Re: linux-next: KVM/s390x regression

Christian Borntraeger
2025-10-20T07:02:04Z

Am 18.10.25 um 00:41 schrieb David Hildenbrand:
> On 18.10.25 00:15, David Hildenbrand wrote:
>> On 17.10.25 23:56, Balbir Singh wrote:
>>> On 10/18/25 04:07, David Hildenbrand wrote:
>>>> On 17.10.25 17:20, Christian Borntraeger wrote:
>>>>>
>>>>>
>>>>> Am 17.10.25 um 17:07 schrieb David Hildenbrand:
>>>>>> On 17.10.25 17:01, Christian Borntraeger wrote:
>>>>>>> Am 17.10.25 um 16:54 schrieb David Hildenbrand:
>>>>>>>> On 17.10.25 16:49, Christian Borntraeger wrote:
>>>>>>>>> This patch triggers a regression for s390x kvm as qemu guests can no longer start
>>>>>>>>>
>>>>>>>>> error: kvm run failed Cannot allocate memory
>>>>>>>>> PSW=mask 0000000180000000 addr 000000007fd00600
>>>>>>>>> R00=0000000000000000 R01=0000000000000000 R02=0000000000000000 R03=0000000000000000
>>>>>>>>> R04=0000000000000000 R05=0000000000000000 R06=0000000000000000 R07=0000000000000000
>>>>>>>>> R08=0000000000000000 R09=0000000000000000 R10=0000000000000000 R11=0000000000000000
>>>>>>>>> R12=0000000000000000 R13=0000000000000000 R14=0000000000000000 R15=0000000000000000
>>>>>>>>> C00=00000000000000e0 C01=0000000000000000 C02=0000000000000000 C03=0000000000000000
>>>>>>>>> C04=0000000000000000 C05=0000000000000000 C06=0000000000000000 C07=0000000000000000
>>>>>>>>> C08=0000000000000000 C09=0000000000000000 C10=0000000000000000 C11=0000000000000000
>>>>>>>>> C12=0000000000000000 C13=0000000000000000 C14=00000000c2000000 C15=0000000000000000
>>>>>>>>>
>>>>>>>>> KVM on s390x does not use THP so far, will investigate. Does anyone have a quick idea?
>>>>>>>>
>>>>>>>> Only when running KVM guests and apart from that everything else seems to be fine?
>>>>>>>
>>>>>>> We have other weirdness in linux-next but in different areas. Could that somehow be
>>>>>>> related to use disabling THP for the kvm address space?
>>>>>>
>>>>>> Not sure ... it's a bit weird. I mean, when KVM disables THPs we essentially just remap everything to be mapped by PTEs. So there shouldn't be any PMDs in that whole process.
>>>>>>
>>>>>> Remapping a file THP (shmem) implies zapping the THP completely.
>>>>>>
>>>>>>
>>>>>> I assume in your kernel config has CONFIG_ZONE_DEVICE and CONFIG_ARCH_ENABLE_THP_MIGRATION set, right?
>>>>>
>>>>> yes.
>>>>>
>>>>>>
>>>>>> I'd rule out copy_huge_pmd(), zap_huge_pmd() a well.
>>>>>>
>>>>>>
>>>>>> What happens if you revert the change in mm/pgtable-generic.c?
>>>>>
>>>>> That partial revert seems to fix the issue
>>>>> diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c
>>>>> index 0c847cdf4fd3..567e2d084071 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_migration_entry(pmdval)))
>>>>
>>>> Okay, but that means that effectively we stumble over a PMD entry that is not a migration entry but still non-present.
>>>>
>>>> And I would expect that it's a page table, because otherwise the change
>>>> wouldn't make a difference.
>>>>
>>>> And the weird thing is that this only triggers sometimes, because if
>>>> it would always trigger nothing would ever work.
>>>>
>>>> Is there some weird scenario where s390x might set a left page table mapped in a PMD to non-present?
>>>>
>>>
>>> Good point
>>>
>>>> Staring at the definition of pmd_present() on s390x it's really just
>>>>
>>>>       return (pmd_val(pmd) & _SEGMENT_ENTRY_PRESENT) != 0;
>>>>
>>>>
>>>> Maybe this is happening in the gmap code only and not actually in the core-mm code?
>>>>
>>>
>>>
>>> I am not an s390 expert, but just looking at the code
>>>
>>> So the check on s390 effectively
>>>
>>> segment_entry/present = false or segment_entry_empty/invalid = true
>>
>> pmd_present() == true iff _SEGMENT_ENTRY_PRESENT is set
>>
>> because
>>
>>     return (pmd_val(pmd) & _SEGMENT_ENTRY_PRESENT) != 0;
>>
>> is the same as
>>
>>     return pmd_val(pmd) & _SEGMENT_ENTRY_PRESENT;
>>
>> But that means we have something where _SEGMENT_ENTRY_PRESENT is not set.
>>
>> I suspect that can only be the gmap tables.
>>
>> Likely __gmap_link() does not set _SEGMENT_ENTRY_PRESENT, which is fine
>> because it's a software managed bit for "ordinary" page tables, not gmap
>> tables.
>>
>> Which raises the question why someone would wrongly use
>> pte_offset_map()/__pte_offset_map() on the gmap tables.
>>
>> I cannot immediately spot any such usage in kvm/gmap code, though.
>>
>
> Ah, it's all that pte_alloc_map_lock() stuff in gmap.c.
>
> Oh my.
>
> So we're mapping a user PTE table that is linked into the gmap tables through a PMD table that does not have the right sw bits set we would expect in a user PMD table.
>
> What's also scary is that pte_alloc_map_lock() would try to pte_alloc() a user page table in the gmap, which sounds completely wrong?
>
> Yeah, when walking the gmap and wanting to lock the linked user PTE table, we should probably never use the pte_*map variants but obtain
> the lock through pte_lockptr().
>
> All magic we end up doing with RCU etc in __pte_offset_map_lock()
> does not apply to the gmap PMD table.
>

CC Claudio.