Subject: Re: linux-next: KVM/s390x regression

From: Balbir Singh <[email-masked]>
Date: Tue, 28 Oct 2025 20:24:07 +1100

On 10/28/25 04:06, Christian Borntraeger wrote:
> Am 27.10.25 um 17:47 schrieb Claudio Imbrenda:
>> On Mon, 20 Oct 2025 10:41:28 +0200
>> David Hildenbrand <[email-masked]> wrote:
>>
>>> On 20.10.25 09:00, Christian Borntraeger wrote:
>>>> Am 17.10.25 um 23:56 schrieb Balbir Singh:
>>>>   
>>>>> In the meanwhile, does this fix/workaround work?
>>>>>
>>>>> diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c
>>>>> index 0c847cdf4fd3..31c1754d5bd4 100644
>>>>> --- a/mm/pgtable-generic.c
>>>>> +++ b/mm/pgtable-generic.c
>>>>> @@ -290,7 +290,7 @@ pte_t *___pte_offset_map(pmd_t *pmd, unsigned long addr, pmd_t *pmdvalp)
>>>>>             if (pmdvalp)
>>>>>             *pmdvalp = pmdval;
>>>>> -    if (unlikely(pmd_none(pmdval) || !pmd_present(pmdval)))
>>>>> +    if (unlikely(pmd_none(pmdval) || is_pmd_non_present_folio_entry(pmdval)))
>>>>>             goto nomap;
>>>>>         if (unlikely(pmd_trans_huge(pmdval)))
>>>>>             goto nomap;
>>>>>   
>>>>
>>>> Yes, this seems to work.
>>>
>>> Right, but that's not what we will want here. We'll have to adjust s390x
>>> gmap code (which is getting redesigned either way) to only take the page
>>> lock.
>>>
>>> In the end, we'll want here later a single
>>>
>>> if (!pmd_present(pmdval))
>>>     goto nomap;
>>>
>>
>> this seems to do the trick:
>>
>> diff --git a/arch/s390/mm/gmap.c b/arch/s390/mm/gmap.c
>> index 8ff6bba107e8..22c448b32340 100644
>> --- a/arch/s390/mm/gmap.c
>> +++ b/arch/s390/mm/gmap.c
>> @@ -599,8 +599,9 @@ int __gmap_link(struct gmap *gmap, unsigned long
>> gaddr, unsigned long vmaddr) | _SEGMENT_ENTRY_GMAP_UC
>>                                          | _SEGMENT_ENTRY;
>>                          } else
>> -                               *table = pmd_val(*pmd) &
>> -                                       _SEGMENT_ENTRY_HARDWARE_BITS;
>> +                               *table = (pmd_val(*pmd) &
>> +                                       _SEGMENT_ENTRY_HARDWARE_BITS)
>> +                                       | _SEGMENT_ENTRY;
>>                  }
>>          } else if (*table & _SEGMENT_ENTRY_PROTECT &&
>>                     !(pmd_val(*pmd) & _SEGMENT_ENTRY_PROTECT)) {
>>
>>
> 
> Tested-by: Christian Borntraeger <[email-masked]>
> Acked-by: Christian Borntraeger <[email-masked]>
> 
> can you send a proper patch? I guess we should add it to Andrews mm true to keep it close to the patch that uncovered the issue.
> s390 maintainers cced.

Thanks for finding the fix. Ideally, we want this fix just before my series if possible!

Balbir