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@xxxxxxxxx> wrote:
On Thu, Dec 1, 2022 at 8:58 AM David Hildenbrand <david@xxxxxxxxxx> 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).
I just sent a fix.
--
Thanks,
David / dhildenb
