Re: [PATCH 1/7] kthread: Don't allocate kthread_struct for init and umh

[Thomas Gleixner <tglx@linutronix.de>]

On Fri, May 06 2022 at 09:15, Eric W. Biederman wrote:
>  	 * the init task will end up wanting to create kthreads, which, if
>  	 * we schedule it before we create kthreadd, will OOPS.
>  	 */
> -	pid = kernel_thread(kernel_init, NULL, CLONE_FS);
> +	pid = user_mode_thread(kernel_init, NULL, CLONE_FS);

So init does not have PF_KTHREAD set anymore, which causes this to go
sideways with a NULL pointer dereference in get_mm_counter() on next:

 get_mm_counter include/linux/mm.h:1996 [inline]
 get_mm_rss include/linux/mm.h:2049 [inline]
 task_nr_scan_windows.isra.0+0x23/0x120 kernel/sched/fair.c:1123
 task_scan_min kernel/sched/fair.c:1144 [inline]
 task_scan_start+0x6c/0x400 kernel/sched/fair.c:1150
 task_tick_numa kernel/sched/fair.c:2944 [inline]
 task_tick_fair+0xaeb/0xef0 kernel/sched/fair.c:11186
 scheduler_tick+0x20a/0x5e0 kernel/sched/core.c:5380

 https://lore.kernel.org/lkml/0000000000008a9fbb05dea76400@google.com

because the fence in task_tick_numa():

 	if ((curr->flags & (PF_EXITING | PF_KTHREAD)) || work->next != work)
		return;

is not longer sufficient. It needs also to bail if !curr->mm.

I'm worried that there are more of these issues lurking. Haven't looked
yet.

Thanks,

        tglx