Adding upstream version 5.10.209.upstream/5.10.209upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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diff --git a/Documentation/vm/active_mm.rst b/Documentation/vm/active_mm.rst
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+.. _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_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)