84 lines
3.7 KiB
Plaintext
84 lines
3.7 KiB
Plaintext
|
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)
|