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

[Andy Lutomirski <luto@amacapital.net>]

> On Jul 29, 2019, at 8:03 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> 
>> 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.

I thought the current status was that we don’t always switch to init_mm on idle and instead we use a fancier and actually correct flushing routine that only flushed idle CPUs when pagetables are freed.  I still think we should be able to kill active_mm in favor of explicit refcounting in the arch code.