So lazy MMU mode is vulnerable to interrupts coming in and issuing 
kmap_atomic, which does not work when under lazy MMU mode.  The window 
for this is small, but it means highmem kernels, especially with heavy 
network, USB, or AIO workloads are vulnerable to getting invariably 
fatal pagefaults in interrupt handlers.  For now, the best fix is to 
simply disable and re-enable interrupts when entering and exiting lazy 
mode (which, btw, is already guaranteed to have preempt disabled).  For 
the future, a better fix is to simply exit lazy mode when issuing 
kmap_atomic, but I do not want to touch any generic code now for 2.6.21.

Hopefully there is still time to apply it.  Thanks to Jeremy 
Fitzhardinge for pointing this out.

Zach