mm, mmu_notifier: be explicit about range invalition non-blocking mode

If invalidate_range_start() is called for !blocking mode then all
callbacks have to guarantee they will no block/sleep.  The same obviously
applies to invalidate_range_end because this operation pairs with the
former and they are called from the same context.  Make sure this is
appropriately documented.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Michal Hocko <[email protected]>
Reviewed-by: Jerome Glisse <[email protected]>
Cc: Boris Ostrovsky <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Juergen Gross <[email protected]>
Cc: Tetsuo Handa <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

include/linux/mmu_notifier.h

@@ -153,7 +153,9 @@ struct mmu_notifier_ops {
 	 *
 	 * If blockable argument is set to false then the callback cannot
 	 * sleep and has to return with -EAGAIN. 0 should be returned
-	 * otherwise.
+	 * otherwise. Please note that if invalidate_range_start approves
+	 * a non-blocking behavior then the same applies to
+	 * invalidate_range_end.
 	 *
 	 */
 	int (*invalidate_range_start)(struct mmu_notifier *mn,