1List: linux-kernel
2Subject: Re: active_mm
3From: Linus Torvalds <torvalds () transmeta ! com>
4Date: 1999-07-30 21:36:24
6Cc'd to linux-kernel, because I don't write explanations all that often,
7and when I do I feel better about more people reading them.
9On Fri, 30 Jul 1999, David Mosberger wrote:
11> Is there a brief description someplace on how "mm" vs. "active_mm" in
12> the task_struct are supposed to be used? (My apologies if this was
13> discussed on the mailing lists---I just returned from vacation and
14> wasn't able to follow linux-kernel for a while).
16Basically, the new setup is:
18 - we have "real address spaces" and "anonymous address spaces". The
19   difference is that an anonymous address space doesn't care about the
20   user-level page tables at all, so when we do a context switch into an
21   anonymous address space we just leave the previous address space
22   active.
24   The obvious use for a "anonymous address space" is any thread that
25   doesn't need any user mappings - all kernel threads basically fall into
26   this category, but even "real" threads can temporarily say that for
27   some amount of time they are not going to be interested in user space,
28   and that the scheduler might as well try to avoid wasting time on
29   switching the VM state around. Currently only the old-style bdflush
30   sync does that.
32 - "tsk->mm" points to the "real address space". For an anonymous process,
33   tsk->mm will be NULL, for the logical reason that an anonymous process
34   really doesn't _have_ a real address space at all.
36 - however, we obviously need to keep track of which address space we
37   "stole" for such an anonymous user. For that, we have "tsk->active_mm",
38   which shows what the currently active address space is.
40   The rule is that for a process with a real address space (ie tsk->mm is
41   non-NULL) the active_mm obviously always has to be the same as the real
42   one.
44   For a anonymous process, tsk->mm == NULL, and tsk->active_mm is the
45   "borrowed" mm while the anonymous process is running. When the
46   anonymous process gets scheduled away, the borrowed address space is
47   returned and cleared.
49To support all that, the "struct mm_struct" now has two counters: a
50"mm_users" counter that is how many "real address space users" there are,
51and a "mm_count" counter that is the number of "lazy" users (ie anonymous
52users) plus one if there are any real users.
54Usually there is at least one real user, but it could be that the real
55user exited on another CPU while a lazy user was still active, so you do
56actually get cases where you have a address space that is _only_ used by
57lazy users. That is often a short-lived state, because once that thread
58gets scheduled away in favour of a real thread, the "zombie" mm gets
59released because "mm_users" becomes zero.
61Also, a new rule is that _nobody_ ever has "init_mm" as a real MM any
62more. "init_mm" should be considered just a "lazy context when no other
63context is available", and in fact it is mainly used just at bootup when
64no real VM has yet been created. So code that used to check
66    if (current->mm == &init_mm)
68should generally just do
70    if (!current->mm)
72instead (which makes more sense anyway - the test is basically one of "do
73we have a user context", and is generally done by the page fault handler
74and things like that).
76Anyway, I put a pre-patch-2.3.13-1 on just a moment ago,
77because it slightly changes the interfaces to accommodate the alpha (who
78would have thought it, but the alpha actually ends up having one of the
79ugliest context switch codes - unlike the other architectures where the MM
80and register state is separate, the alpha PALcode joins the two, and you
81need to switch both together).

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