Just add a label for cross-referencing and indent the text to make it ``literal`` Signed-off-by: Mike Rapoport <rppt@xxxxxxxxxxxxxxxxxx> --- Documentation/vm/active_mm.txt | 174 +++++++++++++++++++++-------------------- 1 file changed, 91 insertions(+), 83 deletions(-) diff --git a/Documentation/vm/active_mm.txt b/Documentation/vm/active_mm.txt index dbf4581..c84471b 100644 --- a/Documentation/vm/active_mm.txt +++ b/Documentation/vm/active_mm.txt @@ -1,83 +1,91 @@ -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_users" 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) +.. _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_users" 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) -- 2.7.4
