Linux doesn't create /proc/vmcore

[matt stuart]

Why doesn't linux create /proc/vmcore while kernel conf marked as does?

my running kernel configuration:
CONFIG_DEBUG_INFO=y
CONFIG_KEXEC=y
CONFIG_CRASH_DUMP=y
CONFIG_MAGIC_SYSRQ=y
CONFIG_PROC_VMCORE=y

my error on the kdump:
kdump-config savecore
running makedumpfile -c -d 31 /proc/vmcore
/var/crash/201305180259/dump-incomplete.
open_dump_memory: Can't open the dump memory(/proc/vmcore). No such
file or directory

makedumpfile Failed.
kdump-config: makedumpfile failed, falling back to 'cp' ... failed!
cp: cannot stat `/proc/vmcore': No such file or directory
kdump-config: failed to save vmcore in /var/crash/201305180259 ... failed!
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