I got Panic message of "kernel BUG at mm/rmap.c:500!", why?

Issue

• A x86 server running Red Hat Enterprise Linux 4.7 got panic with back trace printed in /var/log/messages ”BUG at mm/rmap.c:500!“
• With these call trace in vmcore:

  [c4437ca8] die at c0106049
  [c4437cdc] do_invalid_op at c0106424
  [c4437d8c] error_code (via invalid_op) at c02e14d1
  EAX: ffffffff  EBX: c201fb60  ECX: c38551c0  EDX: c033cff0  EBP: fff690a8
  DS:  007b      ESI: 00000000  ES:  007b      EDI: d994d338
  CS:  0060      EIP: c01542cb  ERR: ffffffff  EFLAGS: 00210286
  [c4437dc8] page_remove_rmap at c01542cb
  [c4437dd4] zap_pte_range at c014dfb5
  [c4437e08] zap_pmd_range at c014e0d7
  [c4437e24] unmap_page_range at c014e136
  [c4437e40] unmap_vmas at c014e24a
  [c4437e78] exit_mmap at c0152577
  [c4437ea0] mmput at c0120995
  [c4437ea8] do_exit at c0124a7b
  [c4437ec0] do_group_exit at c0124d7f
  [c4437ed8] get_signal_to_deliver at c012d1c9
  [c4437f00] do_signal at c0105bd4
  [c4437fb8] do_notify_resume at c0105c80
  [c4437fc0] system_call at c02e0a39
  EAX: 89f85d43  EBX: b746ac90  ECX: b746ad0f  EDX: b746aaf8
  DS:  007b      ESI: 0000000d  ES:  007b      EDI: 0000000e
  SS:  007b      ESP: b7468ff2  EBP: b746ac7b
  CS:  0073      EIP: 65756d93  ERR: ffffffff  EFLAGS: 0021029