[libvirt] Problem with setting up KVM guests to use HugePages

[Vivi L]

Configure hugepages and then start virtual guest via "virsh start". However, 
virtual guest failed to use hugepages although it's configured

The initial usage of system memory
[root at local ~]# free
            total        used        free      shared  buff/cache   available
Mem:   263767352  1900628  261177892  9344    688832   261431872
Swap:       4194300           0     4194300

After configuring hugepages, 
[root at local ~]# cat /proc/meminfo | grep uge
AnonHugePages:      2048 kB
HugePages_Total:     140
HugePages_Free:      140
HugePages_Rsvd:        0
HugePages_Surp:        0
Hugepagesize:    1048576 kB

[root at local ~]# mount | grep huge
cgroup on /sys/fs/cgroup/hugetlb type cgroup (rw,nosuid,nodev,noexec,
relatime,hugetlb)hugetlbfs on /dev/hugepages type hugetlbfs (rw,realtime)

On the guest side, its config xml contains
<domain type='kvm'>
  <name>test-01</name>
  <uuid>e1b72349-4a0b-4b91-aedc-fd34e92251e4</uuid>
  <description>test-01</description>
  <memory unit='KiB'>134217728</memory>
  <currentMemory unit='KiB'>134217728</currentMemory>
  <memoryBacking>
    <hugepages/>
    <nosharepages/>
  </memoryBacking>


Tried to virsh start the guest, but it failed although the hugepages memory 
was sufficient
virsh # start test-01
error: Failed to start domain test-01
error: internal error: early end of file from monitor: possible problem:
Cannot set up guest memory 'pc.ram': Cannot allocate memory


however, if decrease the number of hugepages to smaller(say 100) that 
makes the memory not used by hugepages more than required by guest, 
then guest can start. But memory is not allocated from hugepages. 
[root at local ~]# cat /proc/meminfo | grep uge
AnonHugePages:  134254592 kB
HugePages_Total:     100
HugePages_Free:      100
HugePages_Rsvd:        0
HugePages_Surp:        0
Hugepagesize:    1048576 kB

In summary, although hugepages is configured, the guest seemed not 
instructed to use hugepage, instead, the guest is allocated memory from 
the memory pool leftover by hugepages.