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

00-INDEX

00-INDEX

	- this file.

	- this file.

active_mm.txt

	- An explanation from Linus about tsk->active_mm vs tsk->mm.

balance

balance

	- various information on memory balancing.

	- various information on memory balancing.

hugetlbpage.txt

hugetlbpage.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)