== Test on a VM with 384Mb of memory ==

1)Original setup - crashkernel=384M-2G:64M,2G-:128M
ubuntu@SaucyS:~$ free
             total       used       free     shared    buffers     cached
Mem:        306572     144412     162160          0      21900      73716
-/+ buffers/cache:      48796     257776
Swap:      1044476          0    1044476
ubuntu@SaucyS:~$ dmesg | grep crash
[    0.000000] Command line: BOOT_IMAGE=/vmlinuz-3.11.0-12-generic root=/dev/mapper/SaucyS--vg-root ro crashkernel=384M-2G:64M,2G-:128M console=ttyS0,115200
[    0.000000] Reserving 64MB of memory at 288MB for crashkernel (System RAM: 384MB)

2) Proposed setup - crashkernel=384-:128M
root@SaucyS:~# free
             total       used       free     shared    buffers     cached
Mem:        241036     146528      94508          0      21888      74100
-/+ buffers/cache:      50540     190496
Swap:      1044476          0    1044476
root@SaucyS:~# dmesg | grep crash
[    0.000000] Command line: BOOT_IMAGE=/vmlinuz-3.11.0-12-generic root=/dev/mapper/SaucyS--vg-root ro crashkernel=384M-:128M console=ttyS0,115200
[    0.000000] Reserving 128MB of memory at 224MB for crashkernel (System RAM: 384MB)

3) Proposed setup on a 285Mb VM
cat /proc/cmdline
BOOT_IMAGE=/vmlinuz-3.11.0-12-generic root=/dev/mapper/SaucyS--vg-root ro crashkernel=384M-:128M console=ttyS0,115200

part of boot log:
 * Loading crashkernel...                                                       Memory for crashkernel is not reserved
Please reserve memory by passing "crashkernel=X@Y" parameter to the kernel
Then try loading kdump kernel                                                                         [fail]

= Alternative tests =

1) Forcing 128Mb reservation on 285Mb VM

root@SaucyS:~# free
             total       used       free     shared    buffers     cached
Mem:        140684     125108      15576          0       7988      67296
-/+ buffers/cache:      49824      90860
Swap:      1044476          0    1044476
root@SaucyS:~# dmesg | grep crash
[    0.000000] Command line: BOOT_IMAGE=/vmlinuz-3.11.0-12-generic root=/dev/mapper/SaucyS--vg-root ro crashkernel=128M console=ttyS0,115200
[    0.000000] Reserving 128MB of memory at 128MB for crashkernel (System RAM: 284MB)
[    0.000000] Kernel command line: BOOT_IMAGE=/vmlinuz-3.11.0-12-generic root=/dev/mapper/SaucyS--vg-root ro crashkernel=128M console=ttyS0,115200

2) Forcing 128Mb reservation on 255Mb VM
root@SaucyS:~# free
             total       used       free     shared    buffers     cached
Mem:        113048      99292      13756          0       6624      44132
-/+ buffers/cache:      48536      64512
Swap:      1044476          0    1044476
root@SaucyS:~# dmesg | grep crash
[    0.000000] Command line: BOOT_IMAGE=/vmlinuz-3.11.0-12-generic root=/dev/mapper/SaucyS--vg-root ro crashkernel=128M console=ttyS0,115200
[    0.000000] Reserving 128MB of memory at 96MB for crashkernel (System RAM: 255MB)
[    0.000000] Kernel command line: BOOT_IMAGE=/vmlinuz-3.11.0-12-generic root=/dev/mapper/SaucyS--vg-root ro crashkernel=128M console=ttyS0,115200

So even for a VM with 256Mb of RAM, it could boot with crashkernel=128M but I think it is safe to restrict it to 384Mb onward.