Description of problem:
Balloon the guest's memory to a very low value(about 240M) which was less than the" MemTotal-MemFree".The guest's used memory continued to reduce but the guest's free memory maintained at a certain value(about 70M),and the guest got unavailable at last,but the QEMU-monitor still alive.


Version-Release number of selected component (if applicable):
# uname -r
2.6.32-169.el6.x86_64
# rpm -q qemu-kvm
qemu-kvm-0.12.1.2-2.171.el6.x86_64

How reproducible:
100%

Steps to Reproduce:
1.Start the VM with the CLI:
# /usr/libexec/qemu-kvm -M rhel6.2.0 -enable-kvm -m 2G -smp 2,sockets=2,cores=1,threads=1 -name sluo -uuid fbf37ccb-2278-15d3-310a-baa5316e65b4 -rtc base=utc,clock=host,driftfix=slew -boot order=c -drive file=/home/RHEL6.1-Server-x86_64.raw,if=none,id=disk1,format=raw,cache=none,aio=threads,rerror=stop,werror=stop -device ide-drive,drive=disk1,id=device_disk1 -netdev tap,script=/etc/qemu-ifup,id=hostnet0 -device virtio-net-pci,netdev=hostnet0,id=net0,mac=52:54:00:73:43:59 -device usb-tablet -usb -qmp tcp:0:4444,server,nowait -usbdevice tablet -vnc 0:10 -vga qxl -device virtio-balloon-pci,id=balloon0,bus=pci.0 -monitor stdio
2.After guest booting up ,in the guest run:
[root@localhost ~]# grep MemFree /proc/meminfo
MemFree: 1529404 kB
[root@localhost ~]# grep MemTotal /proc/meminfo
MemTotal: 2075068 kB
And in the qemu-monitor evict the guest's unused memory value from 2G to  a very low value.
3.In the guest's Terminal run "# swapoff -a" at first,and then run "# free -m" repeatly to a certain extent,we can see: The guest's used memory continued to reduce but the guest's free memory maintained at a certain value(about 70M).
4.Balloon the guest's memory to a very low value(about 240M) which was less than the" MemTotal-MemFree",the guest got unavailable.
  
Actual results:
The guest's used memory continued to reduce but the guest's free memory maintained at a certain value(about 70M),and the guest got unavailable at last,but the QEMU-monitor still alive.

Expected results:
The guest's memory value should not continue reduce and less than the" MemTotal-MemFree".

Additional info:
processor	: 3
vendor_id	: AuthenticAMD
cpu family	: 16
model		: 4
model name	: AMD Phenom(tm) II X4 B95 Processor
stepping	: 2
cpu MHz		: 800.000
cache size	: 512 KB
physical id	: 0
siblings	: 4
core id		: 3
cpu cores	: 4
apicid		: 3
initial apicid	: 3
fpu		: yes
fpu_exception	: yes
cpuid level	: 5
wp		: yes
flags		: fpu vme de pse tsc msr pae mce cx8 apic mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx mmxext fxsr_opt pdpe1gb rdtscp lm 3dnowext 3dnow constant_tsc rep_good nonstop_tsc extd_apicid pni monitor cx16 popcnt lahf_lm cmp_legacy svm extapic cr8_legacy abm sse4a misalignsse 3dnowprefetch osvw ibs skinit wdt npt lbrv svm_lock nrip_save
bogomips	: 5985.05
TLB size	: 1024 4K pages
clflush size	: 64
cache_alignment	: 64
address sizes	: 48 bits physical, 48 bits virtual
power management: ts ttp tm stc 100mhzsteps hwpstate