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[2003:d8:2f0c:c200:fa4a:c4ff:1b32:21ce]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-471144239bdsm123442385e9.3.2025.10.17.10.07.56 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 17 Oct 2025 10:07:58 -0700 (PDT) Message-ID: <84349344-b127-41f6-99f1-10f907c2bd07@redhat.com> Date: Fri, 17 Oct 2025 19:07:56 +0200 Precedence: bulk X-Mailing-List: linux-next@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: linux-next: KVM/s390x regression To: Christian Borntraeger , balbirs@nvidia.com Cc: Liam.Howlett@oracle.com, airlied@gmail.com, akpm@linux-foundation.org, apopple@nvidia.com, baohua@kernel.org, baolin.wang@linux.alibaba.com, byungchul@sk.com, dakr@kernel.org, dev.jain@arm.com, dri-devel@lists.freedesktop.org, francois.dugast@intel.com, gourry@gourry.net, joshua.hahnjy@gmail.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, lorenzo.stoakes@oracle.com, lyude@redhat.com, matthew.brost@intel.com, mpenttil@redhat.com, npache@redhat.com, osalvador@suse.de, rakie.kim@sk.com, rcampbell@nvidia.com, ryan.roberts@arm.com, simona@ffwll.ch, ying.huang@linux.alibaba.com, ziy@nvidia.com, kvm@vger.kernel.org, linux-s390@vger.kernel.org, linux-next@vger.kernel.org References: <20251001065707.920170-4-balbirs@nvidia.com> <20251017144924.10034-1-borntraeger@linux.ibm.com> <9beff9d6-47c7-4a65-b320-43efd1e12687@redhat.com> <8c778cd0-5608-4852-9840-4d98828d7b33@redhat.com> <74272098-cfb7-424b-a55e-55e94f04524e@linux.ibm.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit 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? 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? -- Cheers David / dhildenb