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>

Documentation/vm/00-INDEX

 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

Documentation/vm/active_mm.txt

+List:       linux-kernel
+Subject:    Re: active_mm
+From:       Linus Torvalds <torvalds () transmeta ! com>
+Date:       1999-07-30 21:36:24
+
+Cc'd to linux-kernel, because I don't write explanations all that often,
+and when I do I feel better about more people reading them.
+
+On Fri, 30 Jul 1999, David Mosberger wrote:
+>
+> Is there a brief description someplace on how "mm" vs. "active_mm" in
+> the task_struct are supposed to be used?  (My apologies if this was
+> discussed on the mailing lists---I just returned from vacation and
+> wasn't able to follow linux-kernel for a while).
+
+Basically, the new setup is:
+
+ - we have "real address spaces" and "anonymous address spaces". The
+   difference is that an anonymous address space doesn't care about the
+   user-level page tables at all, so when we do a context switch into an
+   anonymous address space we just leave the previous address space
+   active.
+
+   The obvious use for a "anonymous address space" is any thread that
+   doesn't need any user mappings - all kernel threads basically fall into
+   this category, but even "real" threads can temporarily say that for
+   some amount of time they are not going to be interested in user space,
+   and that the scheduler might as well try to avoid wasting time on
+   switching the VM state around. Currently only the old-style bdflush
+   sync does that.
+
+ - "tsk->mm" points to the "real address space". For an anonymous process,
+   tsk->mm will be NULL, for the logical reason that an anonymous process
+   really doesn't _have_ a real address space at all.
+
+ - however, we obviously need to keep track of which address space we
+   "stole" for such an anonymous user. For that, we have "tsk->active_mm",
+   which shows what the currently active address space is.
+
+   The rule is that for a process with a real address space (ie tsk->mm is
+   non-NULL) the active_mm obviously always has to be the same as the real
+   one.
+
+   For a anonymous process, tsk->mm == NULL, and tsk->active_mm is the
+   "borrowed" mm while the anonymous process is running. When the
+   anonymous process gets scheduled away, the borrowed address space is
+   returned and cleared.
+
+To support all that, the "struct mm_struct" now has two counters: a
+"mm_users" counter that is how many "real address space users" there are,
+and a "mm_count" counter that is the number of "lazy" users (ie anonymous
+users) plus one if there are any real users.
+
+Usually there is at least one real user, but it could be that the real
+user exited on another CPU while a lazy user was still active, so you do
+actually get cases where you have a address space that is _only_ used by
+lazy users. That is often a short-lived state, because once that thread
+gets scheduled away in favour of a real thread, the "zombie" mm gets
+released because "mm_users" becomes zero.
+
+Also, a new rule is that _nobody_ ever has "init_mm" as a real MM any
+more. "init_mm" should be considered just a "lazy context when no other
+context is available", and in fact it is mainly used just at bootup when
+no real VM has yet been created. So code that used to check
+
+	if (current->mm == &init_mm)
+
+should generally just do
+
+	if (!current->mm)
+
+instead (which makes more sense anyway - the test is basically one of "do
+we have a user context", and is generally done by the page fault handler
+and things like that).
+
+Anyway, I put a pre-patch-2.3.13-1 on ftp.kernel.org just a moment ago,
+because it slightly changes the interfaces to accomodate the alpha (who
+would have thought it, but the alpha actually ends up having one of the
+ugliest context switch codes - unlike the other architectures where the MM
+and register state is separate, the alpha PALcode joins the two, and you
+need to switch both together).
+
+(From http://marc.info/?l=linux-kernel&m=93337278602211&w=2)