Message-ID: <3b75765f-d4d7-fd0e-366d-59e18ab42766@redhat.com>
Date:   Mon, 5 Dec 2022 16:14:41 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Yang Shi <shy828301@...il.com>
Cc:     "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Hugh Dickins <hughd@...gle.com>, Peter Xu <peterx@...hat.com>
Subject: Re: kernel BUG at include/linux/swapops.h:497!

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 <shy828301@...il.com> wrote:
>>>
>>> On Thu, Dec 1, 2022 at 8:58 AM David Hildenbrand <david@...hat.com> 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@...ian:/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).

I just sent a fix.
-- 
Thanks,

David / dhildenb

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