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Date:   Mon, 14 Jun 2021 10:45:22 +1000
From:   Nicholas Piggin <npiggin@gmail.com>
Subject: Re: [PATCH v4 2/4] lazy tlb: allow lazy tlb mm refcounting to be
 configurable
To:     Andrew Morton <akpm@linux-foundation.org>,
        Andy Lutomirski <luto@kernel.org>
Cc:     Anton Blanchard <anton@ozlabs.org>, linux-arch@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        linuxppc-dev@lists.ozlabs.org,
        Randy Dunlap <rdunlap@infradead.org>,
        Linus Torvalds <torvalds@linux-foundation.org>
References: <20210605014216.446867-1-npiggin@gmail.com>
        <20210605014216.446867-3-npiggin@gmail.com>
        <8ac1d420-b861-f586-bacf-8c3949e9b5c4@kernel.org>
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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.
>>=20
>> Rename rq->prev_mm to rq->prev_lazy_mm, because that's what it is.
>=20
> 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.
>=20
> 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=20
kernel threads are using, even in the unmerged CONFIG_LAZY_TLB=3Dn patch,
active_mm still points to init_mm for kernel threads.

We could hide that idea behind an active_mm() function that would always=20
return &init_mm if mm=3D=3DNULL, but you still have the concept of an activ=
e
mm and a pointer that callers must not access after free (because some
cases will be CONFIG_LAZY_TLB=3Dy).

> [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=20
done this way, maybe it was just convenient and works well for mm and=20
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=3Dn patch if you were thinking of=20
building on that I can send it to you.

Thanks,
Nick