installworld error: ad0: FAILURE - READ_DMA status=51<READY, DSC, ERROR> error=40<UNCORRECTABLE> LBA=28071584

I got this error on make installworld:

===> sbin/dhclient (install)
install -s -o root -g wheel -m 555   dhclient /sbin
ad0: FAILURE - READ_DMA status=51<READY,DSC,ERROR> error=40<UNCORRECTABLE> LBA=28071584
g_vfs_done():ad0a[READ(offset=14372651008, length=16384)]error = 5
vnode_pager_getpages: I/O read error
vm_fault: pager read error, pid 12794 (install)
install: /sbin/dhclient: Bad address
*** [_proginstall] Error code 71

Stop in /usr/src/sbin/dhclient.
*** [realinstall] Error code 1

Stop in /usr/src/sbin.
*** [realinstall] Error code 1

Stop in /usr/src.
*** [reinstall] Error code 1

Stop in /usr/src.
*** [installworld] Error code 1

Stop in /usr/src.
*** Error code 1

Stop in /usr/src.


What's going on?

This is on sparc64 SunBlade 1500 silver,
updating to r244988.

The disk is:

# gpart show ad0
=>        0  234432720  ad0  VTOC8  (111G)
          0   41946480    1  !0  (20G)
   41946480   33558000    2  !0  (16G)
   75504480  158928240    4  !0  (75G)

# 

# df
Filesystem 512-blocks     Used    Avail Capacity  Mounted on
/dev/ad0a    40620236 21769888 15600732    58%    /
devfs               2        2        0   100%    /dev
# 

I've run fsck -p before installworld,
and didn't see any problems.

Thanks

Anton