ext4: slow unmount with large clean page cache; is fsfreeze→umount recommended?

["Richter, Rafael" <rafael.richter.extern@gin.de>]

Hi,

we consistently see slow unmounts (~6–8s) on ext4 after heavy buffered writes
(e.g., dd ~30 GiB) that grow the page cache. Same test on XFS unmounts <1s on
the same hardware, but we must stay on ext4.

Env:
  - Kernel: 6.6.36 (Yocto-based)
  - Device: SSD via mdraid (/dev/md0p1)
  - Mount: ext4 on /mnt/disk (defaults)

Repro:
  dd if=/dev/zero of=/mnt/disk/big.bin bs=1M count=30720 status=progress
  sync -f /mnt/disk
  time umount /mnt/disk      # ext4: ~6–8s

Observations:
  - Dirty/Writeback are ~0 before unmount.
  - `fsfreeze -f /mnt/disk` immediately before `umount` makes unmount very fast.
  - `mount -o remount,ro` before `umount` does NOT improve unmount time.

Questions:
  1) Is this unmount time expected on ext4 with a large *clean* page cache?
  2) Why is XFS unmount much faster here—any ext4-side reasons or regressions?
  3) Is `fsfreeze -f` → `umount` acceptable/recommended for shutdown on ext4?
     Any ext4/VFS options or per-filesystem methods to reduce unmount time?


Looking forward for an answer!

Thx,

Rafael