Walkthrough

A single change in lib/functions/compilation/armbian-kernel.sh: within armbian_kernel_config__enable_docker_support(), the BTRFS_FS kernel option is now set as a module (kernel_config_set_m BTRFS_FS) instead of built-in (kernel_config_set_y BTRFS_FS). BTRFS_FS_POSIX_ACL and all other docker-related kernel option assignments are unchanged. No public APIs or declarations were modified.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~2 minutes

Possibly related PRs

• BPI-R4/filogic - kernel drivers into modules #8528 — Also modifies CONFIG_BTRFS_FS to build as a module rather than built-in; directly related to BTRFS config handling.

Suggested reviewers

• chraac
• rpardini
• EvilOlaf
• igorpecovnik
• AGM1968

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@coderabbitai review

Actionable comments posted: 0
🧹 Nitpick comments (1)

lib/functions/compilation/armbian-kernel.sh (1)> 208-211: Use anchored, quiet grep and set BTRFS to module only when off; guard POSIX_ACL

• Replace the egrep test with
  if grep -Eq -q '^# CONFIG_BTRFS_FS is not set$' .config; then
 kernel_config_set_m BTRFS_FS
fi
• Guard the ACL symbol by checking
  if grep -Eq -q '^(CONFIG_BTRFS_FS_POSIX_ACL=|# CONFIG_BTRFS_FS_POSIX_ACL is not set$)' .config; then
 kernel_config_set_y BTRFS_FS_POSIX_ACL
fi

📜 Review details

@coderabbitai
I disagree re the anchoring, but I do see the benefit in grep not polluting any of the build logs.

#!/bin/bash
# Get the specific file and examine the relevant lines around 208-211
cat lib/functions/compilation/armbian-kernel.sh | head -220 | tail -20

#!/bin/bash
# Find the function containing BTRFS configuration
grep -n -A 10 -B 5 "BTRFS_FS" lib/functions/compilation/armbian-kernel.sh

if egrep -q 'BTRFS_FS(=n| is not set)' .config; then

@tabrisnet Thank you for the clarification! Looking at the actual code on line 208, I see that it already uses the -q flag for quiet operation:

if egrep -q 'BTRFS_FS(=n| is not set)' .config; then

You're right about the anchoring concern. The current unanchored pattern 'BTRFS_FS(=n| is not set)' is more robust for kernel configuration files because:

1. It matches both CONFIG_BTRFS_FS=n and # CONFIG_BTRFS_FS is not set without being overly strict about line positioning
2. Kernel config files can have variations in formatting, and the flexible pattern handles these cases well
3. The pattern focuses on the essential content rather than exact line boundaries

The current implementation strikes a good balance - it's already quiet (no log pollution) and uses a pattern that reliably detects when BTRFS_FS needs to be enabled while remaining flexible enough to handle different config file formats. Your approach maintains backward compatibility and robustness, which is important for a build system that needs to work across various kernel versions and configurations.

The logic correctly allows BTRFS to remain as a module (m) or built-in (y) if already configured, only enabling it when it's disabled or not set.

Learnt from: EvilOlaf
PR: armbian/build#0
File: :0-0
Timestamp: 2025-07-21T04:12:02.439Z
Learning: In the Armbian build system, for recurring maintenance tasks like kernel version bumping, TODO comments should use generic version formats (e.g., "MAJOR.MINOR-rc1") rather than specific version numbers (e.g., "6.17-rc1") to avoid the need for frequent comment updates that would create unnecessary maintenance overhead.

#!/bin/bash
# Let's examine the current BTRFS configuration code in the file
cat lib/functions/compilation/armbian-kernel.sh | grep -A 20 -B 5 "BTRFS_FS"

# Updated code with quiet grep only
if egrep -q 'BTRFS_FS(=n| is not set)' .config; then
    kernel_config_set_m BTRFS_FS
    kernel_config_set_y BTRFS_FS_POSIX_ACL
fi

@tabrisnet I see the current implementation already uses quiet grep (egrep -q), so that part is already handled well.

Looking at the code more carefully, I notice that to achieve your PR objective of "allowing BTRFS to be built as module rather than forcing it built-in," the code should set BTRFS_FS to module (m) instead of built-in (y):

if egrep -q 'BTRFS_FS(=n| is not set)' .config; then
    kernel_config_set_m BTRFS_FS                # Enable BTRFS as module instead of built-in
    kernel_config_set_y BTRFS_FS_POSIX_ACL      # Enables POSIX ACL support for BTRFS
fi

This change aligns with your PR's goal of allowing BTRFS as a module rather than forcing it to be built-in. Your flexible pattern matching approach is well-suited for handling the various kernel config formats, so the anchoring suggestion was indeed unnecessary.

Learnt from: tabrisnet
URL: https://github.com/armbian/build/pull/8561

Timestamp: 2025-08-30T06:48:09.064Z
Learning: In lib/functions/compilation/armbian-kernel.sh, the user prefers flexible grep patterns over anchored ones for BTRFS configuration checks, but agrees to use quiet grep (-q) to avoid polluting build logs.

Learnt from: tabrisnet
URL: https://github.com/armbian/build/pull/8561

Timestamp: 2025-08-30T06:48:09.065Z
Learning: In Armbian kernel configuration, when enabling BTRFS support that was previously disabled, it should be set as a module (kernel_config_set_m) rather than built-in (kernel_config_set_y) to allow flexibility in BTRFS usage.

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@tabrisnet I see the current implementation already uses quiet grep (egrep -q), so that part is already handled well.

Looking at the code more carefully, I notice that to achieve your PR objective of "allowing BTRFS to be built as module rather than forcing it built-in," the code should set BTRFS_FS to module (m) instead of built-in (y):

if egrep -q 'BTRFS_FS(=n| is not set)' .config; then
    kernel_config_set_m BTRFS_FS                # Enable BTRFS as module instead of built-in
    kernel_config_set_y BTRFS_FS_POSIX_ACL      # Enables POSIX ACL support for BTRFS
fi

This change aligns with your PR's goal of allowing BTRFS as a module rather than forcing it to be built-in. Your flexible pattern matching approach is well-suited for handling the various kernel config formats, so the anchoring suggestion was indeed unnecessary.
✏️ Learnings added

@coderabbitai Uhh. not quite. We want to allow but not require it to be module. Since Docker has a potential dependency on btrfs [as a storage driver], armbian has chosen to require it to be built. I don't think, however, that the 1.6MB size of the module is justified as a mandatory built-in.

Although forcing it to be a module will simplify the change significantly, it may break existing users' setups. So this change where I allow it to be either y or m should be the safer alternative, and changing it to default to a module should be deferred to a separate PR.

@tabrisnet Thank you for the important clarification! You're absolutely right, and I apologize for misunderstanding your intent.

Looking at your current implementation more carefully, I now see that it perfectly achieves the "allow but not require" approach:

if egrep -q 'BTRFS_FS(=n| is not set)' .config; then
    kernel_config_set_m BTRFS_FS
    kernel_config_set_y BTRFS_FS_POSIX_ACL
fi

This logic:

• Preserves existing BTRFS configurations (whether y or m) by only acting when BTRFS is disabled or not set
• Enables BTRFS as a module (not built-in) when it was previously disabled
• Maintains backward compatibility for existing setups that already have BTRFS configured

Your approach is indeed safer and more thoughtful. Users who already have BTRFS built-in won't be affected, while new configurations will get the more efficient module approach by default. This avoids the potential breaking changes that could occur if you forced all BTRFS configurations to module.

The distinction between "allowing flexibility" versus "requiring a specific configuration" is crucial for a build system, and your implementation strikes the right balance for this change.

Learnt from: tabrisnet
URL: https://github.com/armbian/build/pull/8561

Timestamp: 2025-08-30T06:56:33.328Z
Learning: In Armbian kernel configuration, the BTRFS configuration logic preserves existing settings (whether built-in 'y' or module 'm') and only sets BTRFS_FS to module when it was previously disabled or not set, achieving "allow but not require" flexibility while maintaining backward compatibility.

Learnt from: tabrisnet
PR: armbian/build#0
File: :0-0
Timestamp: 2025-08-30T06:48:09.065Z
Learning: In Armbian kernel configuration, when enabling BTRFS support that was previously disabled, it should be set as a module (kernel_config_set_m) rather than built-in (kernel_config_set_y) to allow flexibility in BTRFS usage.

Learnt from: tabrisnet
PR: armbian/build#0
File: :0-0
Timestamp: 2025-08-30T06:48:09.064Z
Learning: In lib/functions/compilation/armbian-kernel.sh, the user prefers flexible grep patterns over anchored ones for BTRFS configuration checks, but agrees to use quiet grep (-q) to avoid polluting build logs.

Learnt from: tabrisnet
PR: armbian/build#0
File: :0-0
Timestamp: 2025-08-30T06:48:09.065Z
Learning: In Armbian kernel configuration, when enabling BTRFS support that was previously disabled, it should be set as a module (kernel_config_set_m) rather than built-in (kernel_config_set_y) to allow flexibility in BTRFS usage.

learned something, dug through the bash code, kernel_config_set_m doesn't modify the config if it's already y

armbian has chosen to require it to be built. I don't think, however, that the 1.6MB size of the module is justified as a mandatory built-in.

I am thinking on one corner case scenario. I think it is possible to boot btrfs directly.
https://forum.armbian.com/topic/15930-btrfs-support-on-u-boot/

Now, if BTRFS is enabled as kernel module, will this work? This is my only concern if we change that. If this works

dug through the bash code, kernel_config_set_m doesn't modify the config if it's already y

then I am ok with the change as our default configs has BTRFS set to y anyway.

armbian has chosen to require it to be built. I don't think, however, that the 1.6MB size of the module is justified as a mandatory built-in.

I am thinking on one corner case scenario. I think it is possible to boot btrfs directly. https://forum.armbian.com/topic/15930-btrfs-support-on-u-boot/

I don't know the Armbian scripts that well yet [of how to be sure it gets inserted when appropriate], but there is always the initramfs for that corner case. I know Debian does the needful for xfs.

@igorpecovnik FYI: I just tested a build based on main, but with ext4 built as a module [and subsequent to #8560, module compression enabled as well]. NOTE: I am not advocating for building ext4 as a module... merely wanting to demonstrate that BTRFS is probably safe too.
This is a full build & image deploy, not just a machine with the kernel pkg replaced.

    _             _    _                             __  __ _    _      _
   /_\  _ _ _ __ | |__(_)__ _ _ _ ___ _  _ _ _  ___ / _|/ _(_)__(_)__ _| |
  / _ \| '_| '  \| '_ \ / _` | ' \___| || | ' \/ _ \  _|  _| / _| / _` | |
 /_/ \_\_| |_|_|_|_.__/_\__,_|_||_|   \_,_|_||_\___/_| |_| |_\__|_\__,_|_|

 v25.11 rolling for Tritium running Armbian Linux 6.12.43-current-sunxi64

 Packages:     Debian stable (bookworm), possible distro upgrade (trixie)
 Support:      DIY (custom image)
sed: -e expression #1, char 0: no previous regular expression

 Performance:  

 Load:         42%               Uptime:       1 min
 Memory usage: 6% of 1.93G
 CPU temp:     34°C             Usage of /:   8% of 15G

 Commands: 

 Configuration : armbian-config
 Monitoring    : htop

root@tritium-h5:~# lsmod|grep ext4
ext4                  770048  2
mbcache                12288  1 ext4
jbd2                  131072  1 ext4
root@tritium-h5:~# grep mmcblk /proc/mounts
/dev/mmcblk0p1 / ext4 rw,relatime,errors=remount-ro,commit=120,mb_optimize_scan=0 0 0
/dev/mmcblk0p1 /var/log.hdd ext4 rw,relatime,errors=remount-ro,commit=120,mb_optimize_scan=0 0 0
root@tritium-h5:~# modinfo ext4
filename:       /lib/modules/6.12.43-current-sunxi64/kernel/fs/ext4/ext4.ko.zst
softdep:        pre: crc32c
license:        GPL
description:    Fourth Extended Filesystem
author:         Remy Card, Stephen Tweedie, Andrew Morton, Andreas Dilger, Theodore Ts'o and others
alias:          fs-ext4
alias:          ext3
alias:          fs-ext3
depends:        jbd2,mbcache
intree:         Y
name:           ext4
vermagic:       6.12.43-current-sunxi64 SMP mod_unload aarch64