Fedora - Libvirt - unable to map backing store for guest RAM: Cannot allocate memory

Hi Guys, first post and hoping for some help :grinning:

I have a VM set up in Fedora. After my last update I can no longer start it up as I get the following error:

Error starting domain: internal error: process exited while connecting to monitor: 2020-05-28T22:44:52.041442Z qemu-system-x86_64: -numa node,nodeid=0,cpus=0-15,mem=16384: warning: Parameter -numa node,mem is deprecated, use -numa node,memdev instead
2020-05-28T22:44:52.048448Z qemu-system-x86_64: unable to map backing store for guest RAM: Cannot allocate memory

My issue is that I am unsure how to specify memdev in the libvirt xml as I am struggling to make sense of the libvirt docs. Machine details are below. Any help appreciated.

<domain type="kvm">
    <libosinfo:libosinfo xmlns:libosinfo="http://libosinfo.org/xmlns/libvirt/domain/1.0">
      <libosinfo:os id="http://microsoft.com/win/10"/>
  <memory unit="KiB">16777216</memory>
  <currentMemory unit="KiB">16777216</currentMemory>
  <vcpu placement="static" cpuset="8-31">16</vcpu>
    <vcpupin vcpu="0" cpuset="8"/>
    <vcpupin vcpu="1" cpuset="24"/>
    <vcpupin vcpu="2" cpuset="9"/>
    <vcpupin vcpu="3" cpuset="25"/>
    <vcpupin vcpu="4" cpuset="10"/>
    <vcpupin vcpu="5" cpuset="26"/>
    <vcpupin vcpu="6" cpuset="11"/>
    <vcpupin vcpu="7" cpuset="27"/>
    <vcpupin vcpu="8" cpuset="12"/>
    <vcpupin vcpu="9" cpuset="28"/>
    <vcpupin vcpu="10" cpuset="13"/>
    <vcpupin vcpu="11" cpuset="29"/>
    <vcpupin vcpu="12" cpuset="14"/>
    <vcpupin vcpu="13" cpuset="30"/>
    <vcpupin vcpu="14" cpuset="15"/>
    <vcpupin vcpu="15" cpuset="31"/>
    <emulatorpin cpuset="8-31"/>
    <memory mode="strict" nodeset="0"/>
    <type arch="x86_64" machine="pc-q35-4.1">hvm</type>
    <loader readonly="yes" type="pflash">/usr/share/edk2/ovmf/OVMF_CODE.fd</loader>
      <relaxed state="on"/>
      <vapic state="on"/>
      <spinlocks state="on" retries="8191"/>
      <vendor_id state="on" value="10DE"/>
      <hidden state="on"/>
    <vmport state="off"/>
  <cpu mode="host-passthrough" check="none">
    <topology sockets="1" cores="16" threads="1"/>
      <cell id="0" cpus="0-15" memory="16777216" unit="KiB"/>
  <clock offset="localtime">
    <timer name="rtc" tickpolicy="catchup"/>
    <timer name="pit" tickpolicy="delay"/>
    <timer name="hpet" present="no"/>
    <timer name="hypervclock" present="yes"/>
    <suspend-to-mem enabled="no"/>
    <suspend-to-disk enabled="no"/>
    <controller type="usb" index="0" model="qemu-xhci" ports="15">
      <address type="pci" domain="0x0000" bus="0x02" slot="0x00" function="0x0"/>
    <controller type="sata" index="0">
      <address type="pci" domain="0x0000" bus="0x00" slot="0x1f" function="0x2"/>
    <controller type="pci" index="0" model="pcie-root"/>
    <controller type="pci" index="1" model="pcie-root-port">
      <model name="pcie-root-port"/>
      <target chassis="1" port="0x10"/>
      <address type="pci" domain="0x0000" bus="0x00" slot="0x02" function="0x0" multifunction="on"/>
    <controller type="pci" index="2" model="pcie-root-port">
      <model name="pcie-root-port"/>
      <target chassis="2" port="0x11"/>
      <address type="pci" domain="0x0000" bus="0x00" slot="0x02" function="0x1"/>
    <controller type="pci" index="3" model="pcie-root-port">
      <model name="pcie-root-port"/>
      <target chassis="3" port="0x12"/>
      <address type="pci" domain="0x0000" bus="0x00" slot="0x02" function="0x2"/>
    <controller type="pci" index="4" model="pcie-root-port">
      <model name="pcie-root-port"/>
      <target chassis="4" port="0x13"/>
      <address type="pci" domain="0x0000" bus="0x00" slot="0x02" function="0x3"/>
    <controller type="pci" index="5" model="pcie-root-port">
      <model name="pcie-root-port"/>
      <target chassis="5" port="0x14"/>
      <address type="pci" domain="0x0000" bus="0x00" slot="0x02" function="0x4"/>
    <controller type="pci" index="6" model="pcie-root-port">
      <model name="pcie-root-port"/>
      <target chassis="6" port="0x15"/>
      <address type="pci" domain="0x0000" bus="0x00" slot="0x02" function="0x5"/>
    <controller type="pci" index="7" model="pcie-root-port">
      <model name="pcie-root-port"/>
      <target chassis="7" port="0x16"/>
      <address type="pci" domain="0x0000" bus="0x00" slot="0x02" function="0x6"/>
    <controller type="pci" index="8" model="pcie-root-port">
      <model name="pcie-root-port"/>
      <target chassis="8" port="0x17"/>
      <address type="pci" domain="0x0000" bus="0x00" slot="0x02" function="0x7"/>
    <controller type="pci" index="9" model="pcie-root-port">
      <model name="pcie-root-port"/>
      <target chassis="9" port="0x18"/>
      <address type="pci" domain="0x0000" bus="0x00" slot="0x03" function="0x0" multifunction="on"/>
    <controller type="pci" index="10" model="pcie-root-port">
      <model name="pcie-root-port"/>
      <target chassis="10" port="0x19"/>
      <address type="pci" domain="0x0000" bus="0x00" slot="0x03" function="0x1"/>
    <controller type="pci" index="11" model="pcie-to-pci-bridge">
      <model name="pcie-pci-bridge"/>
      <address type="pci" domain="0x0000" bus="0x09" slot="0x00" function="0x0"/>
    <controller type="pci" index="12" model="pcie-root-port">
      <model name="pcie-root-port"/>
      <target chassis="12" port="0x1a"/>
      <address type="pci" domain="0x0000" bus="0x00" slot="0x03" function="0x2"/>
    <controller type="virtio-serial" index="0">
      <address type="pci" domain="0x0000" bus="0x03" slot="0x00" function="0x0"/>
    <interface type="direct">
      <mac address="52:54:00:cf:d6:ce"/>
      <source dev="enp4s0" mode="bridge"/>
      <model type="e1000e"/>
      <address type="pci" domain="0x0000" bus="0x01" slot="0x00" function="0x0"/>
    <serial type="pty">
      <target type="isa-serial" port="0">
        <model name="isa-serial"/>
    <console type="pty">
      <target type="serial" port="0"/>
    <channel type="spicevmc">
      <target type="virtio" name="com.redhat.spice.0"/>
      <address type="virtio-serial" controller="0" bus="0" port="1"/>
    <input type="mouse" bus="ps2"/>
    <input type="keyboard" bus="ps2"/>
    <graphics type="spice" autoport="yes">
      <listen type="address"/>
      <image compression="off"/>
    <sound model="ich9">
      <address type="pci" domain="0x0000" bus="0x00" slot="0x1b" function="0x0"/>
      <model type="qxl" ram="65536" vram="65536" vgamem="16384" heads="1" primary="yes"/>
      <address type="pci" domain="0x0000" bus="0x00" slot="0x01" function="0x0"/>
    <hostdev mode="subsystem" type="pci" managed="yes">
        <address domain="0x0000" bus="0x41" slot="0x00" function="0x0"/>
      <boot order="2"/>
      <address type="pci" domain="0x0000" bus="0x0a" slot="0x00" function="0x0"/>
    <hostdev mode="subsystem" type="usb" managed="yes">
        <vendor id="0x046d"/>
        <product id="0xc539"/>
      <address type="usb" bus="0" port="1"/>
    <hostdev mode="subsystem" type="usb" managed="yes">
        <vendor id="0x046d"/>
        <product id="0x0a5b"/>
      <address type="usb" bus="0" port="4"/>
    <hostdev mode="subsystem" type="usb" managed="yes">
        <vendor id="0x04b4"/>
        <product id="0x120d"/>
      <address type="usb" bus="0" port="5"/>
    <hostdev mode="subsystem" type="pci" managed="yes">
        <address domain="0x0000" bus="0x09" slot="0x00" function="0x0"/>
      <address type="pci" domain="0x0000" bus="0x04" slot="0x00" function="0x0"/>
    <hostdev mode="subsystem" type="pci" managed="yes">
        <address domain="0x0000" bus="0x09" slot="0x00" function="0x1"/>
      <address type="pci" domain="0x0000" bus="0x05" slot="0x00" function="0x0"/>
    <hostdev mode="subsystem" type="pci" managed="yes">
        <address domain="0x0000" bus="0x09" slot="0x00" function="0x2"/>
      <address type="pci" domain="0x0000" bus="0x06" slot="0x00" function="0x0"/>
    <hostdev mode="subsystem" type="pci" managed="yes">
        <address domain="0x0000" bus="0x09" slot="0x00" function="0x3"/>
      <address type="pci" domain="0x0000" bus="0x07" slot="0x00" function="0x0"/>
    <redirdev bus="usb" type="spicevmc">
      <address type="usb" bus="0" port="2"/>
    <redirdev bus="usb" type="spicevmc">
      <address type="usb" bus="0" port="3"/>
    <memballoon model="virtio">
      <address type="pci" domain="0x0000" bus="0x08" slot="0x00" function="0x0"/>

It’s related to a deprecated feature in Qemu 4.1 according to the changelog but I have no idea how this would be set up in virt-manager / libvirt

Why are you specifying numa?

      <cell id="0" cpus="0-15" memory="16777216" unit="KiB"/>

Remove this and you’ll be fine.

You just want hugepages working? Or what specifically are you trying to achieve by providing the numa directives there?

Also, what CPU/memory configuration are you running? I have a suspicion about about what you’re trying to do, but I want to be sure.

THanks for the response.

I specified numa as I am running on a Threadripper 2950x. One of the guides I found mentioned specifying this.

Memory is quad channel in 4 slots.

Hugepages was added to improve performance from another guide.

Okay, so huge pages should be set up differently for threadripper. I am not in front of my 1950x right now, but I’ll share it in the morning.

You can specify which Numa node huge pages are created on, and you must do that at boot, or when you manually allocate them. I’ve got a script that runs on a systemd timer to auto create them.

The error isn’t happening because of memdev but because when you define huge pages the original method, it spreads them across all Numa nodes.

Then when you tell it to allocate all the memory from one node, it doesn’t have enough pages to assign, and throws the error.

Excellent thank you for the explanation.

If you could share your script that would be really helpful.

In the meantime I have something to work with.

Sorted. I tweaked the hugepage size and it is now booting again. a 2950x only has a single node anyway and I already had hugepages configured.

Thanks for pointing me in the right direction.

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Sorry I never got back to you, this weekend’s been absolutely nuts

If you want more info, I actually made a thread on threadripper, numa and memory assignment:

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