linux-kernel - Re: [f2fs-dev] [PATCH] f2fs: fix to handle looped node chain during recovery

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Date:   Sat, 3 Feb 2018 18:35:53 +0800
From:   Gao Xiang <hsiangkao@....com>
To:     Chao Yu <yuchao0@...wei.com>, jaegeuk@...nel.org,
        Yunlei He <heyunlei@...wei.com>
Cc:     linux-kernel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net
Subject: Re: [f2fs-dev] [PATCH] f2fs: fix to handle looped node chain during
 recovery

Hi Chao and YunLei,


On 2018/2/3 17:44, Chao Yu wrote:
> There is no checksum in node block now, so bit-transition from hardware
> can make node_footer.next_blkaddr being corrupted w/o any detection,
> result in node chain becoming looped one.
>
> For this condition, during recovery, in order to avoid running into dead
> loop, let's detect it and just skip out.
>
> Signed-off-by: Yunlei He <heyunlei@...wei.com>
> Signed-off-by: Chao Yu <yuchao0@...wei.com>
> ---
>   fs/f2fs/recovery.c | 14 ++++++++++++++
>   1 file changed, 14 insertions(+)
>
> diff --git a/fs/f2fs/recovery.c b/fs/f2fs/recovery.c
> index b6d1ec620a8c..60dd0cee4820 100644
> --- a/fs/f2fs/recovery.c
> +++ b/fs/f2fs/recovery.c
> @@ -243,6 +243,9 @@ static int find_fsync_dnodes(struct f2fs_sb_info *sbi, struct list_head *head,
>   	struct curseg_info *curseg;
>   	struct page *page = NULL;
>   	block_t blkaddr;
> +	unsigned int loop_cnt = 0;
> +	unsigned int free_blocks = sbi->user_block_count -
> +					valid_user_blocks(sbi);
There exists another way to detect loop more faster but only using two 
variables.
The algorithm is described as simply "B goes forward a steps only A goes 
forwards 2 steps".
For example:
1)
    1   2  3  4   5   6     7
    |             \             /
    |                \------/
   A, B
2)
    1  2  3  4   5   6     7
     |   |        \             /
    B   A        \------/
  3)
    1  2  3  4   5   6     7
        |    |     \             /
       B   A      \------/
   4)
    1  2  3  4   5   6     7
        |       |\             /
       B      A \------/
5)....

B will equal A or beyoud A if and only if there has a cycle.
It's a more faster algorithm. :D

Thanks,

>   	int err = 0;
>   
>   	/* get node pages in the current segment */
> @@ -295,6 +298,17 @@ static int find_fsync_dnodes(struct f2fs_sb_info *sbi, struct list_head *head,
>   		if (IS_INODE(page) && is_dent_dnode(page))
>   			entry->last_dentry = blkaddr;
>   next:
> +		/* sanity check in order to detect looped node chain */
> +		if (++loop_cnt >= free_blocks ||
> +			blkaddr == next_blkaddr_of_node(page)) {
> +			f2fs_msg(sbi->sb, KERN_NOTICE,
> +				"%s: detect looped node chain, "
> +				"blkaddr:%u, next:%u",
> +				__func__, blkaddr, next_blkaddr_of_node(page));
> +			err = -EINVAL;
> +			break;
> +		}
> +
>   		/* check next segment */
>   		blkaddr = next_blkaddr_of_node(page);
>   		f2fs_put_page(page, 1);