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

[Zi Yan <ziy@nvidia.com>]

On 9 Oct 2025, at 3:39, Miaohe Lin wrote:

> On 2025/10/3 1:47, jane.chu@oracle.com wrote:
>>
>>
>> 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.
>>>
>
> Sorry for late. I just got back from my vacation. :)
>
>>> 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.
>
> It might be because even if split is skipped, the folio is still accessible (thus available)
> from user sapce and premature split might lead to potential performance loss. But it's fine
> to me to split folio first in soft offline case. No strong opinion.

Thanks for the explanation. I will add a comment about this. We can always do the
split at soft offline when we find it useful. I will send the patches out.

--
Best Regards,
Yan, Zi