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(p200300cbc7054f0041f1185d4f9fd1c2.dip0.t-ipconnect.de. [2003:cb:c705:4f00:41f1:185d:4f9f:d1c2]) by smtp.gmail.com with ESMTPSA id m41-20020a05600c3b2900b003d1e3b1624dsm3781426wms.2.2022.12.06.01.00.24 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 06 Dec 2022 01:00:24 -0800 (PST) Message-ID: <24acd798-4e54-ca82-5ff9-e84a4795742e@redhat.com> Date: Tue, 6 Dec 2022 10:00:23 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.5.0 Subject: Re: kernel BUG at include/linux/swapops.h:497! Content-Language: en-US To: Yang Shi Cc: "linux-mm@kvack.org" , "linux-kernel@vger.kernel.org" , Hugh Dickins , Peter Xu References: <0efeec33-8030-df1e-b055-2e0748fab4e0@redhat.com> <082e64a7-7340-b1fb-b819-43bc4df0584f@redhat.com> <3b75765f-d4d7-fd0e-366d-59e18ab42766@redhat.com> From: David Hildenbrand Organization: Red Hat In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 06.12.22 00:14, Yang Shi wrote: > On Mon, Dec 5, 2022 at 7:14 AM David Hildenbrand wrote: >> >> On 02.12.22 13:36, David Hildenbrand wrote: >>> On 01.12.22 19:14, Yang Shi wrote: >>>> On Thu, Dec 1, 2022 at 9:48 AM Yang Shi wrote: >>>>> >>>>> On Thu, Dec 1, 2022 at 8:58 AM David Hildenbrand wrote: >>>>>> >>>>>> Hi, >>>>>> >>>>>> running COW tests (in mm-unstable) on x86-pae with 8GiB, I am able to trigger the >>>>>> following BUG on latest upstream: >>>>>> >>>>>> root@debian:/mnt/scratch/linux/tools/testing/selftests/vm# ./cow >>>>>> # [INFO] detected THP size: 2048 KiB >>>>>> # [INFO] detected hugetlb size: 2048 KiB >>>>>> # [INFO] huge zeropage is enabled >>>>>> TAP version 13 >>>>>> 1..147 >>>>>> # [INFO] Anonymous memory tests in private mappings >>>>>> # [RUN] Basic COW after fork() ... with base page >>>>>> ok 1 No leak from parent into child >>>>>> # [RUN] Basic COW after fork() ... with swapped out base page >>>>>> ok 2 No leak from parent into child >>>>>> # [RUN] Basic COW after fork() ... with THP >>>>>> ok 3 No leak from parent into child >>>>>> # [RUN] Basic COW after fork() ... with swapped-out THP >>>>>> Segmentation fault >>>>>> >>>>>> >>>>>> [ 879.314600] kernel BUG at include/linux/swapops.h:497! >>>>>> [ 879.314615] invalid opcode: 0000 [#1] PREEMPT SMP NOPTI >>>>>> [ 879.314624] CPU: 7 PID: 746 Comm: cow Tainted: G E 6.1.0-rc7+ #5 >>>>>> [ 879.314631] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.0-1.fc36 04/01/2014 >>>>>> [ 879.314634] EIP: pagemap_pmd_range+0x644/0x650 >>>>>> [ 879.314645] Code: 00 00 00 00 66 90 89 ce b9 00 f0 ff ff e9 ff fb ff ff 89 d8 31 db e8 1b c2 52 00 e9 23 fb ff ff e8 51 80 56 00 e9 b6 fe ff ff <0f> 0b bf 00 f0 ff ff e9 38 fa ff ff 3e 8d 74 26 00 55 89 e5 57 31 >>>>>> [ 879.314651] EAX: ee2bd000 EBX: 00000002 ECX: ee2bd000 EDX: 00000000 >>>>>> [ 879.314656] ESI: f54b9ed4 EDI: 0001f400 EBP: f54b9db4 ESP: f54b9d68 >>>>>> [ 879.314660] DS: 007b ES: 007b FS: 00d8 GS: 0033 SS: 0068 EFLAGS: 00010246 >>>>>> [ 879.314670] CR0: 80050033 CR2: b7a00000 CR3: 357452a0 CR4: 00350ef0 >>>>>> [ 879.314675] Call Trace: >>>>>> [ 879.314681] ? madvise_free_pte_range+0x720/0x720 >>>>>> [ 879.314689] ? smaps_pte_range+0x4b0/0x4b0 >>>>>> [ 879.314694] walk_pgd_range+0x325/0x720 >>>>>> [ 879.314701] ? mt_find+0x1d6/0x3a0 >>>>>> [ 879.314710] __walk_page_range+0x164/0x170 >>>>>> [ 879.314716] walk_page_range+0xf9/0x170 >>>>>> [ 879.314720] ? __kmem_cache_alloc_node+0x2a8/0x340 >>>>>> [ 879.314729] pagemap_read+0x124/0x280 >>>>>> [ 879.314738] ? default_llseek+0xf1/0x160 >>>>>> [ 879.314747] ? smaps_account+0x1d0/0x1d0 >>>>>> [ 879.314754] vfs_read+0x90/0x290 >>>>>> [ 879.314760] ? do_madvise.part.0+0x24b/0x390 >>>>>> [ 879.314765] ? debug_smp_processor_id+0x12/0x20 >>>>>> [ 879.314773] ksys_pread64+0x58/0x90 >>>>>> [ 879.314778] __ia32_sys_ia32_pread64+0x1b/0x20 >>>>>> [ 879.314787] __do_fast_syscall_32+0x4c/0xc0 >>>>>> [ 879.314796] do_fast_syscall_32+0x29/0x60 >>>>>> [ 879.314803] do_SYSENTER_32+0x15/0x20 >>>>>> [ 879.314809] entry_SYSENTER_32+0x98/0xf1 >>>>>> [ 879.314815] EIP: 0xb7f36559 >>>>>> [ 879.314820] Code: 03 74 c0 01 10 05 03 74 b8 01 10 06 03 74 b4 01 10 07 03 74 b0 01 10 08 03 74 d8 01 00 00 00 00 00 51 52 55 89 e5 0f 34 cd 80 <5d> 5a 59 c3 90 90 90 90 8d 76 00 58 b8 77 00 00 00 cd 80 90 8d 76 >>>>>> [ 879.314825] EAX: ffffffda EBX: 00000003 ECX: bff00a50 EDX: 00000008 >>>>>> [ 879.314829] ESI: 005bd000 EDI: 00000000 EBP: b7f1c000 ESP: bff00a00 >>>>>> [ 879.314833] DS: 007b ES: 007b FS: 0000 GS: 0033 SS: 007b EFLAGS: 00000246 >>>>>> [ 879.314840] Modules linked in: intel_rapl_msr(E) intel_rapl_common(E) intel_pmc_core(E) kvm_intel(E) kvm(E) irqbypass(E) aesni_intel(E) libaes(E) crypto_simd(E) cryptd(E) rfkill(E) snd_pcm(E) snd_timer(E) joydev(E) snd(E) soundcore(E) sg(E) evdev(E) pcspkr(E) serio_raw(E) qemu_fw_cfg(E) parport_pc(E) ppdev(E) lp(E) parport(E) fuse(E) configfs(E) ip_tables(E) x_tables(E) autofs4(E) ext4(E) crc32c_generic(E) crc16(E) mbcache(E) jbd2(E) sd_mod(E) t10_pi(E) crc64_rocksoft(E) crc64(E) crc_t10dif(E) sr_mod(E) crct10dif_generic(E) cdrom(E) crct10dif_common(E) bochs(E) drm_vram_helper(E) drm_ttm_helper(E) ttm(E) drm_kms_helper(E) ata_generic(E) ata_piix(E) crc32_pclmul(E) libata(E) crc32c_intel(E) drm(E) e1000(E) scsi_mod(E) psmouse(E) i2c_piix4(E) scsi_common(E) floppy(E) button(E) >>>>>> [ 879.314936] ---[ end trace 0000000000000000 ]--- >>>>>> [ 879.314940] EIP: pagemap_pmd_range+0x644/0x650 >>>>>> [ 879.314944] Code: 00 00 00 00 66 90 89 ce b9 00 f0 ff ff e9 ff fb ff ff 89 d8 31 db e8 1b c2 52 00 e9 23 fb ff ff e8 51 80 56 00 e9 b6 fe ff ff <0f> 0b bf 00 f0 ff ff e9 38 fa ff ff 3e 8d 74 26 00 55 89 e5 57 31 >>>>>> [ 879.314949] EAX: ee2bd000 EBX: 00000002 ECX: ee2bd000 EDX: 00000000 >>>>>> [ 879.314953] ESI: f54b9ed4 EDI: 0001f400 EBP: f54b9db4 ESP: f54b9d68 >>>>>> [ 879.314957] DS: 007b ES: 007b FS: 00d8 GS: 0033 SS: 0068 EFLAGS: 00010246 >>>>>> [ 879.314961] CR0: 80050033 CR2: b7a00000 CR3: 357452a0 CR4: 00350ef0 >>>>>> >>>>>> >>>>>> Reading /proc/self/pagemap in THP test case seems to trigger the >>>>>> BUG_ON(is_migration_entry(entry) && !PageLocked(p)); >>>>>> in pfn_swap_entry_to_page(). >>>>>> >>>>>> I did not have time to cherry pick (slow machine) or look into details. >>>>>> And I don't remember seeing that BUG 64bit yet during my tests. >>>>>> >>>>>> Having a migration entry in the swap testcase is kind-of weird. But maybe it's >>>>>> related to THP splitting (which would, however, also be weird). I'd have expected >>>>>> a swap entry ... hopefully our swap type doesn't get corrupted. >>>>> >>>>> I'm on a slow machine too... anyway some hints off the top of my head. >>>>> >>>>> First of all, I don't think we will see a real swap PMD entry since >>>>> even though THP swap is supported the transhuge PMD is split by >>>>> try_to_unmap() if I remember correctly. So we should just be able to >>>>> see a regular PMD, a transhuge PMD, a migration PMD or a PROT_NONE PMD >>>>> (if autonuma is on). >>> >>> Yes. >>> >>>>> >>>>> Secondly, THP splitting doesn't convert transhuge PMD to migration PMD >>>>> either, it just splits transhuge PMD then converts every single PTEs >>>>> to migration PTEs. >>> >>> Right. >>> >>>>> >>>>> Thirdly, before pfn_swap_entry_to_page() is called, it does check >>>>> whether the swap PMD is migration PMD or not, if it is not a VM_BUG is >>>>> triggered. >>>>> >>>>> So it seems like a migration PMD is fine. The problem seems like the >>>>> page is not locked when doing migration IIUC. >>>> >>>> A quick look at the migration code, I don't see the page is unlocked >>>> if I don't miss something. So it may be helpful to dump the page. >>> >>> It is highly unlikely that we have migration happening here, because >>> >>> 1) This triggers 100% on the first try >>> 2) The machine is essentially idle with 7 GiB of free memory. >>> >>> I'll try digging a bit what exactly is happening here, dumping the PMD >>> entry first. >> >> Turns out that 32bit x86 doesn't even support PMD migration. We're >> stumbling over a PTE holding a migration entry and the underlying page >> was indeed unlocked. Turns out we fail to remove the migration entries >> we temporarily installed while splitting the THP. Splitting code >> doesn't/cannot notice that and unlocks the now-split page(s). > > Thanks for debugging this. IIUC the real call trace should be: > > pagemap_pmd_range -> > pte_to_pagemap_entry -> > pfn_swap_entry_to_page <--- BUG > > I thought it was due to the migration PMD entry in the first place. Yeah, me too. Unfortunately, the system swallowed that part of the call trace. -- Thanks, David / dhildenb