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

[Andy Lutomirski <luto@kernel.org>]

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.

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.

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

--Andy