Re: [RFC 06/10] Reclaim memory from blocked kernel stacks

[Peter Zijlstra]

On Fri, Aug 28, 2026 at 03:59:47PM +0200, Peter Zijlstra wrote:

> > What also should be simpler (and I am not saying just to move you away
> > from the scheduler) is to have a shrinker which iterates over all tasks
> > which are marked for reclaim and then similar to swap just unmap both
> > stack pages and release the second page which is not used.
> > Upon wake up the task should create a page_fault which would be used to
> > allocate the second stack page and map the whole stack again.
> 
> Right, so you can FREEZE the task, unmap its stack and then thaw it or
> something. But there should be a definite opt-out on all this, because
> taking faults on your stack will be horrible.
> 
> Not to mention you'll suffer wakeup latencies while frozen.
> 
> This all really sounds like what should be addressed is this insane
> number of tasks rather than trying to cope with the consequences of
> that.

Taking faults on the task-stack is not going to work. That fault will
happen while you have locks held and IRQs disabled. Ideally, it'll
happen when you're knee deep in the allocator.

That fault will then again call the allocator to allocator your stack
page, and...