[REGRESSION] fs/overlayfs/ext4: Severe jbd2 lock contention and journal starvation on concurrent copy-up (v6.6 -> v6.12)

Hi all,

We are tracking a severe performance regression in Google's
Container-Optimized OS (COS) that appeared when moving from the 6.6
LTS kernel to the 6.12 LTS kernel.

Under concurrent CI workloads (specifically, many containers doing
Python package compilation / .pyc generation simultaneously), the 6.12
kernel suffers from massive jbd2 journal contention. Processes hang
for 20-30 seconds waiting for VFS locks and journal space. On 6.6, the
exact same workload completes in ~4 seconds.

# Environment:
* Host FS: ext4 (backed by standard cloud block storage)
* Container FS: OverlayFS (Docker)
* Machine: n2d-highmem-96 (96 vCPU, high memory)
* Good Kernel: 6.6.87
* Bad Kernels: 6.12.55, 6.12.68

# The Bottleneck
During the 20+ second hang, `cat /proc/<pid>/stack` reveals three
distinct groups of blocked processes thrashing on the jbd2 journal.
The OverlayFS copy-up mechanism seems to be generating so many
synchronous ext4 transactions that it exhausts the jbd2 transaction
buffers.

1. Journal Space Exhaustion (Waiting to start transaction):
[<0>] __jbd2_log_wait_for_space+0xa3/0x240
[<0>] start_this_handle+0x42d/0x8a0
[<0>] jbd2__journal_start+0x103/0x1e0
[<0>] __ext4_journal_start_sb+0x129/0x1c0
[<0>] __ext4_new_inode+0x7cd/0x1290
[<0>] ext4_create+0xbc/0x1b0
[<0>] vfs_create+0x192/0x250
[<0>] ovl_create_real+0xd5/0x170
[<0>] ovl_create_or_link+0x1d7/0x7f0

2. VFS Rename / Copy-up Contention (Blocked by the slow sync):
[<0>] lock_rename+0x29/0x50
[<0>] ovl_copy_up_flags+0x84c/0x12e0
[<0>] ovl_create_object+0x4a/0x120
[<0>] vfs_mkdir+0x1aa/0x260
[<0>] do_mkdirat+0xb9/0x240

3. Synchronous Flush Blocking:
[<0>] jbd2_log_wait_commit+0x107/0x150
[<0>] jbd2_journal_force_commit+0x9c/0xc0
[<0>] ext4_sync_file+0x278/0x310
[<0>] ovl_sync_file+0x2f/0x50
[<0>] ovl_copy_up_metadata+0x455/0x4b0

# Minimal Reproducer
The issue is easily reproducible by triggering 20 concurrent cold
Python imports in Docker, which forces OverlayFS to copy-up the
`__pycache__` directories and write the `.pyc` files.

```bash
# 1. Build a clean image with no pre-compiled bytecode
cat << 'EOF' > Dockerfile
FROM python:3.10-slim
RUN pip install --quiet google-cloud-compute
RUN find /usr/local -type d -name "__pycache__" -exec rm -rf {} +
EOF
docker build -t clean-import-test .

# 2. Fire 20 concurrent imports
for i in {1..20}; do
  docker run --rm clean-import-test bash -c 'time python -c "import
google.cloud.compute_v1"' > clean_test_cold_$i.log 2>&1 &
done
wait
grep "real" clean_test_cold_*.log
```

On 6.6.87, all 20 containers finish in ~4.3s.
On 6.12.x, they hang and finish between 17s and 27s. Bypassing disk
writes completely mitigates the regression on 6.12 (using
PYTHONDONTWRITEBYTECODE=1), confirming it is an ext4/overlayfs I/O
contention issue rather than a CPU scheduling one.

Because the regression spans from 6.6 to 6.12, bisection is quite
heavy. Before we initiate a full kernel bisect, does this symptom ring
a bell for any ext4 fast_commit, jbd2 locking, or OverlayFS
metacopy/sync changes introduced during this window?

Any pointers or patches you'd like us to test would be greatly appreciated.

Thanks,
Chenglong Tang
Google Container-Optimized OS Team