Re: [MOCKUP] x86/mm: Lightweight lazy mm refcounting

[Rik van Riel]

On Thu, 2020-12-03 at 12:31 +0000, Matthew Wilcox wrote:

> And this just makes me think RCU freeing of mm_struct.  I'm sure it's
> more complicated than that (then, or now), but if an anonymous
> process
> is borrowing a freed mm, and the mm is freed by RCU then it will not
> go
> away until the task context switches.  When we context switch back to
> the anon task, it'll borrow some other task's MM and won't even
> notice
> that the MM it was using has gone away.

One major complication here is that most of the
active_mm borrowing is done by the idle task,
but RCU does not wait for idle tasks to context
switch.

That means RCU, as it is today, is not a
mechanism that mm_struct freeing could just
piggyback off.

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