I'm sure everyone knows this, but I didn't, so I googled it, and found a
nice explanation from Linus.  Might be worth sticking in Documentation.
Signed-off-by: NMichael Ellerman <michael@ellerman.id.au>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>