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[2003:d8:2f0c:c200:fa4a:c4ff:1b32:21ce]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-427f00ce56csm1338836f8f.50.2025.10.17.15.15.02 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 17 Oct 2025 15:15:04 -0700 (PDT) Message-ID: <3a2db8fc-d289-415b-ae67-5a35c9c32a76@redhat.com> Date: Sat, 18 Oct 2025 00:15:01 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: linux-next: KVM/s390x regression To: Balbir Singh , Christian Borntraeger 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> <84349344-b127-41f6-99f1-10f907c2bd07@redhat.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-BeenThere: dri-devel@lists.freedesktop.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Direct Rendering Infrastructure - Development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dri-devel-bounces@lists.freedesktop.org Sender: "dri-devel" 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. -- Cheers David / dhildenb