Weird delay introduced in v6.16-rc only, possible regression

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

Hi,

Recently I'm hitting a very weird delay when doing development inside a 
x86_64 VM.

The dmesg shows the delay (10+ sec) between virtio blk and device-mapper:

[    3.651377] virtio_blk virtio4: 10/0/0 default/read/poll queues
[    3.653075] virtio_scsi virtio2: 10/0/0 default/read/poll queues
[    3.670269] virtio_blk virtio4: [vda] 83886080 512-byte logical 
blocks (42.9 GB/40.0 GiB)
[    3.672096] scsi host6: Virtio SCSI HBA
[    3.708452]  vda: vda1 vda2
[    3.711073] virtio_blk virtio5: 10/0/0 default/read/poll queues
[    3.729879] virtio_blk virtio5: [vdb] 167772160 512-byte logical 
blocks (85.9 GB/80.0 GiB)
[    3.737535] virtio_blk virtio8: 10/0/0 default/read/poll queues
[    3.747045] virtio_blk virtio8: [vdc] 83886080 512-byte logical 
blocks (42.9 GB/40.0 GiB)
[   17.453833] device-mapper: uevent: version 1.0.3
[   17.455689] device-mapper: ioctl: 4.50.0-ioctl (2025-04-28) 
initialised: dm-devel@lists.linux.dev
:: performing fsck on '/dev/os/root'
/dev/mapper/os-root: clean, 240299/1048576 files, 3372218/4194304 blocks
:: mounting '/dev/os/root' on real root
[   17.871671] EXT4-fs (dm-0): mounted filesystem 
00a85626-d289-4817-8183-ee828e221f76 r/w with ordered data mode. Quota 
mode: none.

The VM is running kernel based on upstream commit 78f4e737a53e ("Merge 
tag 'for-6.16/dm-fixes' of 
git://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm"), 
with a lot of extra btrfs patches.


The v6.15 kernel from Archlinux is totally fine without any delay.

The v6.16-rc kernel may have some different configs, but the config is 
used for a long long time, way before v6.15, so it looks like it's 
something in the v6.16 cycle causing problems.

I can definitely do a bisection, but any clue would be appreciated.

Thanks,
Qu