Message-ID: <CAL3q7H6Jc9-NMyk49Dykt291jpuVfWAEDnZtfnC3jjc13FLVcw@mail.gmail.com>
Date: Thu, 6 Nov 2025 11:53:54 +0000
From: Filipe Manana <fdmanana@...nel.org>
To: syzbot <syzbot+feba382c68462d76be14@...kaller.appspotmail.com>
Cc: clm@...com, dsterba@...e.com, josef@...icpanda.com, 
	linux-btrfs@...r.kernel.org, linux-kernel@...r.kernel.org, 
	syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [btrfs?] kernel BUG in reserve_bytes

On Wed, Nov 5, 2025 at 10:54 PM syzbot
<syzbot+feba382c68462d76be14@...kaller.appspotmail.com> wrote:
>
> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit:    98bd8b16ae57 Add linux-next specific files for 20251031
> git tree:       linux-next
> console output: https://syzkaller.appspot.com/x/log.txt?x=17abfe7c580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=63d09725c93bcc1c
> dashboard link: https://syzkaller.appspot.com/bug?extid=feba382c68462d76be14
> compiler:       Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
>
> Unfortunately, I don't have any reproducer for this issue yet.
>
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/975261746f29/disk-98bd8b16.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/ad565c6cf272/vmlinux-98bd8b16.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/1816a55a8d5f/bzImage-98bd8b16.xz
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+feba382c68462d76be14@...kaller.appspotmail.com
>
> assertion failed: !(ticket->bytes == 0 && ticket->error) :: 0, in fs/btrfs/space-info.c:1671
> ------------[ cut here ]------------
> kernel BUG at fs/btrfs/space-info.c:1671!

It's a race introduced with the patch: "btrfs: avoid space_info
locking when checking if tickets are served"

I'll fold the following diff to patch and push it to the for-next branch.

diff --git a/fs/btrfs/space-info.c b/fs/btrfs/space-info.c
index 63d14b5dfc6c..85c96b8bef7f 100644
--- a/fs/btrfs/space-info.c
+++ b/fs/btrfs/space-info.c
@@ -527,7 +527,12 @@ static void remove_ticket(struct btrfs_space_info
*space_info,
        }

        spin_lock(&ticket->lock);
-       if (error)
+       /*
+        * If we are called from a task waiting on the ticket, it may happen
+        * that before it sets an error on the ticket, a reclaim task was able
+        * to satisfy the ticket. In that case ignore the error.
+        */
+       if (error && ticket->bytes > 0)
                ticket->error = error;
        else
                ticket->bytes = 0;

Thanks.


> Oops: invalid opcode: 0000 [#1] SMP KASAN PTI
> CPU: 0 UID: 0 PID: 7313 Comm: syz.1.358 Not tainted syzkaller #0 PREEMPT(full)
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/02/2025
> RIP: 0010:handle_reserve_ticket fs/btrfs/space-info.c:1671 [inline]
> RIP: 0010:reserve_bytes+0x129e/0x1410 fs/btrfs/space-info.c:1864
> Code: 0f 0b e8 15 f0 cc fd 48 c7 c7 60 c9 b0 8b 48 c7 c6 80 d5 b0 8b 31 d2 48 c7 c1 40 c6 b0 8b 41 b8 87 06 00 00 e8 f3 f8 33 fd 90 <0f> 0b f3 0f 1e fa 65 8b 1d 59 4a 80 0e bf 07 00 00 00 89 de e8 19
> RSP: 0018:ffffc9000f2ff180 EFLAGS: 00010246
> RAX: 000000000000005c RBX: 0000000000000000 RCX: 6a0e5d27bc457900
> RDX: ffffc9000d29b000 RSI: 00000000000080c9 RDI: 00000000000080ca
> RBP: ffffc9000f2ff3c0 R08: ffffc9000f2feea7 R09: 1ffff92001e5fdd4
> R10: dffffc0000000000 R11: fffff52001e5fdd5 R12: ffff88814e273000
> R13: dffffc0000000000 R14: 00000000fffffffc R15: ffffc9000f2ff220
> FS:  00007f71682106c0(0000) GS:ffff888125ee2000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007fe3d8bb6d60 CR3: 0000000029df0000 CR4: 00000000003526f0
> Call Trace:
>  <TASK>
>  btrfs_reserve_metadata_bytes+0x28/0x150 fs/btrfs/space-info.c:1887
>  btrfs_reserve_trans_metadata fs/btrfs/transaction.c:577 [inline]
>  start_transaction+0x102c/0x1610 fs/btrfs/transaction.c:658
>  btrfs_replace_file_extents+0x2b1/0x1de0 fs/btrfs/file.c:2432
>  insert_prealloc_file_extent fs/btrfs/inode.c:9004 [inline]
>  __btrfs_prealloc_file_range+0x48d/0xcf0 fs/btrfs/inode.c:9071
>  btrfs_prealloc_file_range+0x40/0x60 fs/btrfs/inode.c:9149
>  btrfs_zero_range+0xb9a/0xe00 fs/btrfs/file.c:3073
>  btrfs_fallocate+0xb95/0x1c10 fs/btrfs/file.c:3187
>  vfs_fallocate+0x669/0x7e0 fs/open.c:342
>  ksys_fallocate fs/open.c:366 [inline]
>  __do_sys_fallocate fs/open.c:371 [inline]
>  __se_sys_fallocate fs/open.c:369 [inline]
>  __x64_sys_fallocate+0xc0/0x110 fs/open.c:369
>  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>  do_syscall_64+0xfa/0xfa0 arch/x86/entry/syscall_64.c:94
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> RIP: 0033:0x7f716738efc9
> Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 a8 ff ff ff f7 d8 64 89 01 48
> RSP: 002b:00007f7168210038 EFLAGS: 00000246 ORIG_RAX: 000000000000011d
> RAX: ffffffffffffffda RBX: 00007f71675e6090 RCX: 00007f716738efc9
> RDX: 0000000000003ffd RSI: 0000000000000010 RDI: 000000000000000a
> RBP: 00007f7167411f91 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000008000 R11: 0000000000000246 R12: 0000000000000000
> R13: 00007f71675e6128 R14: 00007f71675e6090 R15: 00007fff6c300678
>  </TASK>
> Modules linked in:
> ---[ end trace 0000000000000000 ]---
> RIP: 0010:handle_reserve_ticket fs/btrfs/space-info.c:1671 [inline]
> RIP: 0010:reserve_bytes+0x129e/0x1410 fs/btrfs/space-info.c:1864
> Code: 0f 0b e8 15 f0 cc fd 48 c7 c7 60 c9 b0 8b 48 c7 c6 80 d5 b0 8b 31 d2 48 c7 c1 40 c6 b0 8b 41 b8 87 06 00 00 e8 f3 f8 33 fd 90 <0f> 0b f3 0f 1e fa 65 8b 1d 59 4a 80 0e bf 07 00 00 00 89 de e8 19
> RSP: 0018:ffffc9000f2ff180 EFLAGS: 00010246
> RAX: 000000000000005c RBX: 0000000000000000 RCX: 6a0e5d27bc457900
> RDX: ffffc9000d29b000 RSI: 00000000000080c9 RDI: 00000000000080ca
> RBP: ffffc9000f2ff3c0 R08: ffffc9000f2feea7 R09: 1ffff92001e5fdd4
> R10: dffffc0000000000 R11: fffff52001e5fdd5 R12: ffff88814e273000
> R13: dffffc0000000000 R14: 00000000fffffffc R15: ffffc9000f2ff220
> FS:  00007f71682106c0(0000) GS:ffff888125ee2000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007ffdaa1f0eb8 CR3: 0000000029df0000 CR4: 00000000003526f0
>
>
> ---
> This report is generated by a bot. It may contain errors.
> See https://goo.gl/tpsmEJ for more information about syzbot.
> syzbot engineers can be reached at syzkaller@...glegroups.com.
>
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>
> If the report is already addressed, let syzbot know by replying with:
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>
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>
> If the report is a duplicate of another one, reply with:
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>
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>

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