bio

buffer_cache 안쓰는 이유?
buffer_cache의 메타데이터가 하나의 섹터를 관리하는데, 메타데이터의 크기가 너무 크다.
그래서, bio구조체를 이용해서 여러개의 섹터를 관리할 수 있는 자료구조를 만들어냈다.

struct bio {
        sector_t                bi_sector;      /* device address in 512 byte
                                                   sectors */
        struct bio              *bi_next;       /* request queue link */
        struct block_device     *bi_bdev;
        unsigned long           bi_flags;       /* status, command, etc */
        unsigned long           bi_rw;          /* bottom bits READ/WRITE,
                                                 * top bits priority
                                                 */

        unsigned short          bi_vcnt;        /* how many bio_vec's */
        unsigned short          bi_idx;         /* current index into bvl_vec */

        /* Number of segments in this BIO after
         * physical address coalescing is performed.
         */
        unsigned int            bi_phys_segments;

        unsigned int            bi_size;        /* residual I/O count */

        /*
         * To keep track of the max segment size, we account for the
         * sizes of the first and last mergeable segments in this bio.
         */
        unsigned int            bi_seg_front_size;
        unsigned int            bi_seg_back_size;

        unsigned int            bi_max_vecs;    /* max bvl_vecs we can hold */

        unsigned int            bi_comp_cpu;    /* completion CPU */

        atomic_t                bi_cnt;         /* pin count */

        struct bio_vec          *bi_io_vec;     /* the actual vec list */

        bio_end_io_t            *bi_end_io;

        void                    *bi_private;
#if defined(CONFIG_BLK_DEV_INTEGRITY)
        struct bio_integrity_payload *bi_integrity;  /* data integrity */
#endif

        bio_destructor_t        *bi_destructor; /* destructor */

        /*
         * We can inline a number of vecs at the end of the bio, to avoid
         * double allocations for a small number of bio_vecs. This member
         * MUST obviously be kept at the very end of the bio.
         */
        struct bio_vec          bi_inline_vecs[0]; 

83};

  

struct block_device {
        dev_t                   bd_dev;  /* not a kdev_t - it's a search key */
        int                     bd_openers;
        struct inode *          bd_inode;       /* will die */
        struct super_block *    bd_super;
        struct mutex            bd_mutex;       /* open/close mutex */
        struct list_head        bd_inodes;
        void *                  bd_claiming;
        void *                  bd_holder;
        int                     bd_holders;
        bool                    bd_write_holder;
#ifdef CONFIG_SYSFS
        struct list_head        bd_holder_disks;
#endif
        struct block_device *   bd_contains;
        unsigned                bd_block_size;
        struct hd_struct *      bd_part;
        /* number of times partitions within this device have been opened. */
        unsigned                bd_part_count;
        int                     bd_invalidated;
        struct gendisk *        bd_disk;
        struct list_head        bd_list;
        /*
         * Private data.  You must have bd_claim'ed the block_device
         * to use this.  NOTE:  bd_claim allows an owner to claim
         * the same device multiple times, the owner must take special
         * care to not mess up bd_private for that case.
         */
        unsigned long           bd_private;

        /* The counter of freeze processes */
        int                     bd_fsfreeze_count;
        /* Mutex for freeze */
        struct mutex            bd_fsfreeze_mutex; 

693}; 

struct gendisk {
        /* major, first_minor and minors are input parameters only,
         * don't use directly.  Use disk_devt() and disk_max_parts().
         */
        int major;                      /* major number of driver */
        int first_minor;
        int minors;                     /* maximum number of minors, =1 for
                                         * disks that can't be partitioned. */

        char disk_name[DISK_NAME_LEN];  /* name of major driver */
        char *(*devnode)(struct gendisk *gd, mode_t *mode);

        unsigned int events;            /* supported events */
        unsigned int async_events;      /* async events, subset of all */

        /* Array of pointers to partitions indexed by partno.
         * Protected with matching bdev lock but stat and other
         * non-critical accesses use RCU.  Always access through
         * helpers.
         */
        struct disk_part_tbl __rcu *part_tbl;
        struct hd_struct part0;

        const struct block_device_operations *fops;
        struct request_queue *queue;
        void *private_data;

        int flags;
        struct device *driverfs_dev;  // FIXME: remove
        struct kobject *slave_dir;

        struct timer_rand_state *random;
        atomic_t sync_io;               /* RAID */
        struct disk_events *ev;
#ifdef  CONFIG_BLK_DEV_INTEGRITY
        struct blk_integrity *integrity;
#endif
        int node_id; 

193}; 

struct root_device_t *root_dev = (struct root_device_t *)bio->bi_bdev->bd_disk->private_data;

strucr root_device_t 는 블록 디바이스를 다루기위해 모듈내에 선언한 자료구조