Re: [f2fs-dev] [PATCH v6] fsck.f2fs: Detect and fix looped node chain efficiently

[Chao Yu <chao@kernel.org>]

On 2023/5/22 18:34, Chunhai Guo wrote:
> find_fsync_inode() detect the looped node chain by comparing the loop
> counter with free blocks. While it may take tens of seconds to quit when
> the free blocks are large enough. We can use Floyd's cycle detection
> algorithm to make the detection more efficient, and fix the issue by
> filling a NULL address in the last node of the chain.
> 
> Below is the log we encounter on a 256GB UFS storage and it takes about 25
> seconds to detect looped node chain. After changing the algorithm, it takes
> about 20ms to finish the same job.
> 
>      [   10.822904] fsck.f2fs: Info: version timestamp cur: 17, prev: 430
>      [   10.822949] fsck.f2fs: [update_superblock: 762] Info: Done to update
> superblock
>      [   10.822953] fsck.f2fs: Info: superblock features = 1499 :  encrypt
> verity extra_attr project_quota quota_ino casefold
>      [   10.822956] fsck.f2fs: Info: superblock encrypt level = 0, salt =
> 00000000000000000000000000000000
>      [   10.822960] fsck.f2fs: Info: total FS sectors = 59249811 (231444 MB)
>      [   35.852827] fsck.f2fs:	detect looped node chain, blkaddr:1114802,
> next:1114803
>      [   35.852842] fsck.f2fs: [f2fs_do_mount:3846] record_fsync_data failed
>      [   35.856106] fsck.f2fs: fsck.f2fs terminated by exit(255)
> 
> Signed-off-by: Chunhai Guo <guochunhai@vivo.com>
> ---
> v5 -> v6 : Simplify the code by removing unnecessary retry logic.
> v4 -> v5 : Use IS_INODE() to make the code more clear.
> v3 -> v4 : Set c.bug_on with ASSERT_MSG() when issue is detected and fix
> 	it only if c.fix_on is 1.
> v2 -> v3 : Write inode with write_inode() to avoid chksum being broken.
> v1 -> v2 : Fix looped node chain directly after it is detected.
> ---
>   fsck/mount.c | 129 ++++++++++++++++++++++++++++++++++++++++++++-------
>   1 file changed, 111 insertions(+), 18 deletions(-)
> 
> diff --git a/fsck/mount.c b/fsck/mount.c
> index 4c7488840c7c..2b5937cdba7f 100644
> --- a/fsck/mount.c
> +++ b/fsck/mount.c
> @@ -3518,22 +3518,90 @@ static void destroy_fsync_dnodes(struct list_head *head)
>   		del_fsync_inode(entry);
>   }
>   
> +static int find_node_blk_fast(struct f2fs_sb_info *sbi, block_t *blkaddr_fast,
> +		struct f2fs_node *node_blk_fast, bool *is_detecting)
> +{
> +	int i, err;
> +
> +	for (i = 0; i < 2; i++) {
> +		if (!f2fs_is_valid_blkaddr(sbi, *blkaddr_fast, META_POR)) {
> +			*is_detecting = false;
> +			return 0;
> +		}
> +
> +		err = dev_read_block(node_blk_fast, *blkaddr_fast);
> +		if (err)
> +			return err;
> +
> +		if (!is_recoverable_dnode(sbi, node_blk_fast)) {
> +			*is_detecting = false;
> +			return 0;
> +		}
> +
> +		*blkaddr_fast = next_blkaddr_of_node(node_blk_fast);
> +	}
> +
> +	return 0;
> +}
> +
> +static int loop_node_chain_fix(struct f2fs_sb_info *sbi,
> +		block_t blkaddr_fast, struct f2fs_node *node_blk_fast,
> +		block_t blkaddr, struct f2fs_node *node_blk)
> +{
> +	block_t blkaddr_entry, blkaddr_tmp;
> +	int err;
> +
> +	/* find the entry point of the looped node chain */
> +	while (blkaddr_fast != blkaddr) {
> +		err = dev_read_block(node_blk_fast, blkaddr_fast);
> +		if (err)
> +			return err;
> +		blkaddr_fast = next_blkaddr_of_node(node_blk_fast);
> +
> +		err = dev_read_block(node_blk, blkaddr);
> +		if (err)
> +			return err;
> +		blkaddr = next_blkaddr_of_node(node_blk);
> +	}
> +	blkaddr_entry = blkaddr;
> +
> +	/* find the last node of the chain */
> +	do {
> +		blkaddr_tmp = blkaddr;
> +		err = dev_read_block(node_blk, blkaddr);
> +		if (err)
> +			return err;
> +		blkaddr = next_blkaddr_of_node(node_blk);
> +	} while (blkaddr != blkaddr_entry);
> +
> +	/* fix the blkaddr of last node with NULL_ADDR. */
> +	node_blk->footer.next_blkaddr = NULL_ADDR;
> +	if (IS_INODE(node_blk))
> +		err = write_inode(node_blk, blkaddr_tmp);
> +	else
> +		err = dev_write_block(node_blk, blkaddr_tmp);
> +	if (!err)
> +		FIX_MSG("Fix looped node chain on blkaddr %u\n",
> +				blkaddr_tmp);
> +	return err;
> +}
> +
>   static int find_fsync_inode(struct f2fs_sb_info *sbi, struct list_head *head)
>   {
>   	struct curseg_info *curseg;
> -	struct f2fs_node *node_blk;
> -	block_t blkaddr;
> -	unsigned int loop_cnt = 0;
> -	unsigned int free_blocks = MAIN_SEGS(sbi) * sbi->blocks_per_seg -
> -						sbi->total_valid_block_count;
> +	struct f2fs_node *node_blk, *node_blk_fast;
> +	block_t blkaddr, blkaddr_fast;
> +	bool is_detecting = true;
>   	int err = 0;
>   
> +	node_blk = calloc(F2FS_BLKSIZE, 1);
> +	node_blk_fast = calloc(F2FS_BLKSIZE, 1);
> +	ASSERT(node_blk && node_blk_fast);
> +
>   	/* get node pages in the current segment */
>   	curseg = CURSEG_I(sbi, CURSEG_WARM_NODE);
>   	blkaddr = NEXT_FREE_BLKADDR(sbi, curseg);
> -
> -	node_blk = calloc(F2FS_BLKSIZE, 1);
> -	ASSERT(node_blk);
> +	blkaddr_fast = blkaddr;
>   
>   	while (1) {
>   		struct fsync_inode_entry *entry;
> @@ -3564,19 +3632,44 @@ static int find_fsync_inode(struct f2fs_sb_info *sbi, struct list_head *head)
>   		if (IS_INODE(node_blk) && is_dent_dnode(node_blk))
>   			entry->last_dentry = blkaddr;
>   next:
> -		/* sanity check in order to detect looped node chain */
> -		if (++loop_cnt >= free_blocks ||
> -			blkaddr == next_blkaddr_of_node(node_blk)) {
> -			MSG(0, "\tdetect looped node chain, blkaddr:%u, next:%u\n",
> -				    blkaddr,
> -				    next_blkaddr_of_node(node_blk));
> -			err = -1;
> -			break;
> -		}
> -
>   		blkaddr = next_blkaddr_of_node(node_blk);
> +
> +		/* sanity check to detect looped node chain with Floyd's cycle
> +		 * detection algorithm
> +		 */
> +		if (is_detecting) {
> +			err = find_node_blk_fast(sbi, &blkaddr_fast,
> +					node_blk_fast, &is_detecting);
> +			if (err)
> +				break;

if (!is_detecing)
	continue;

Otherwise, in first round, blkaddr moves forward by one step, and blkaddr_fast may
moves forward by one step as well, if find_node_blk_fast() breaks when
f2fs_is_valid_blkaddr() or is_recoverable_dnode() returns false, then blkaddr_fast
will be equal to blkaddr?

Thanks,

> +
> +			if (blkaddr_fast != blkaddr)
> +				continue;
> +
> +			ASSERT_MSG("\tdetect looped node chain, blkaddr:%u\n",
> +					blkaddr);
> +
> +			if (!c.fix_on) {
> +				err = -1;
> +				break;
> +			}
> +
> +			err = loop_node_chain_fix(sbi,
> +					NEXT_FREE_BLKADDR(sbi, curseg),
> +					node_blk_fast, blkaddr, node_blk);
> +			if (err)
> +				break;
> +
> +			/* Since we call get_fsync_inode() to ensure there are
> +			 * no duplicate inodes in the inode_list even if there
> +			 * are duplicate blkaddr, we can continue running from
> +			 * here after fixing the looped node chain.
> +			 */
> +			is_detecting = false;
> +		}
>   	}
>   
> +	free(node_blk_fast);
>   	free(node_blk);
>   	return err;
>   }


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