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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 Given that the revert works, the check changes to segment_entry/present = false or pmd_migration_entry (PAGE_INVALID | PAGE_PROTECT) So it isn't the first check of segment_entry/present = false sounds like for s390 we would want __pte_offset_map to allow mappings with segment_entry_empty/invalid entries? Any chance we can get the stack trace and a dump of the PMD entry when the issue occurs? 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; Thanks David and Christian! Balbir