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-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 just a moment ago,
-because it slightly changes the interfaces to accommodate 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).