Note, the mm_count refcount may no longer include the "lazy" users (running tasks with ->active_mm == mm && ->mm == NULL) on kernels with CONFIG_MMU_LAZY_TLB_REFCOUNT=n. Taking and releasing these lazy references must be done with mmgrab_lazy_tlb() and mmdrop_lazy_tlb() helpers, which abstract this config option.
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)