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From:   "Eric W. Biederman" <ebiederm@xmission.com>
To:     Linus Torvalds <torvalds@linuxfoundation.org>
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Date:   Wed, 11 May 2022 12:53:00 -0500
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        (Linus Torvalds's message of "Wed, 11 May 2022 10:42:21 -0700")
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Subject: Re: [PATCH 8/7] sched: Update task_tick_numa to ignore tasks
 without an mm
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Linus Torvalds <torvalds@linuxfoundation.org> writes:

 On Wed, May 11, 2022 at 10:37 AM Eric W. Biederman
> <ebiederm@xmission.com> wrote:
>>
>> With the change to init and the user mode helper processes to not have
>> PF_KTHREAD set before they call kernel_execve the PF_KTHREAD test in
>> task_tick_numa became insufficient to detect all tasks that have
>> "->mm == NULL".  Correct that by testing for "->mm == NULL" directly.
>
> If you end up rebasing at any time for other reasons (I didn't even
> check if you keep this series in a public git branch), please just
> fold this fix into the original commit, so that we don't have
> unnecessary bisection issues.

I do have it in a public git branch.  The testing in linux-next
is what revealed this.

However it is a topic branch that as far as I know no one depends
on so I should be able to rebase it.

I can rearrange the patches and tweak the description a bit.
Say:

    sched: Update task_tick_numa to ignore tasks without an mm

    With the change to init and the user mode helper processes to not have
    PF_KTHREAD set before they call kernel_execve the PF_KTHREAD test in
    task_tick_numa became insufficient to detect all tasks that have
    "->mm == NULL".  Correct that by testing for "->mm == NULL" directly.

    During testing Qian Cai <quic_qiancai@quicinc.com> found this and wrote:
    > Reverting the last 3 commits of the series fixed a boot crash.
    >
    > 1b2552cbdbe0 fork: Stop allowing kthreads to call execve
    > 753550eb0ce1 fork: Explicitly set PF_KTHREAD
    > 68d85f0a33b0 init: Deal with the init process being a user mode process
    >
    >  BUG: KASAN: null-ptr-deref in task_nr_scan_windows.isra.0
    >  arch_atomic_long_read at ./include/linux/atomic/atomic-long.h:29
    >  (inlined by) atomic_long_read at ./include/linux/atomic/atomic-instrumented.h:1266
    >  (inlined by) get_mm_counter at ./include/linux/mm.h:1996
    >  (inlined by) get_mm_rss at ./include/linux/mm.h:2049
    >  (inlined by) task_nr_scan_windows at kernel/sched/fair.c:1123
    >  Read of size 8 at addr 00000000000003d0 by task swapper/0/1
    
    Reported-by: Qian Cai <quic_qiancai@quicinc.com>
    Tested-by: Qian Cai <quic_qiancai@quicinc.com>
    Link: https://lkml.kernel.org/r/87r150ug1l.fsf_-_@email.froward.int.ebiederm.org
    Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

Does that sound reasonable?

Eric