一. 用户空间 f2fs文件系统 Mount(挂载)

     在挂载之前，我们需要使用mkfs.f2fs工具对块设备进行格式化为f2fs文件系统(如果在Mount之前不格式化成f2fs文件系统，后续Mount挂载的时候会报错，不识别f2fs文件系统)

     mkfs.f2fs工具作用就是在块设备上建立f2fs文件系统

     1. # mkfs.f2fs -l label /dev/block_device 

    将块设备挂载到/mnt/f2fs目录，挂载的文件系统格式为F2fs文件系统

     2. # mount -t f2fs /dev/block_device /mnt/f2fs

二. 用户空间Mount 系统调用进入内核空间f2fs_mount

三. 内核空间f2fs文件系统Mount (f2fs_mount)

用户空间执行Mount命令---->系统调用 ---->内核空间f2fs文件系统 f2fs_mount

1. f2fs文件系统 f2fs_fs_type 结构体含有 mount成员，当用户在用户空间执行mount操作时，会回调到这个mount位置，由f2fs_mount 执行接下去的Mount动作,

static struct file_system_type f2fs_fs_type = {
	.owner		= THIS_MODULE,
	.name		= "f2fs",
	.mount		= f2fs_mount,
	.kill_sb	= kill_f2fs_super,
	.fs_flags	= FS_REQUIRES_DEV,
};

2. f2fs_mount是针对块设备挂载成f2fs文件系统的函数

static struct dentry *f2fs_mount(struct file_system_type *fs_type, int flags,
			const char *dev_name, void *data)
{
	return mount_bdev(fs_type, flags, dev_name, data, f2fs_fill_super);
}

f2fs_fill_super：填充f2fs super block信息

3.mount_bdev是针对块设备挂载时使用的函数，执行块设备挂载，这里块设备指的时 /dev/block_device ，此外还有mount_nodev, mount_single等函数，分别用于不同的挂载情况

struct dentry *mount_bdev(struct file_system_type *fs_type,
	int flags, const char *dev_name, void *data,
	int (*fill_super)(struct super_block *, void *, int))
{
	struct block_device *bdev;
	struct super_block *s;
	fmode_t mode = FMODE_READ | FMODE_EXCL;
	int error = 0;

	if (!(flags & SB_RDONLY))
		mode |= FMODE_WRITE;

    /* 打开由dev_name/mode/fs_type描述的块设备*/ 
	bdev = blkdev_get_by_path(dev_name, mode, fs_type);
	if (IS_ERR(bdev))
		return ERR_CAST(bdev);

	/*
	 * once the super is inserted into the list by sget, s_umount
	 * will protect the lockfs code from trying to start a snapshot
	 * while we are mounting
	 */
	mutex_lock(&bdev->bd_fsfreeze_mutex);
	if (bdev->bd_fsfreeze_count > 0) {
		mutex_unlock(&bdev->bd_fsfreeze_mutex);
		error = -EBUSY;
		goto error_bdev;
	}
    /* find or create a superblock */
	s = sget(fs_type, test_bdev_super, set_bdev_super, flags | SB_NOSEC,
		 bdev);
	mutex_unlock(&bdev->bd_fsfreeze_mutex);
	if (IS_ERR(s))
		goto error_s;

	if (s->s_root) {
		if ((flags ^ s->s_flags) & SB_RDONLY) {
			deactivate_locked_super(s);
			error = -EBUSY;
			goto error_bdev;
		}

		/*
		 * s_umount nests inside bd_mutex during
		 * __invalidate_device().  blkdev_put() acquires
		 * bd_mutex and can't be called under s_umount.  Drop
		 * s_umount temporarily.  This is safe as we're
		 * holding an active reference.
		 */
		up_write(&s->s_umount);
		blkdev_put(bdev, mode);
		down_write(&s->s_umount);
	} else {
		s->s_mode = mode;
		snprintf(s->s_id, sizeof(s->s_id), "%pg", bdev);
		sb_set_blocksize(s, block_size(bdev));
        /* 填充f2fs super block信息*/
		error = fill_super(s, data, flags & SB_SILENT ? 1 : 0);
		if (error) {
			deactivate_locked_super(s);
			goto error;
		}

		s->s_flags |= SB_ACTIVE;
		bdev->bd_super = s;
	}

	return dget(s->s_root);

error_s:
	error = PTR_ERR(s);
error_bdev:
	blkdev_put(bdev, mode);
error:
	return ERR_PTR(error);
}
EXPORT_SYMBOL(mount_bdev);

4. f2fs_fill_super是填充f2fs super block信息

Superblock区域

Superblock保存了F2FS的核心元数据的结构，包括磁盘大小，元区域的各个部分的起始地址等。

Superblock在元数据区域的物理结构

Superblock区域是由两个struct f2fs_super_block结构组成，互为备份

(1)Superblock物理存放区域结构

struct f2fs_super_block是F2FS对Superblock的具体数据结构实现，它保存在磁盘的最开始的位置，F2FS进行挂载时从磁盘的前端直接读取出来，然后转换为struct f2fs_super_block结构。它的定义如下:
 

struct f2fs_super_block {
	__le32 magic;			/* Magic Number */
	__le16 major_ver;		/* Major Version */
	__le16 minor_ver;		/* Minor Version */
	__le32 log_sectorsize;		/* log2 sector size in bytes */
	__le32 log_sectors_per_block;	/* log2 # of sectors per block */
	__le32 log_blocksize;		/* log2 block size in bytes */
	__le32 log_blocks_per_seg;	/* log2 # of blocks per segment */
	__le32 segs_per_sec;		/* # of segments per section */
	__le32 secs_per_zone;		/* # of sections per zone */
	__le32 checksum_offset;		/* checksum offset inside super block */
	__le64 block_count;		/* total # of user blocks */
	__le32 section_count;		/* total # of sections */
	__le32 segment_count;		/* total # of segments */
	__le32 segment_count_ckpt;	/* # of segments for checkpoint */
	__le32 segment_count_sit;	/* # of segments for SIT */
	__le32 segment_count_nat;	/* # of segments for NAT */
	__le32 segment_count_ssa;	/* # of segments for SSA */
	__le32 segment_count_main;	/* # of segments for main area */
	__le32 segment0_blkaddr;	/* start block address of segment 0 */
	__le32 cp_blkaddr;		/* start block address of checkpoint */
	__le32 sit_blkaddr;		/* start block address of SIT */
	__le32 nat_blkaddr;		/* start block address of NAT */
	__le32 ssa_blkaddr;		/* start block address of SSA */
	__le32 main_blkaddr;		/* start block address of main area */
	__le32 root_ino;		/* root inode number */
	__le32 node_ino;		/* node inode number */
	__le32 meta_ino;		/* meta inode number */
	__u8 uuid[16];			/* 128-bit uuid for volume */
	__le16 volume_name[MAX_VOLUME_NAME];	/* volume name */
	__le32 extension_count;		/* # of extensions below */
	__u8 extension_list[F2FS_MAX_EXTENSION][F2FS_EXTENSION_LEN];/* extension array */
	__le32 cp_payload;
	__u8 version[VERSION_LEN];	/* the kernel version */
	__u8 init_version[VERSION_LEN];	/* the initial kernel version */
	__le32 feature;			/* defined features */
	__u8 encryption_level;		/* versioning level for encryption */
	__u8 encrypt_pw_salt[16];	/* Salt used for string2key algorithm */
	struct f2fs_device devs[MAX_DEVICES];	/* device list */
	__le32 qf_ino[F2FS_MAX_QUOTAS];	/* quota inode numbers */
	__u8 hot_ext_count;		/* # of hot file extension */
	__u8 reserved[246];		/* valid reserved region */
	__u8 mount_opts[64];            /* default mount option for SEC */
	__le32 crc;			/* checksum of superblock */
} __packed;

(2)Superblock内存管理结构：

f2fs_super_block在内存中的对应的结构是struct f2fs_sb_info，它除了包含了struct