mm: add documentation describing what tsk->active_mm means vs tsk->mm

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: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 2 insertions, 0 deletions

diff --git a/Documentation/vm/00-INDEX b/Documentation/vm/00-INDEX
index 2131b00b63f6..2f77ced35df7 100644
--- a/Documentation/vm/00-INDEX
+++ b/Documentation/vm/00-INDEX
@@ -1,5 +1,7 @@
 00-INDEX
         - this file.
+active_mm.txt
+        - An explanation from Linus about tsk->active_mm vs tsk->mm.
 balance
         - various information on memory balancing.
 hugetlbpage.txt