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- .. _active_mm:
- =========
- Active MM
- =========
- ::
- 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_count" 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 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).
- (From http://marc.info/?l=linux-kernel&m=93337278602211&w=2)
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