92 lines
3.8 KiB
ReStructuredText
92 lines
3.8 KiB
ReStructuredText
.. _active_mm:
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=========
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Active MM
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=========
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::
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List: linux-kernel
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Subject: Re: active_mm
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From: Linus Torvalds <torvalds () transmeta ! com>
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Date: 1999-07-30 21:36:24
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Cc'd to linux-kernel, because I don't write explanations all that often,
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and when I do I feel better about more people reading them.
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On Fri, 30 Jul 1999, David Mosberger wrote:
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>
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> Is there a brief description someplace on how "mm" vs. "active_mm" in
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> the task_struct are supposed to be used? (My apologies if this was
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> discussed on the mailing lists---I just returned from vacation and
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> wasn't able to follow linux-kernel for a while).
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Basically, the new setup is:
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- we have "real address spaces" and "anonymous address spaces". The
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difference is that an anonymous address space doesn't care about the
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user-level page tables at all, so when we do a context switch into an
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anonymous address space we just leave the previous address space
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active.
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The obvious use for a "anonymous address space" is any thread that
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doesn't need any user mappings - all kernel threads basically fall into
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this category, but even "real" threads can temporarily say that for
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some amount of time they are not going to be interested in user space,
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and that the scheduler might as well try to avoid wasting time on
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switching the VM state around. Currently only the old-style bdflush
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sync does that.
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- "tsk->mm" points to the "real address space". For an anonymous process,
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tsk->mm will be NULL, for the logical reason that an anonymous process
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really doesn't _have_ a real address space at all.
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- however, we obviously need to keep track of which address space we
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"stole" for such an anonymous user. For that, we have "tsk->active_mm",
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which shows what the currently active address space is.
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The rule is that for a process with a real address space (ie tsk->mm is
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non-NULL) the active_mm obviously always has to be the same as the real
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one.
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For a anonymous process, tsk->mm == NULL, and tsk->active_mm is the
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"borrowed" mm while the anonymous process is running. When the
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anonymous process gets scheduled away, the borrowed address space is
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returned and cleared.
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To support all that, the "struct mm_struct" now has two counters: a
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"mm_users" counter that is how many "real address space users" there are,
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and a "mm_count" counter that is the number of "lazy" users (ie anonymous
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users) plus one if there are any real users.
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Usually there is at least one real user, but it could be that the real
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user exited on another CPU while a lazy user was still active, so you do
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actually get cases where you have a address space that is _only_ used by
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lazy users. That is often a short-lived state, because once that thread
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gets scheduled away in favour of a real thread, the "zombie" mm gets
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released because "mm_users" becomes zero.
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Also, a new rule is that _nobody_ ever has "init_mm" as a real MM any
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more. "init_mm" should be considered just a "lazy context when no other
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context is available", and in fact it is mainly used just at bootup when
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no real VM has yet been created. So code that used to check
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if (current->mm == &init_mm)
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should generally just do
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if (!current->mm)
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instead (which makes more sense anyway - the test is basically one of "do
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we have a user context", and is generally done by the page fault handler
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and things like that).
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Anyway, I put a pre-patch-2.3.13-1 on ftp.kernel.org just a moment ago,
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because it slightly changes the interfaces to accommodate the alpha (who
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would have thought it, but the alpha actually ends up having one of the
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ugliest context switch codes - unlike the other architectures where the MM
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and register state is separate, the alpha PALcode joins the two, and you
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need to switch both together).
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(From http://marc.info/?l=linux-kernel&m=93337278602211&w=2)
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