

[Daniel Henrique Barboza]

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(-)
>