linux-kernel - Re: [f2fs-dev] [PATCH v2] f2fs: fix to skip f2fs_balance

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Message-ID: <CAHJ8P3J8cX5+pVkE4TT24zh+wvW06KrpKXT2-7cRx3i8XzBbNw@mail.gmail.com>
Date: Fri, 23 May 2025 09:39:21 +0800
From: Zhiguo Niu <niuzhiguo84@...il.com>
To: Chao Yu <chao@...nel.org>
Cc: jaegeuk@...nel.org, syzbot+aa5bb5f6860e08a60450@...kaller.appspotmail.com, 
	Qi Han <hanqi@...o.com>, linux-kernel@...r.kernel.org, 
	linux-f2fs-devel@...ts.sourceforge.net
Subject: Re: [f2fs-dev] [PATCH v2] f2fs: fix to skip f2fs_balance_fs() if
 checkpoint is disabled

Chao Yu via Linux-f2fs-devel <linux-f2fs-devel@...ts.sourceforge.net>
于2025年5月21日周三 20:02写道：
>
> INFO: task syz-executor328:5856 blocked for more than 144 seconds.
>       Not tainted 6.15.0-rc6-syzkaller-00208-g3c21441eeffc #0
> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> task:syz-executor328 state:D stack:24392 pid:5856  tgid:5832  ppid:5826   task_flags:0x400040 flags:0x00004006
> Call Trace:
>  <TASK>
>  context_switch kernel/sched/core.c:5382 [inline]
>  __schedule+0x168f/0x4c70 kernel/sched/core.c:6767
>  __schedule_loop kernel/sched/core.c:6845 [inline]
>  schedule+0x165/0x360 kernel/sched/core.c:6860
>  io_schedule+0x81/0xe0 kernel/sched/core.c:7742
>  f2fs_balance_fs+0x4b4/0x780 fs/f2fs/segment.c:444
>  f2fs_map_blocks+0x3af1/0x43b0 fs/f2fs/data.c:1791
>  f2fs_expand_inode_data+0x653/0xaf0 fs/f2fs/file.c:1872
>  f2fs_fallocate+0x4f5/0x990 fs/f2fs/file.c:1975
>  vfs_fallocate+0x6a0/0x830 fs/open.c:338
>  ioctl_preallocate fs/ioctl.c:290 [inline]
>  file_ioctl fs/ioctl.c:-1 [inline]
>  do_vfs_ioctl+0x1b8f/0x1eb0 fs/ioctl.c:885
>  __do_sys_ioctl fs/ioctl.c:904 [inline]
>  __se_sys_ioctl+0x82/0x170 fs/ioctl.c:892
>  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>  do_syscall_64+0xf6/0x210 arch/x86/entry/syscall_64.c:94
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
>
> The root cause is after commit 84b5bb8bf0f6 ("f2fs: modify
> f2fs_is_checkpoint_ready logic to allow more data to be written with the
> CP disable"), we will get chance to allow f2fs_is_checkpoint_ready() to
> return true once below conditions are all true:
> 1. checkpoint is disabled
> 2. there are not enough free segments
> 3. there are enough free blocks
>
> Then it will cause f2fs_balance_fs() to trigger foreground GC.
>
> void f2fs_balance_fs(struct f2fs_sb_info *sbi, bool need)
> ...
>         if (!f2fs_is_checkpoint_ready(sbi))
>                 return;
>
> And it mounts f2fs image w/ gc_merge,checkpoint=disable, so below deadloop
> will happen:
>
> - f2fs_do_shutdown              - vfs_fallocate                         - gc_thread_func
>                                  - file_start_write
>                                   - __sb_start_write(SB_FREEZE_WRITE)
>                                  - f2fs_fallocate
>                                   - f2fs_expand_inode_data
>                                    - f2fs_map_blocks
>                                     - f2fs_balance_fs
>                                      - prepare_to_wait
>                                      - wake_up(gc_wait_queue_head)
>                                      - io_schedule
>  - bdev_freeze
>   - freeze_super
>    - sb->s_writers.frozen = SB_FREEZE_WRITE;
>    - sb_wait_write(sb, SB_FREEZE_WRITE);
>                                                                          - if (sbi->sb->s_writers.frozen >= SB_FREEZE_WRITE) continue;
>                                                                          : cause deadloop
>
> This patch fix to add check condition in f2fs_balance_fs(), so that if
> checkpoint is disabled, we will just skip trigger foreground GC to
> avoid above deadloop issue.
>
> Reported-by: syzbot+aa5bb5f6860e08a60450@...kaller.appspotmail.com
> Closes: https://lore.kernel.org/linux-f2fs-devel/682d743a.a00a0220.29bc26.0289.GAE@google.com
> Fixes: 84b5bb8bf0f6 ("f2fs: modify f2fs_is_checkpoint_ready logic to allow more data to be written with the CP disable")
> Cc: Qi Han <hanqi@...o.com>
> Signed-off-by: Chao Yu <chao@...nel.org>
> ---
> v2:
> - add missing Closes line
> - fix to use git commit description style
>
>  fs/f2fs/segment.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
> index 5ff0111ed974..19b716fda72a 100644
> --- a/fs/f2fs/segment.c
> +++ b/fs/f2fs/segment.c
> @@ -433,6 +433,8 @@ void f2fs_balance_fs(struct f2fs_sb_info *sbi, bool need)
>         if (need && excess_cached_nats(sbi))
>                 f2fs_balance_fs_bg(sbi, false);
>
> +       if (unlikely(is_sbi_flag_set(sbi, SBI_CP_DISABLED)))
> +               return;
>         if (!f2fs_is_checkpoint_ready(sbi))
>                 return;
Hi Chao,
When I read this patch, I feel that it is somewhat redundant with the
following checking about SBI_CP_DISABLED in f2fs_is_checkpoint_ready.
Can we reorganize these checks for the f2fs_balance_fs case?
This will make the code easier to read and understand.
thanks!
>
> --
> 2.49.0
>
>
>
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