[f2fs-dev] [PATCH] f2fs: fix to skip f2fs_balance_fs() if checkpoint is disabled

[f2fs-dev] [PATCH] f2fs: fix to skip f2fs_balance_fs() if checkpoint is disabled

[Chao Yu via Linux-f2fs-devel]

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 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@syzkaller.appspotmail.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@vivo.com>
Signed-off-by: Chao Yu <chao@kernel.org>
---
 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;
 
-- 
2.49.0



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