Re: [PATCH v2] sched/core: Don't use dying mm as active_mm of kthreads

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Jul 29, 2019 at 10:51:51AM -0400, Waiman Long wrote:
> On 7/29/19 4:52 AM, Peter Zijlstra wrote:
> > On Sat, Jul 27, 2019 at 01:10:47PM -0400, Waiman Long wrote:
> >> It was found that a dying mm_struct where the owning task has exited
> >> can stay on as active_mm of kernel threads as long as no other user
> >> tasks run on those CPUs that use it as active_mm. This prolongs the
> >> life time of dying mm holding up memory and other resources like swap
> >> space that cannot be freed.
> > Sure, but this has been so 'forever', why is it a problem now?
> 
> I ran into this probem when running a test program that keeps on
> allocating and touch memory and it eventually fails as the swap space is
> full. After the failure, I could not rerun the test program again
> because the swap space remained full. I finally track it down to the
> fact that the mm stayed on as active_mm of kernel threads. I have to
> make sure that all the idle cpus get a user task to run to bump the
> dying mm off the active_mm of those cpus, but this is just a workaround,
> not a solution to this problem.

The 'sad' part is that x86 already switches to init_mm on idle and we
only keep the active_mm around for 'stupid'.

Rik and Andy were working on getting that 'fixed' a while ago, not sure
where that went.