key_buffer_size at 6GB but ipcs -m only shows 5GB (no replies)

OS: Linux 2.6.34.7 x86_64
MySQL: 5.1.52-1 (Binary RPM)
RAM: 24GB

I have the key_buffer_size set to 6144M in /etc/my.cnf. It is also set up to use LARGE PAGES. In /proc/meminfo, it showed that of the 8GB of pages available, it only reserved and used 5GB:

HugePages_Total: 4096
HugePages_Free: 1598
HugePages_Rsvd: 0
HugePages_Surp: 0
Hugepagesize: 2048 kB

The output of 'ipcs -m' shows the following:

------ Shared Memory Segments --------
key shmid owner perms bytes nattch status
0x00000000 32768 mysql 600 5238685696 1 dest

However, MySQL believes it has 6GB of key_buffer as shown by the output of mysqlreport:

MySQL 5.1.52-log uptime 1 16:51:47 Fri Dec 31 09:42:51 2010

__ Key _________________________________________________________________
Buffer used 4.88G of 6.00G %Used: 81.30
Current 6.00G %Usage: 100.00
Write hit 14.34%
Read hit 100.00%

/proc/<PID>/numa_maps for this process shows the following for the huge pages portion of its address space:

7fbf6f400000 default file=/SYSV00000000\040(deleted) huge dirty=2498 N0=2498

Of course, 2498 * 2MB (page size) is 5GB.

So what gives? There aren't any errors to speak of in the mysql error log, nor in the system log. Have I run into some hard limit of the software?