Re: [syzbot] [mm?] WARNING in memory_failure

[jane.chu@oracle.com]

On 10/2/2025 6:54 AM, Zi Yan wrote:
> On 2 Oct 2025, at 1:23, jane.chu@oracle.com wrote:
> 
>> On 10/1/2025 7:04 PM, Zi Yan wrote:
>>> On 1 Oct 2025, at 20:38, Zi Yan wrote:
>>>
>>>> On 1 Oct 2025, at 19:58, jane.chu@oracle.com wrote:
>>>>
>>>>> Hi, Zi Yan,
>>>>>
>>>>> On 9/30/2025 9:51 PM, syzbot wrote:
>>>>>> Hello,
>>>>>>
>>>>>> syzbot has tested the proposed patch but the reproducer is still triggering an issue:
>>>>>> lost connection to test machine
>>>>>>
>>>>>>
>>>>>>
>>>>>> Tested on:
>>>>>>
>>>>>> commit:         d8795075 mm/huge_memory: do not change split_huge_page..
>>>>>> git tree:       https://github.com/x-y-z/linux-dev.git fix_split_page_min_order-for-kernelci
>>>>>> console output: https://syzkaller.appspot.com/x/log.txt?x=17ce96e2580000
>>>>>> kernel config:  https://syzkaller.appspot.com/x/.config?x=714d45b6135c308e
>>>>>> dashboard link: https://syzkaller.appspot.com/bug?extid=e6367ea2fdab6ed46056
>>>>>> compiler:       Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
>>>>>> userspace arch: arm64
>>>>>>
>>>>>> Note: no patches were applied.
>>>>>>
>>>>>
>>>>
>>>> Thank you for looking into this.
>>>>
>>>>> My hunch is that
>>>>> https://github.com/x-y-z/linux-dev.git fix_split_page_min_order-for-kernelci
>>>>> alone is not enough.  Perhaps on ARM64, the page cache pages of /dev/nullb0 in
>>>> Yes, it only has the first patch, which fails a split if it cannot be
>>>> split to the intended order (order-0 in this case).
>>>>
>>>>
>>>>> the test case are probably with min_order > 0, therefore THP split fails, as the console message show:
>>>>> [  200.378989][T18221] Memory failure: 0x124d30: recovery action for unsplit thp: Failed
>>>>>
>>>>> With lots of poisoned THP pages stuck in the page cache, OOM could trigger too soon.
>>>>
>>>> That is my understanding too. Thanks for the confirmation.
>>>>
>>>>>
>>>>> I think it's worth to try add the additional changes I suggested earlier -
>>>>> https://lore.kernel.org/lkml/7577871f-06be-492d-b6d7-8404d7a045e0@oracle.com/
>>>>>
>>>>> So that in the madvise HWPOISON cases, large huge pages are splitted to smaller huge pages, and most of them remain usable in the page cache.
>>>>
>>>> Yep, I am going to incorporate your suggestion as the second patch and make
>>>> syzbot check it again.
>>>
>>>
>>> #syz test: https://github.com/x-y-z/linux-dev.git fix_split_page_min_order_and_opt_memory_failure-for-kernelci
>>>
>>
>> There is a bug here,
>>
>> 		if (try_to_split_thp_page(p, new_order, false) || new_order) {
>> 			res = -EHWPOISON;
>> 			kill_procs_now(p, pfn, flags, folio);  <---
>>
>> If try_to_split_thp_page() succeeded on min_order, 'folio' should be retaken:  folio = page_folio(page) before moving on to kill_procs_now().
> 
> Thank you for pointing it out. Let me fix it and let syzbot test it again.
> 
> BTW, do you mind explaining why soft offline case does not want to split?
> Like memory failure case, splitting it would make other after-split folios
> available.

That's exactly what I think.  Let's wait for Miaohe, not sure if he has 
other concern.

thanks,
-jane

> 
> Thanks.
> 
> Best Regards,
> Yan, Zi