fsck.f2fs: Detect and fix looped node chain efficiently

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>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

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(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 +3632,47 @@ 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));
+		blkaddr = next_blkaddr_of_node(node_blk);
+
+		/* Below we will detect looped node chain with Floyd's cycle
+		 * detection algorithm.
+		 */
+		if (!is_detecting)
+			continue;
+
+		err = find_node_blk_fast(sbi, &blkaddr_fast,
+				node_blk_fast, &is_detecting);
+		if (err)
+			break;
+
+		if (!is_detecting)
+			continue;
+
+		if (blkaddr_fast != blkaddr)
+			continue;
+
+		ASSERT_MSG("\tdetect looped node chain, blkaddr:%u\n",
+				blkaddr);
+
+		if (!c.fix_on) {
 			err = -1;
 			break;
 		}
 
-		blkaddr = next_blkaddr_of_node(node_blk);
+		err = loop_node_chain_fix(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 after fixing the
+		 * looped node chain.
+		 */
+		is_detecting = false;
 	}
 
+	free(node_blk_fast);
 	free(node_blk);
 	return err;
 }