ACK: [SRU][N][O][PATCH 0/1] btrfs will WARN_ON() in btrfs_remove_qgroup() unnecessarily

[Tim Whisonant]

On Fri, Jan 24, 2025 at 02:39:36PM +1300, Matthew Ruffell wrote:
> BugLink: https://bugs.launchpad.net/bugs/2091719
> 
> [Impact]
> 
> The following commit for noble and oracular introduced two new WARN_ON() calls
> in btrfs qgroup removals, and even though the author at the time believed they
> would not be reachable, it turns out it can happen quite frequently in the
> right conditions.
> 
> ubuntu-noble b2ad25ba539452f492805e5f7d94e80894aa860f
> commit a776bf5f3c2300cfdf8a195663460b1793ac9847
> Author: Qu Wenruo <wqu at suse.com>
> Date: Fri Apr 19 14:29:32 2024 +0930
> Subject: btrfs: slightly loosen the requirement for qgroup removal
> Link: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=a776bf5f3c2300cfdf8a195663460b1793ac9847
> 
> $ git describe --contains b2ad25ba539452f492805e5f7d94e80894aa860f
> Ubuntu-6.8.0-50.51~143
> 
> This primarily affects the systemd CI that runs integration tests on merge:
> https://github.com/systemd/systemd/actions/runs/12297539029/job/34318915884?pr=35589
> 
> Kernel panic - not syncing: kernel: panic_on_warn set ...
> CPU: 0 PID: 1316 Comm: (sd-clean) Not tainted 6.8.0-50-generic #51-Ubuntu
> Call Trace:
>  <TASK>
>  dump_stack_lvl+0x27/0xa0
>  dump_stack+0x10/0x20
>  panic+0x366/0x3c0
>  ? btrfs_remove_qgroup+0x271/0x490 [btrfs]
>  check_panic_on_warn+0x4f/0x60
>  __warn+0x95/0x160
>  ? btrfs_remove_qgroup+0x271/0x490 [btrfs]
>  report_bug+0x17e/0x1b0
>  handle_bug+0x51/0xa0
>  exc_invalid_op+0x18/0x80
>  asm_exc_invalid_op+0x1b/0x20
> RIP: 0010:btrfs_remove_qgroup+0x271/0x490 [btrfs]
> Code: c0 0f 85 27 fe ff ff 48 8b 43 b0 4c 39 f0 75 d5 4d 8d b5 e0 08 00 00 4c 89 f7 e8 8a 45 19 e2 48 83 7b 98 00 0f 84 52 01 00 00 <0f> 0b 49 8b 45 10 a8 10 74 42 41 f6 85 d0 08 00 00 0c 75 38 48 83
>  ? btrfs_remove_qgroup+0x266/0x490 [btrfs]
>  btrfs_ioctl+0x12b9/0x13a0 [btrfs]
>  ? srso_alias_return_thunk+0x5/0xfbef5
>  ? __seccomp_filter+0x368/0x570
>  ? __fput+0x15e/0x2e0
>  __x64_sys_ioctl+0xa3/0xf0
>  x64_sys_call+0x12a3/0x25a0
>  do_syscall_64+0x7f/0x180
>  entry_SYSCALL_64_after_hwframe+0x78/0x80
> 
> [Fix]
> 
> The fix just landed in mainline as:
> 
> commit c0def46dec9c547679a25fe7552c4bcbec0b0dd2
> Author: Qu Wenruo <wqu at suse.com>
> Date:   Mon Nov 11 07:29:07 2024 +1030
> Subject: btrfs: improve the warning and error message for btrfs_remove_qgroup()
> Link: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=c0def46dec9c547679a25fe7552c4bcbec0b0dd2
> 
> The commit places the WARN_ON behind CONFIG_BTRFS_DEBUG, which silences the
> warning for most users. It is safe to do so, as noted by the Author, as
> the user space tool managing the qgroups would rescan them, to fix the
> inconsistent view.
> 
> This is needed for both noble and oracular.
> 
> [Testcase]
> 
> The upstream systemd CI tests can consistently reproduce the issue, so the test
> and proposed kernels will be run against the systemd CI for verification.
> 
> There is a test kernel available in the following ppa:
> 
> https://launchpad.net/~mruffell/+archive/ubuntu/lp2091719-test
> 
> If you install it, the systemd CI will run to completion.
> 
> [Where problems could occur]
> 
> We are changing the WARN_ON() to occur only when CONFIG_BTRFS_DEBUG is enabled.
> There is no other change in logic, so functionality should be the same as what
> we have now.
> 
> If a regression were to occur, it would affect systems with btrfs filesystems
> that are utilising subvolumes. It would not likely cause any data loss or disk
> corruption, as userspace tools should be able to automatically fix up any
> inconsistent views without user interaction.
> 
> [Other info]
> 
> Systemd upstream bisected the issue here:
> https://github.com/systemd/systemd/pull/35567#issuecomment-2538160543
> 
> Qu Wenruo (1):
>   btrfs: improve the warning and error message for btrfs_remove_qgroup()
> 
>  fs/btrfs/qgroup.c | 21 ++++++++++++++++-----
>  1 file changed, 16 insertions(+), 5 deletions(-)
> 
> -- 
> 2.45.2
> 

Acked-by: Tim Whisonant <tim.whisonant at canonical.com>