Re: Ext4 iomap warning during btrfs/136 (yes, it's from btrfs test cases)

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

在 2025/8/8 17:22, Qu Wenruo 写道:
> Hi,
> 
> [BACKGROUND]
> Recently I'm testing btrfs with 16KiB block size.
> 
> Currently btrfs is artificially limiting subpage block size to 4K.
> But there is a simple patch to change it to support all block sizes <= 
> page size in my branch:
> 
> https://github.com/adam900710/linux/tree/larger_bs_support
> 
> [IOMAP WARNING]
> And I'm running into a very weird kernel warning at btrfs/136, with 16K 
> block size and 64K page size.
> 
> The problem is, the problem happens with ext3 (using ext4 modeule) with 
> 16K block size, and no btrfs is involved yet.

The following reproducer is much smaller, and of course, btrfs is not 
involved:

---
#!/bin/bash

dev="/dev/test/scratch1"
mnt="/mnt/btrfs/"
fsstress="/home/adam/xfstests-dev/ltp/fsstress"

mkfs.ext3 -F -b 16k $dev
mount $dev $mnt

$fsstress -w -n 128 -d $mnt
umount $dev
---

And ext4 is fine, so it's ext3 mode causing the problem.

Furthermore, after the kernel wanring, the fsstress will never finish, 
and no blocked process either.

Thanks,
Qu

> 
> The test case btrfs/136 create an ext3 fs first, using the same block 
> size of the btrfs on TEST_DEV (so it's 16K).
> Then populate the fs.
> 
> The hang happens at the ext3 populating part, with the following kernel 
> warning:
> 
> [  989.664270] run fstests btrfs/136 at 2025-08-08 16:57:37
> [  990.551395] EXT4-fs (dm-3): mounting ext3 file system using the ext4 
> subsystem
> [  990.554980] EXT4-fs (dm-3): mounted filesystem d90f4325- 
> e6a6-4787-9da8-150ece277a94 r/w with ordered data mode. Quota mode: none.
> [  990.581540] ------------[ cut here ]------------
> [  990.581551] WARNING: CPU: 3 PID: 434101 at fs/iomap/iter.c:34 
> iomap_iter_done+0x148/0x190
> [  990.583497] Modules linked in: dm_flakey nls_ascii nls_cp437 vfat fat 
> btrfs polyval_ce ghash_ce rtc_efi processor xor xor_neon raid6_pq 
> zstd_compress fuse loop nfnetlink qemu_fw_cfg ext4 crc16 mbcache jbd2 
> dm_mod xhci_pci xhci_hcd virtio_net virtio_scsi net_failover failover 
> virtio_console virtio_balloon virtio_blk virtio_mmio
> [  990.587247] CPU: 3 UID: 0 PID: 434101 Comm: fsstress Not tainted 
> 6.16.0-rc7-custom+ #128 PREEMPT(voluntary)
> [  990.588525] Hardware name: QEMU KVM Virtual Machine, BIOS unknown 
> 2/2/2022
> [  990.589414] pstate: 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS 
> BTYPE=--)
> [  990.590314] pc : iomap_iter_done+0x148/0x190
> [  990.590874] lr : iomap_iter+0x174/0x230
> [  990.591370] sp : ffff8000880af740
> [  990.591800] x29: ffff8000880af740 x28: ffff0000db8e6840 x27: 
> 0000000000000000
> [  990.592716] x26: 0000000000000000 x25: ffff8000880af830 x24: 
> 0000004000000000
> [  990.593631] x23: 0000000000000002 x22: 000001bfdbfa8000 x21: 
> ffffa6a41c002e48
> [  990.594549] x20: 0000000000000001 x19: ffff8000880af808 x18: 
> 0000000000000000
> [  990.595464] x17: 0000000000000000 x16: ffffa6a495ee6cd0 x15: 
> 0000000000000000
> [  990.596379] x14: 00000000000003d4 x13: 00000000fa83b2da x12: 
> 0000b236fc95f18c
> [  990.597295] x11: ffffa6a4978b9c08 x10: 0000000000001da0 x9 : 
> ffffa6a41c1a2a44
> [  990.598210] x8 : ffff8000880af5c8 x7 : 0000000001000000 x6 : 
> 0000000000000000
> [  990.599125] x5 : 0000000000000004 x4 : 000001bfdbfa8000 x3 : 
> 0000000000000000
> [  990.600040] x2 : 0000000000000000 x1 : 0000004004030000 x0 : 
> 0000000000000000
> [  990.600955] Call trace:
> [  990.601273]  iomap_iter_done+0x148/0x190 (P)
> [  990.601829]  iomap_iter+0x174/0x230
> [  990.602280]  iomap_fiemap+0x154/0x1d8
> [  990.602751]  ext4_fiemap+0x110/0x140 [ext4]
> [  990.603350]  do_vfs_ioctl+0x4b8/0xbc0
> [  990.603831]  __arm64_sys_ioctl+0x8c/0x120
> [  990.604346]  invoke_syscall+0x6c/0x100
> [  990.604836]  el0_svc_common.constprop.0+0x48/0xf0
> [  990.605444]  do_el0_svc+0x24/0x38
> [  990.605875]  el0_svc+0x38/0x120
> [  990.606283]  el0t_64_sync_handler+0x10c/0x138
> [  990.606846]  el0t_64_sync+0x198/0x1a0
> [  990.607319] ---[ end trace 0000000000000000 ]---
> [  990.608042] ------------[ cut here ]------------
> [  990.608047] WARNING: CPU: 3 PID: 434101 at fs/iomap/iter.c:35 
> iomap_iter_done+0x164/0x190
> [  990.610842] Modules linked in: dm_flakey nls_ascii nls_cp437 vfat fat 
> btrfs polyval_ce ghash_ce rtc_efi processor xor xor_neon raid6_pq 
> zstd_compress fuse loop nfnetlink qemu_fw_cfg ext4 crc16 mbcache jbd2 
> dm_mod xhci_pci xhci_hcd virtio_net virtio_scsi net_failover failover 
> virtio_console virtio_balloon virtio_blk virtio_mmio
> [  990.619189] CPU: 3 UID: 0 PID: 434101 Comm: fsstress Tainted: G 
>   W           6.16.0-rc7-custom+ #128 PREEMPT(voluntary)
> [  990.620876] Tainted: [W]=WARN
> [  990.621458] Hardware name: QEMU KVM Virtual Machine, BIOS unknown 
> 2/2/2022
> [  990.622507] pstate: 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS 
> BTYPE=--)
> [  990.623911] pc : iomap_iter_done+0x164/0x190
> [  990.624936] lr : iomap_iter+0x174/0x230
> [  990.626747] sp : ffff8000880af740
> [  990.627404] x29: ffff8000880af740 x28: ffff0000db8e6840 x27: 
> 0000000000000000
> [  990.628947] x26: 0000000000000000 x25: ffff8000880af830 x24: 
> 0000004000000000
> [  990.631024] x23: 0000000000000002 x22: 000001bfdbfa8000 x21: 
> ffffa6a41c002e48
> [  990.632278] x20: 0000000000000001 x19: ffff8000880af808 x18: 
> 0000000000000000
> [  990.634189] x17: 0000000000000000 x16: ffffa6a495ee6cd0 x15: 
> 0000000000000000
> [  990.635608] x14: 00000000000003d4 x13: 00000000fa83b2da x12: 
> 0000b236fc95f18c
> [  990.637854] x11: ffffa6a4978b9c08 x10: 0000000000001da0 x9 : 
> ffffa6a41c1a2a44
> [  990.639181] x8 : ffff8000880af5c8 x7 : 0000000001000000 x6 : 
> 0000000000000000
> [  990.642370] x5 : 0000000000000004 x4 : 000001bfdbfa8000 x3 : 
> 0000000000000000
> [  990.644505] x2 : 0000004004030000 x1 : 0000004004030000 x0 : 
> 0000004004030000
> [  990.645493] Call trace:
> [  990.645841]  iomap_iter_done+0x164/0x190 (P)
> [  990.646377]  iomap_iter+0x174/0x230
> [  990.647550]  iomap_fiemap+0x154/0x1d8
> [  990.648052]  ext4_fiemap+0x110/0x140 [ext4]
> [  990.649061]  do_vfs_ioctl+0x4b8/0xbc0
> [  990.649704]  __arm64_sys_ioctl+0x8c/0x120
> [  990.652141]  invoke_syscall+0x6c/0x100
> [  990.653001]  el0_svc_common.constprop.0+0x48/0xf0
> [  990.653909]  do_el0_svc+0x24/0x38
> [  990.654332]  el0_svc+0x38/0x120
> [  990.654736]  el0t_64_sync_handler+0x10c/0x138
> [  990.655295]  el0t_64_sync+0x198/0x1a0
> [  990.655761] ---[ end trace 0000000000000000 ]---
> 
> Considering it's not yet btrfs, and the call trace is from iomap, I 
> guess there is something wrong with ext4's ext3 support?
> 
> The involved ext4 kernel configs are the following:
> 
> # CONFIG_EXT2_FS is not set
> # CONFIG_EXT3_FS is not set
> CONFIG_EXT4_FS=m
> CONFIG_EXT4_USE_FOR_EXT2=y
> CONFIG_EXT4_FS_POSIX_ACL=y
> CONFIG_EXT4_FS_SECURITY=y
> # CONFIG_EXT4_DEBUG is not set
> CONFIG_JBD2=m
> # CONFIG_JBD2_DEBUG is not set
> 
> Thanks,
> Qu
>