Re: [syzbot] [btrfs?] kernel BUG in populate_free_space_tree

[syzbot <syzbot+36fae25c35159a763a2a@syzkaller.appspotmail.com>]

Hello,

syzbot has tested the proposed patch but the reproducer is still triggering an issue:
WARNING in btrfs_rebuild_free_space_tree

BTRFS warning: 'usebackuproot' is deprecated, use 'rescue=usebackuproot' instead
BTRFS info (device loop2 state M): rebuilding free space tree
------------[ cut here ]------------
BTRFS: Transaction aborted (error -17)
WARNING: CPU: 1 PID: 7626 at fs/btrfs/free-space-tree.c:1341 btrfs_rebuild_free_space_tree+0x470/0x54c fs/btrfs/free-space-tree.c:1341
Modules linked in:
CPU: 1 UID: 0 PID: 7626 Comm: syz.2.25 Not tainted 6.15.0-rc7-syzkaller-00085-gd7fa1af5b33e-dirty #0 PREEMPT 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/07/2025
pstate: 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
pc : btrfs_rebuild_free_space_tree+0x470/0x54c fs/btrfs/free-space-tree.c:1341
lr : btrfs_rebuild_free_space_tree+0x470/0x54c fs/btrfs/free-space-tree.c:1341
sp : ffff80009c4f7740
x29: ffff80009c4f77b0 x28: ffff0000d4c3f400 x27: 0000000000000000
x26: dfff800000000000 x25: ffff70001389eee8 x24: 0000000000000003
x23: 1fffe000182b6e7b x22: 0000000000000000 x21: ffff0000c15b73d8
x20: 00000000ffffffef x19: ffff0000c15b7378 x18: 1fffe0003386f276
x17: ffff80008f31e000 x16: ffff80008adbe98c x15: 0000000000000001
x14: 1fffe0001b281550 x13: 0000000000000000 x12: 0000000000000000
x11: ffff60001b281551 x10: 0000000000000003 x9 : 1c8922000a902c00
x8 : 1c8922000a902c00 x7 : ffff800080485878 x6 : 0000000000000000
x5 : 0000000000000001 x4 : 0000000000000001 x3 : ffff80008047843c
x2 : 0000000000000001 x1 : ffff80008b3ebc40 x0 : 0000000000000001
Call trace:
 btrfs_rebuild_free_space_tree+0x470/0x54c fs/btrfs/free-space-tree.c:1341 (P)
 btrfs_start_pre_rw_mount+0xa78/0xe10 fs/btrfs/disk-io.c:3074
 btrfs_remount_rw fs/btrfs/super.c:1319 [inline]
 btrfs_reconfigure+0x828/0x2418 fs/btrfs/super.c:1543
 reconfigure_super+0x1d4/0x6f0 fs/super.c:1083
 do_remount fs/namespace.c:3365 [inline]
 path_mount+0xb34/0xde0 fs/namespace.c:4200
 do_mount fs/namespace.c:4221 [inline]
 __do_sys_mount fs/namespace.c:4432 [inline]
 __se_sys_mount fs/namespace.c:4409 [inline]
 __arm64_sys_mount+0x3e8/0x468 fs/namespace.c:4409
 __invoke_syscall arch/arm64/kernel/syscall.c:35 [inline]
 invoke_syscall+0x98/0x2b8 arch/arm64/kernel/syscall.c:49
 el0_svc_common+0x130/0x23c arch/arm64/kernel/syscall.c:132
 do_el0_svc+0x48/0x58 arch/arm64/kernel/syscall.c:151
 el0_svc+0x58/0x17c arch/arm64/kernel/entry-common.c:767
 el0t_64_sync_handler+0x78/0x108 arch/arm64/kernel/entry-common.c:786
 el0t_64_sync+0x198/0x19c arch/arm64/kernel/entry.S:600
irq event stamp: 330
hardirqs last  enabled at (329): [<ffff80008048590c>] raw_spin_rq_unlock_irq kernel/sched/sched.h:1525 [inline]
hardirqs last  enabled at (329): [<ffff80008048590c>] finish_lock_switch+0xb0/0x1c0 kernel/sched/core.c:5130
hardirqs last disabled at (330): [<ffff80008adb9e60>] el1_dbg+0x24/0x80 arch/arm64/kernel/entry-common.c:511
softirqs last  enabled at (10): [<ffff8000801fbf10>] local_bh_enable+0x10/0x34 include/linux/bottom_half.h:32
softirqs last disabled at (8): [<ffff8000801fbedc>] local_bh_disable+0x10/0x34 include/linux/bottom_half.h:19
---[ end trace 0000000000000000 ]---
BTRFS: error (device loop2 state MA) in btrfs_rebuild_free_space_tree:1341: errno=-17 Object already exists
BTRFS warning (device loop2 state EMA): failed to rebuild free space tree: -17


Tested on:

commit:         d7fa1af5 Merge branch 'for-next/core' into for-kernelci
git tree:       git://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git for-kernelci
console output: https://syzkaller.appspot.com/x/log.txt?x=152cfc0c580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=89c13de706fbf07a
dashboard link: https://syzkaller.appspot.com/bug?extid=36fae25c35159a763a2a
compiler:       Debian clang version 20.1.6 (++20250514063057+1e4d39e07757-1~exp1~20250514183223.118), Debian LLD 20.1.6
userspace arch: arm64
patch:          https://syzkaller.appspot.com/x/patch.diff?x=168d7282580000