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

[jane.chu@oracle.com]

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().

thanks,
-jane

> 
> --
> Best Regards,
> Yan, Zi