fatal error: failed to mmap new message file

[Adam Tauno Williams]

I updated one of my production boxes to Cyrus IMAPd 2.4.12 [using
Simon's excellent packages].

On my test box / replica I was able to reconstruct all the mailboxes.

But [of course] on the production box I have a few mailboxes [important
ones, of course] that fail a reconstruct.

$ /usr/lib/cyrus-imapd/reconstruct -r user.steve
fatal error: failed to mmap new message file
$ /usr/lib/cyrus-imapd/reconstruct -r user.barnosky
fatal error: failed to mmap new message file

Reconstructing other mailboxes is working.  Hints / Tips?  reconstruct
doesn't seem top have a verbose / debug switch.