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a55ce6dc ME |
1 | List: linux-kernel |
2 | Subject: Re: active_mm | |
3 | From: Linus Torvalds <torvalds () transmeta ! com> | |
4 | Date: 1999-07-30 21:36:24 | |
5 | ||
6 | Cc'd to linux-kernel, because I don't write explanations all that often, | |
7 | and when I do I feel better about more people reading them. | |
8 | ||
9 | On Fri, 30 Jul 1999, David Mosberger wrote: | |
10 | > | |
11 | > Is there a brief description someplace on how "mm" vs. "active_mm" in | |
12 | > the task_struct are supposed to be used? (My apologies if this was | |
13 | > discussed on the mailing lists---I just returned from vacation and | |
14 | > wasn't able to follow linux-kernel for a while). | |
15 | ||
16 | Basically, the new setup is: | |
17 | ||
18 | - we have "real address spaces" and "anonymous address spaces". The | |
19 | difference is that an anonymous address space doesn't care about the | |
20 | user-level page tables at all, so when we do a context switch into an | |
21 | anonymous address space we just leave the previous address space | |
22 | active. | |
23 | ||
24 | The obvious use for a "anonymous address space" is any thread that | |
25 | doesn't need any user mappings - all kernel threads basically fall into | |
26 | this category, but even "real" threads can temporarily say that for | |
27 | some amount of time they are not going to be interested in user space, | |
28 | and that the scheduler might as well try to avoid wasting time on | |
29 | switching the VM state around. Currently only the old-style bdflush | |
30 | sync does that. | |
31 | ||
32 | - "tsk->mm" points to the "real address space". For an anonymous process, | |
33 | tsk->mm will be NULL, for the logical reason that an anonymous process | |
34 | really doesn't _have_ a real address space at all. | |
35 | ||
36 | - however, we obviously need to keep track of which address space we | |
37 | "stole" for such an anonymous user. For that, we have "tsk->active_mm", | |
38 | which shows what the currently active address space is. | |
39 | ||
40 | The rule is that for a process with a real address space (ie tsk->mm is | |
41 | non-NULL) the active_mm obviously always has to be the same as the real | |
42 | one. | |
43 | ||
44 | For a anonymous process, tsk->mm == NULL, and tsk->active_mm is the | |
45 | "borrowed" mm while the anonymous process is running. When the | |
46 | anonymous process gets scheduled away, the borrowed address space is | |
47 | returned and cleared. | |
48 | ||
49 | To support all that, the "struct mm_struct" now has two counters: a | |
50 | "mm_users" counter that is how many "real address space users" there are, | |
51 | and a "mm_count" counter that is the number of "lazy" users (ie anonymous | |
52 | users) plus one if there are any real users. | |
53 | ||
54 | Usually there is at least one real user, but it could be that the real | |
55 | user exited on another CPU while a lazy user was still active, so you do | |
56 | actually get cases where you have a address space that is _only_ used by | |
57 | lazy users. That is often a short-lived state, because once that thread | |
58 | gets scheduled away in favour of a real thread, the "zombie" mm gets | |
59 | released because "mm_users" becomes zero. | |
60 | ||
61 | Also, a new rule is that _nobody_ ever has "init_mm" as a real MM any | |
62 | more. "init_mm" should be considered just a "lazy context when no other | |
63 | context is available", and in fact it is mainly used just at bootup when | |
64 | no real VM has yet been created. So code that used to check | |
65 | ||
66 | if (current->mm == &init_mm) | |
67 | ||
68 | should generally just do | |
69 | ||
70 | if (!current->mm) | |
71 | ||
72 | instead (which makes more sense anyway - the test is basically one of "do | |
73 | we have a user context", and is generally done by the page fault handler | |
74 | and things like that). | |
75 | ||
76 | Anyway, I put a pre-patch-2.3.13-1 on ftp.kernel.org just a moment ago, | |
25985edc | 77 | because it slightly changes the interfaces to accommodate the alpha (who |
a55ce6dc ME |
78 | would have thought it, but the alpha actually ends up having one of the |
79 | ugliest context switch codes - unlike the other architectures where the MM | |
80 | and register state is separate, the alpha PALcode joins the two, and you | |
81 | need to switch both together). | |
82 | ||
83 | (From http://marc.info/?l=linux-kernel&m=93337278602211&w=2) |