Re: [PATCH 10/10] mm,sched: conditionally skip lazy TLB mm refcounting

[Rik van Riel]

On Sun, 2018-07-29 at 08:29 -0700, Andy Lutomirski wrote:
> > On Jul 29, 2018, at 5:11 AM, Rik van Riel <riel@xxxxxxxxxxx> wrote:
> > 
> > > On Sat, 2018-07-28 at 21:21 -0700, Andy Lutomirski wrote:
> > > On Sat, Jul 28, 2018 at 2:53 PM, Rik van Riel <riel@xxxxxxxxxxx>
> > > wrote:
> > > > Conditionally skip lazy TLB mm refcounting. When an
> > > > architecture
> > > > has
> > > > CONFIG_ARCH_NO_ACTIVE_MM_REFCOUNTING enabled, an mm that is
> > > > used in
> > > > lazy TLB mode anywhere will get shot down from exit_mmap, and
> > > > there
> > > > in no need to incur the cache line bouncing overhead of
> > > > refcounting
> > > > a lazy TLB mm.
> > > 
> > > Unless I've misunderstood something, this patch results in idle
> > > tasks
> > > whose active_mm has been freed still having active_mm pointing at
> > > freed memory. 
> > 
> > Patch 9/10 is supposed to ensure that the lazy TLB CPUs get
> > switched to init_mm before an mm is freed. No CPU should ever
> > have its active_mm pointing at a freed mm.
> > 
> > Your message made me re-read the code, and now I realize that
> > leave_mm does not actually do that.
> > 
> > Looking at the other callers of leave_mm, I might not be the
> > only one surprised by that; xen_drop_mm_ref comes to mind.
> > 
> > I guess I should some code to leave_mm to have it actually
> > clear active_mm and call the conditional refcount drop helper
> > function.
> > 
> > Does that clear up the confusion?
> 
> Kind of. But whatâs the point of keeping active_mm?  On architectures
> that opt in to the new mode, there wonât be any code that cares about
> itâs value.  Whatâs the benefit of keeping it around?  If you ifdef
> it out, then it canât possibly point to freed memory, and thereâs
> nothing to worry about.

I would like to get to that point, but in a way that does
not leave the code too difficult to follow.

Getting rid of ->active_mm in context_switch() is straightforward,
but I am not sure at all what to do about idle_task_exit() for
example.

All the subtleties I ran into just with this phase of the
code suggests (to me at least) that we should probably do
this one step at a time.

I agree on the same end goal, though :)

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