Block IO (Buffer Head, BIO Struct, Scheduler)

// https://github.com/torvalds/linux/blob/f2e8a57ee9036c7d5443382b6c3c09b51a92ec7e/include/linux/buffer_head.h#L59
/*
* Historically, a buffer_head was used to map a single block
* within a page, and of course as the unit of I/O through the
* filesystem and block layers.  Nowadays the basic I/O unit
* is the bio, and buffer_heads are used for extracting block
* mappings (via a get_block_t call), for tracking state within
* a page (via a page_mapping) and for wrapping bio submission
* for backward compatibility reasons (e.g. submit_bh).
*/
struct buffer_head {
  unsigned long b_state;		/* 해당 버퍼의 현재 상태를 의미하며 여러 플래그 중 하나의 값을 가진다. (see above) */
  struct buffer_head *b_this_page;/* circular list of page's buffers */
  union {
    struct page *b_page;	/* the page this bh is mapped to */
    struct folio *b_folio;	/* the folio this bh is mapped to */
  };

  sector_t b_blocknr;		/* start block number */
  size_t b_size;			/* 블록의 길이(크기) */
  char *b_data;			/* 버퍼가 가리키는 블록 */

  struct block_device *b_bdev;
  bh_end_io_t *b_end_io;		/* I/O completion */
  void *b_private;		/* reserved for b_end_io */
  struct list_head b_assoc_buffers; /* associated with another mapping */
  struct address_space *b_assoc_map;	/* mapping this buffer is
              associated with */
  atomic_t b_count;		/* 버퍼의 사용 횟수를 의미한다. get_bh(), put_bh() 함수로 증감한다. */
  spinlock_t b_uptodate_lock;	/* Used by the first bh in a page, to
          * serialise IO completion of other
          * buffers in the page */
};

// https://litux.nl/mirror/kerneldevelopment/0672327201/ch13lev1sec3.html
struct bio {
        sector_t             bi_sector;         /* associated sector on disk */
        struct bio           *bi_next;          /* list of requests */
        struct block_device  *bi_bdev;          /* associated block device */
        unsigned long        bi_flags;          /* status and command flags */
        unsigned long        bi_rw;             /* read or write? */
        unsigned short       bi_vcnt;           /* number of bio_vecs off */
        unsigned short       bi_idx;            /* current index in bi_io_vec */
        unsigned short       bi_phys_segments;  /* number of segments after coalescing */
        unsigned short       bi_hw_segments;    /* number of segments after remapping */
        unsigned int         bi_size;           /* I/O count */
        unsigned int         bi_hw_front_size;  /* size of the first mergeable segment */
        unsigned int         bi_hw_back_size;   /* size of the last mergeable segment */
        unsigned int         bi_max_vecs;       /* maximum bio_vecs possible */
        struct bio_vec       *bi_io_vec;        /* bio_vec list */
        bio_end_io_t         *bi_end_io;        /* I/O completion method */
        atomic_t             bi_cnt;            /* usage counter */
        void                 *bi_private;       /* owner-private method */
        bio_destructor_t     *bi_destructor;    /* destructor method */
};

// https://github.com/torvalds/linux/blob/f2e8a57ee9036c7d5443382b6c3c09b51a92ec7e/include/linux/blkdev.h#L378
struct request_queue {
	struct request		*last_merge;
	struct elevator_queue	*elevator;

	struct percpu_ref	q_usage_counter;

	struct blk_queue_stats	*stats;
	struct rq_qos		*rq_qos;
	struct mutex		rq_qos_mutex;

	const struct blk_mq_ops	*mq_ops;

	/* sw queues */
	struct blk_mq_ctx __percpu	*queue_ctx;

	unsigned int		queue_depth;

	/* hw dispatch queues */
	struct xarray		hctx_table;
	unsigned int		nr_hw_queues;

	/*
	 * The queue owner gets to use this for whatever they like.
	 * ll_rw_blk doesn't touch it.
	 */
	void			*queuedata;

	/*
	 * various queue flags, see QUEUE_* below
	 */
	unsigned long		queue_flags;
	/*
	 * Number of contexts that have called blk_set_pm_only(). If this
	 * counter is above zero then only RQF_PM requests are processed.
	 */
	atomic_t		pm_only;

	/*
	 * ida allocated id for this queue.  Used to index queues from
	 * ioctx.
	 */
	int			id;

	spinlock_t		queue_lock;

	struct gendisk		*disk;

	refcount_t		refs;

	/*
	 * mq queue kobject
	 */
	struct kobject *mq_kobj;

#ifdef  CONFIG_BLK_DEV_INTEGRITY
	struct blk_integrity integrity;
#endif	/* CONFIG_BLK_DEV_INTEGRITY */

#ifdef CONFIG_PM
	struct device		*dev;
	enum rpm_status		rpm_status;
#endif

	/*
	 * queue settings
	 */
	unsigned long		nr_requests;	/* Max # of requests */

	unsigned int		dma_pad_mask;

#ifdef CONFIG_BLK_INLINE_ENCRYPTION
	struct blk_crypto_profile *crypto_profile;
	struct kobject *crypto_kobject;
#endif

	unsigned int		rq_timeout;

	struct timer_list	timeout;
	struct work_struct	timeout_work;

	atomic_t		nr_active_requests_shared_tags;

	struct blk_mq_tags	*sched_shared_tags;

	struct list_head	icq_list;
#ifdef CONFIG_BLK_CGROUP
	DECLARE_BITMAP		(blkcg_pols, BLKCG_MAX_POLS);
	struct blkcg_gq		*root_blkg;
	struct list_head	blkg_list;
	struct mutex		blkcg_mutex;
#endif

	struct queue_limits	limits;

	unsigned int		required_elevator_features;

	int			node;
#ifdef CONFIG_BLK_DEV_IO_TRACE
	struct blk_trace __rcu	*blk_trace;
#endif
	/*
	 * for flush operations
	 */
	struct blk_flush_queue	*fq;
	struct list_head	flush_list;

	struct list_head	requeue_list;
	spinlock_t		requeue_lock;
	struct delayed_work	requeue_work;

	struct mutex		sysfs_lock;
	struct mutex		sysfs_dir_lock;

	/*
	 * for reusing dead hctx instance in case of updating
	 * nr_hw_queues
	 */
	struct list_head	unused_hctx_list;
	spinlock_t		unused_hctx_lock;

	int			mq_freeze_depth;

#ifdef CONFIG_BLK_DEV_THROTTLING
	/* Throttle data */
	struct throtl_data *td;
#endif
	struct rcu_head		rcu_head;
	wait_queue_head_t	mq_freeze_wq;
	/*
	 * Protect concurrent access to q_usage_counter by
	 * percpu_ref_kill() and percpu_ref_reinit().
	 */
	struct mutex		mq_freeze_lock;

	int			quiesce_depth;

	struct blk_mq_tag_set	*tag_set;
	struct list_head	tag_set_list;

	struct dentry		*debugfs_dir;
	struct dentry		*sched_debugfs_dir;
	struct dentry		*rqos_debugfs_dir;
	/*
	 * Serializes all debugfs metadata operations using the above dentries.
	 */
	struct mutex		debugfs_mutex;

	bool			mq_sysfs_init_done;
};