1========= 2Active MM 3========= 4 5Note, the mm_count refcount may no longer include the "lazy" users 6(running tasks with ->active_mm == mm && ->mm == NULL) on kernels 7with CONFIG_MMU_LAZY_TLB_REFCOUNT=n. Taking and releasing these lazy 8references must be done with mmgrab_lazy_tlb() and mmdrop_lazy_tlb() 9helpers, which abstract this config option. 10 11:: 12 13 List: linux-kernel 14 Subject: Re: active_mm 15 From: Linus Torvalds <torvalds () transmeta ! com> 16 Date: 1999-07-30 21:36:24 17 18 Cc'd to linux-kernel, because I don't write explanations all that often, 19 and when I do I feel better about more people reading them. 20 21 On Fri, 30 Jul 1999, David Mosberger wrote: 22 > 23 > Is there a brief description someplace on how "mm" vs. "active_mm" in 24 > the task_struct are supposed to be used? (My apologies if this was 25 > discussed on the mailing lists---I just returned from vacation and 26 > wasn't able to follow linux-kernel for a while). 27 28 Basically, the new setup is: 29 30 - we have "real address spaces" and "anonymous address spaces". The 31 difference is that an anonymous address space doesn't care about the 32 user-level page tables at all, so when we do a context switch into an 33 anonymous address space we just leave the previous address space 34 active. 35 36 The obvious use for a "anonymous address space" is any thread that 37 doesn't need any user mappings - all kernel threads basically fall into 38 this category, but even "real" threads can temporarily say that for 39 some amount of time they are not going to be interested in user space, 40 and that the scheduler might as well try to avoid wasting time on 41 switching the VM state around. Currently only the old-style bdflush 42 sync does that. 43 44 - "tsk->mm" points to the "real address space". For an anonymous process, 45 tsk->mm will be NULL, for the logical reason that an anonymous process 46 really doesn't _have_ a real address space at all. 47 48 - however, we obviously need to keep track of which address space we 49 "stole" for such an anonymous user. For that, we have "tsk->active_mm", 50 which shows what the currently active address space is. 51 52 The rule is that for a process with a real address space (ie tsk->mm is 53 non-NULL) the active_mm obviously always has to be the same as the real 54 one. 55 56 For a anonymous process, tsk->mm == NULL, and tsk->active_mm is the 57 "borrowed" mm while the anonymous process is running. When the 58 anonymous process gets scheduled away, the borrowed address space is 59 returned and cleared. 60 61 To support all that, the "struct mm_struct" now has two counters: a 62 "mm_users" counter that is how many "real address space users" there are, 63 and a "mm_count" counter that is the number of "lazy" users (ie anonymous 64 users) plus one if there are any real users. 65 66 Usually there is at least one real user, but it could be that the real 67 user exited on another CPU while a lazy user was still active, so you do 68 actually get cases where you have a address space that is _only_ used by 69 lazy users. That is often a short-lived state, because once that thread 70 gets scheduled away in favour of a real thread, the "zombie" mm gets 71 released because "mm_count" becomes zero. 72 73 Also, a new rule is that _nobody_ ever has "init_mm" as a real MM any 74 more. "init_mm" should be considered just a "lazy context when no other 75 context is available", and in fact it is mainly used just at bootup when 76 no real VM has yet been created. So code that used to check 77 78 if (current->mm == &init_mm) 79 80 should generally just do 81 82 if (!current->mm) 83 84 instead (which makes more sense anyway - the test is basically one of "do 85 we have a user context", and is generally done by the page fault handler 86 and things like that). 87 88 Anyway, I put a pre-patch-2.3.13-1 on ftp.kernel.org just a moment ago, 89 because it slightly changes the interfaces to accommodate the alpha (who 90 would have thought it, but the alpha actually ends up having one of the 91 ugliest context switch codes - unlike the other architectures where the MM 92 and register state is separate, the alpha PALcode joins the two, and you 93 need to switch both together). 94 95 (From http://marc.info/?l=linux-kernel&m=93337278602211&w=2) 96