linux-kernel - Re: [PATCH 1/2] f2fs: fix to add missing sb_{start,end}

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Date: Sat, 8 Jun 2024 20:34:47 +0800
From: Chao Yu <chao@...nel.org>
To: jaegeuk@...nel.org
Cc: linux-f2fs-devel@...ts.sourceforge.net, linux-kernel@...r.kernel.org,
 Daeho Jeong <daehojeong@...gle.com>
Subject: Re: [PATCH 1/2] f2fs: fix to add missing sb_{start,end}_intwrite()
 for ckpt thread

On 2024/6/6 17:52, Chao Yu wrote:
> After commit 261eeb9c1585 ("f2fs: introduce checkpoint_merge mount
> option"), checkpoint can be triggered in background thread, it missed
> to cover f2fs inner checkpoint operation w/ sb_{start,end}_intwrite(),
> fix it.

It needs to use sb_start_intwrite_trylock(), otherwise, it will cause
deadlock as below:

- freeze_super
  - sb_wait_write(SB_FREEZE_WRITE)
  - sb_wait_write(SB_FREEZE_PAGEFAULT)
  - sb_wait_write(SB_FREEZE_FS)
					- sync
					 - iterate_supers
					  - super_lock_shared
					   - down_read(&sb->s_umount)
					   - sync_fs_one_sb
					    - f2fs_sync_fs
					     - f2fs_issue_checkpoint
					      - wait_for_completion
									- issue_checkpoint_thread
									 - sb_start_intwrite(sbi->sb);

- thaw_super
  - super_lock_excl
   - down_write(&sb->s_umount)

Thanks,

> 
> Fixes: 261eeb9c1585 ("f2fs: introduce checkpoint_merge mount option")
> Cc: Daeho Jeong <daehojeong@...gle.com>
> Signed-off-by: Chao Yu <chao@...nel.org>
> ---
>   fs/f2fs/checkpoint.c | 5 ++++-
>   1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/f2fs/checkpoint.c b/fs/f2fs/checkpoint.c
> index 55d444bec5c0..66eaad591b60 100644
> --- a/fs/f2fs/checkpoint.c
> +++ b/fs/f2fs/checkpoint.c
> @@ -1828,8 +1828,11 @@ static int issue_checkpoint_thread(void *data)
>   	if (kthread_should_stop())
>   		return 0;
>   
> -	if (!llist_empty(&cprc->issue_list))
> +	if (!llist_empty(&cprc->issue_list)) {
> +		sb_start_intwrite(sbi->sb);
>   		__checkpoint_and_complete_reqs(sbi);
> +		sb_end_intwrite(sbi->sb);
> +	}
>   
>   	wait_event_interruptible(*q,
>   		kthread_should_stop() || !llist_empty(&cprc->issue_list));