Re: [Patch v2 2/2] mm/khugepaged: remove definition of struct khugepaged_mm_slot

[Wei Yang <richard.weiyang@gmail.com>]

On Sat, Sep 20, 2025 at 06:41:30AM -0700, SeongJae Park wrote:
>On Sat, 20 Sep 2025 12:29:58 +0000 Wei Yang <richard.weiyang@gmail.com> wrote:
>
>> On Sat, Sep 20, 2025 at 04:52:33AM -0700, SeongJae Park wrote:
>> [...]
>> >
>> >On latest mm-new tree, I am getting below error while building UML mode kernel
>> >for kunit.  And 'git bisect' points me this patch.  I'm not familiar with this
>> >code and have no time to dive deep for now, so reporting first.
>> 
>> Thanks for reporting.
>
>Thank you for fast reply!
>
>> 
>> >
>> >Oops: general protection fault, probably for non-canonical adI
>> >[  356.456907] CPU: 34 UID: 0 PID: 309 Comm: khugepaged Not tainted 6.17.0-rc4+ #370 PREEMPT(voluntary)
>> >[  356.457702] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-4.el9 04/01/2014
>> >[  356.458484] RIP: 0010:collect_mm_slot (mm/khugepaged.c:1427)
>> 
>> If my understanding is correct, the error happens in following code flow:
>> 
>>   khugepaged_scan_mm_slot()
>>     mm = slot->mm;                (1)
>>     collect_mm_slot()
>>       mm = slot->mm;              (2)
>> 
>> Looks the reason is slot is NULL at (2), but we have already accessed it at (1).
>> 
>> Not find the cause yet.
>> 
>> Would you mind sharing your UML config and test step? So that I can reproduce
>> it.
>
>To reproduce the issue, I do below:
>
>    $ ./tools/testing/kunit/kunit.py run --kunitconfig mm/damon/tests
>
>Then the OOPs happens a few seconds aftr the kunit run, or during the
>kunit-purpose UML build.  The issue sometimes doesn't happen or take time more
>than seconds.  Since the code is related with khugepaged, apparently there are
>some timing factors.  But I was able to reproduce about 3/4 times.

I run Qemu with kernel built from mm-new base commit b8086c280108
"drivers/base: move memory_block_add_nid() into the caller". Then run the
kunit command above.

Not see error for around 20 times of the test.

Is my step correct?

>
>It was also reproducible when I build and install kernel on my QEMU machine
>using 'make install'.
>

Run qemu with the same kernel and do "make clean && make all" several times.

Not spot errors yet.

kselftests/mm/khugepaged pass with this kernel.

Will do kernel build for more time to see the effect.

If my step is not correct, please let me know.

>
>Thanks,
>SJ
>
>[...]

-- 
Wei Yang
Help you, Help me