[PATCH v4 2/4] lazy tlb: allow lazy tlb mm refcounting to be configurable

[Nicholas Piggin]

Excerpts from Andy Lutomirski's message of June 9, 2021 2:20 am:
> On 6/4/21 6:42 PM, Nicholas Piggin wrote:
>> Add CONFIG_MMU_TLB_REFCOUNT which enables refcounting of the lazy tlb mm
>> when it is context switched. This can be disabled by architectures that
>> don't require this refcounting if they clean up lazy tlb mms when the
>> last refcount is dropped. Currently this is always enabled, which is
>> what existing code does, so the patch is effectively a no-op.
>> 
>> Rename rq->prev_mm to rq->prev_lazy_mm, because that's what it is.
> 
> I am in favor of this approach, but I would be a lot more comfortable
> with the resulting code if task->active_mm were at least better
> documented and possibly even guarded by ifdefs.

active_mm is fairly well documented in Documentation/active_mm.rst IMO.
I don't think anything has changed in 20 years, I don't know what more
is needed, but if you can add to documentation that would be nice. Maybe
moving a bit of that into .c and .h files?

> x86 bare metal currently does not need the core lazy mm refcounting, and
> x86 bare metal *also* does not need ->active_mm.  Under the x86 scheme,
> if lazy mm refcounting were configured out, ->active_mm could become a
> dangling pointer, and this makes me extremely uncomfortable.
> 
> So I tend to think that, depending on config, the core code should
> either keep ->active_mm [1] alive or get rid of it entirely.

I don't actually know what you mean.

core code needs the concept of an "active_mm". This is the mm that your 
kernel threads are using, even in the unmerged CONFIG_LAZY_TLB=n patch,
active_mm still points to init_mm for kernel threads.

We could hide that idea behind an active_mm() function that would always 
return &init_mm if mm==NULL, but you still have the concept of an active
mm and a pointer that callers must not access after free (because some
cases will be CONFIG_LAZY_TLB=y).

> [1] I don't really think it belongs in task_struct at all.  It's not a
> property of the task.  It's the *per-cpu* mm that the core code is
> keeping alive for lazy purposes.  How about consolidating it with the
> copy in rq?

I agree it's conceptually a per-cpu property. I don't know why it was 
done this way, maybe it was just convenient and works well for mm and 
active_mm to be adjacent. Linus might have a better insight.

> I guess the short summary of my opinion is that I like making this
> configurable, but I do not like the state of the code.

I don't think I'd object to moving active_mm to rq and converting all
usages to active_mm() while we're there, it would make things a bit
more configurable. But I don't see it making core code fundamentally
less complex... if you're referring to the x86 mm switching monstrosity,
then that's understandable, but I admit I haven't spent enough time
looking at it to make a useful comment. A patch would be enlightening,
I have the leftover CONFIG_LAZY_TLB=n patch if you were thinking of 
building on that I can send it to you.

Thanks,
Nick