Re: active_mm and mm

["Stephen C. Tweedie" <sct@redhat.com>]

Hi,

On Mon, Aug 19, 2002 at 10:09:50PM -0400, Hai Huang wrote:
> In struct task_struct, what's the difference between active_mm and mm?  I
> vaguely remembers it's used for reducing cache overhead during context
> switch, is this right

Yep.  Many context switches don't require us to switch to the mm of
the newly running process.  All processes share exactly the same
kernel address space, so as long as we are only accessing kernel
memory and not per-process memory, we don't need to do the mm switch.

So, for operations such as waiting on an IO event, a process might get
woken up, check some kernel space data structures, and go back to
sleep, all in side a system call and never touching user space.  It's
a waste to switch to the process's mm just for that --- we'd end up
throwing out the tlb cache of the old process for nothing.

So, Linux has a "LAZY_TLB" mode which tasks such as the idle task
(which never touch user space) all have set, and which tasks can enter
if they are spinning in kernel space for a while.  When we switch to a
LAZY_TLB task, we don't get a new mm, so the new task's active_mm
is set to whatever the old task's active_mm was.  For non-LAZY_TLB
running tasks, active_mm and mm should be the same.

--Stephen
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