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

[Nicholas Piggin <npiggin@gmail.com>]

Excerpts from Nicholas Piggin's message of June 14, 2021 2:47 pm:
> Excerpts from Nicholas Piggin's message of June 14, 2021 2:14 pm:
>> Excerpts from Andy Lutomirski's message of June 14, 2021 1:52 pm:
>>> On 6/13/21 5:45 PM, Nicholas Piggin wrote:
>>>> 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?
>>>> 
>>> 
>>> Quoting from that file:
>>> 
>>>   - however, we obviously need to keep track of which address space we
>>>     "stole" for such an anonymous user. For that, we have "tsk->active_mm",
>>>     which shows what the currently active address space is.
>>> 
>>> This isn't even true right now on x86.
>> 
>> From the perspective of core code, it is. x86 might do something crazy 
>> with it, but it has to make it appear this way to non-arch code that
>> uses active_mm.
>> 
>> Is x86's scheme documented?
>> 
>>> With your patch applied:
>>> 
>>>  To support all that, the "struct mm_struct" now has two counters: a
>>>  "mm_users" counter that is how many "real address space users" there are,
>>>  and a "mm_count" counter that is the number of "lazy" users (ie anonymous
>>>  users) plus one if there are any real users.
>>> 
>>> isn't even true any more.
>> 
>> Well yeah but the active_mm concept hasn't changed. The refcounting 
>> change is hopefully reasonably documented?
>> 
>>> 
>>> 
>>>>> 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.
>>> 
>>> Core code does *not* need this concept.  First, it's wrong on x86 since
>>> at least 4.15.  Any core code that actually assumes that ->active_mm is
>>> "active" for any sensible definition of the word active is wrong.
>>> Fortunately there is no such code.
>>> 
>>> I looked through all active_mm references in core code.  We have:
>>> 
>>> kernel/sched/core.c: it's all refcounting, although it's a bit tangled
>>> with membarrier.
>>> 
>>> kernel/kthread.c: same.  refcounting and membarrier stuff.
>>> 
>>> kernel/exit.c: exit_mm() a BUG_ON().
>>> 
>>> kernel/fork.c: initialization code and a warning.
>>> 
>>> kernel/cpu.c: cpu offline stuff.  wouldn't be needed if active_mm went away.
>>> 
>>> fs/exec.c: nothing of interest
>> 
>> I might not have been clear. Core code doesn't need active_mm if 
>> active_mm somehow goes away. I'm saying active_mm can't go away because
>> it's needed to support (most) archs that do lazy tlb mm switching.
>> 
>> The part I don't understand is when you say it can just go away. How? 
>> 
>>> I didn't go through drivers, but I maintain my point.  active_mm is
>>> there for refcounting.  So please don't just make it even more confusing
>>> -- do your performance improvement, but improve the code at the same
>>> time: get rid of active_mm, at least on architectures that opt out of
>>> the refcounting.
>> 
>> powerpc opts out of the refcounting and can not "get rid of active_mm".
>> Not even in theory.
> 
> That is to say, it does do a type of reference management that requires 
> active_mm so you can argue it has not entirely opted out of refcounting.
> But we're not just doing refcounting for the sake of refcounting! That
> would make no sense.
> 
> active_mm is required because that's the mm that we have switched to 
> (from core code's perspective), and it is integral to know when to 
> switch to a different mm. See how active_mm is a fundamental concept
> in core code? It's part of the contract between core code and the
> arch mm context management calls. reference counting follows from there
> but it's not the _reason_ for this code.
> 
> Pretend the reference problem does not exit (whether by refcounting or 
> shootdown or garbage collection or whatever). We still can't remove 
> active_mm! We need it to know how to call into arch functions like 
> switch_mm.
> 
> I don't know if you just forgot that critical requirement in your above 
> list, or you actually are entirely using x86's mental model for this 
> code which is doing something entirely different that does not need it 
> at all. If that is the case I really don't mind some cleanup or wrapper 
> functions for x86 do entirely do its own thing, but if that's the case
> you can't criticize core code's use of active_mm due to the current
> state of x86. It's x86 that needs documentation and cleaning up.

Ah, that must be where your confusion is coming from: x86's switch_mm 
doesn't use prev anywhere, and the reference scheme it is using appears 
to be under-documented, although vague references in changelogs suggest 
it has not actually "opted out" of active_mm refcounting.

That's understandable, but please redirect your objections to the proper 
place. git blame suggests 3d28ebceaffab.

Thanks,
Nick