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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 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 Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Queue-Id: 3DC7080007 X-Rspamd-Server: rspam02 X-Stat-Signature: 3keuymas56hmxejogoxtcp94qw4q99wf X-HE-Tag: 1760720882-601898 X-HE-Meta: U2FsdGVkX18dX6oOr8aDXz1Z4o4AzMe9x03MlyzKGj1l7FqrHctewi+JaP4l7ivcGf+v2+uxRJNm6lnKpBTqmpXXY/zqBxhQ4wwrztjC2C7VhvE6QDFPOqVy5uvh+Gno8b4a8EnxL08oUL41TndZaOoH9YOuUJHwdBWh+z/m7zU8qkJdlSTdBMcfxot2WmZ6RVuYPFAnxiK7Bf0MqYkULQ6/LyQKayYQ9JX2MvHUjAWKXIBnLn6pjFUvQU3wr7Cgwok7m39LUpSPPavxPMi4hzo7fP8KFpGNdH3ZMCCpjsY8EUeSTWcNyu5zWsndRu7FRqzeNvfbRcyia/s/ZOwzY8/oNWNEVir4GpYvdz6xUniorfklqqV+KiDIcI5huswtmOL4u59y2ByXzBtogTc2njPJOJn9XNqPSRfezXlIx+7nTepVbVlIzCQW/mDQLyH+ghN4lzhl37gmTDgcsrk+Ro6z4ogiqkArpuu6OSLkMdayvb5qnronx6hThf1jFYUuliclyYZpb3bqE5iNr9G0gIDTRSIHAH2ObI/l326tGhMKN+vZ6YznqFPVFACyKH6SmPHe1S2qq/UWznQXUwLISqRJZ3Y62qDGg/Zd8Fi+Q174CXFSfKHFLnO/KQ0//UkguGHmtdY5IjJ1RGeTIQ+HRJ2eaIGPuWXbEeg+bhVr68qA/bZGzckxWhp5x5ImND0P7lLtq4D3ASITBZGbGBNCJb7q1JvBDAoqQ4h7NymOc3F+ZxeFhtUfLzGozdDhuqsab9xaC3G6xw0HwPP8jenJI7JPOmvt4QvRFfY/F4iJ67xv/usoL9YJ7Lo4ehVWWkOVl5a2p5LTsAjNKqNpn0m2ga+nD3rs259ZXMjuryuNb2LyyuBRu63+MOv5rNWqHQFZADrPU3E12GRU4WtgsGsRLg5vMz3I8PYsjaoTU1Sj3WDq/QiOKaHIjqy7b/lkBqsE/hd09Xd3ipMyAEJDBP0 hjQ9pExs 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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