This patch is the odd-one-out of the sequence.  The one before adjusted
copy_pte_range from a for loop to a do while loop, and it was therefore
simplest to check for lockbreak before copying pte: possibility that it
might keep getting preempted without making progress under some loads.

Some loads such as startup: 2*HT*P4 with preemption cannot even reach
multiuser login.  Suspect needs_lockbreak is broken, can get in a state
when it remains forever true.  Investigate that later: for now, and for
all time, it makes sense to aim for a little progress before breaking
out; and we can manage more pte_nones than copies.
Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>