Messages in this thread | | | Subject | Re: [PATCH v4 2/4] lazy tlb: allow lazy tlb mm refcounting to be configurable | From | Andy Lutomirski <> | Date | Tue, 8 Jun 2021 09:20:05 -0700 |
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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
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