Search notes:

include/linux/part_stat.h

include/linux/part_stat.h essentially defines the struct disk_stats, a few macros that operate on such stats an the inline function part_stat_set_all. 
struct disk_stats {
        u64           nsecs  [NR_STAT_GROUPS];
        unsigned long sectors[NR_STAT_GROUPS];
        unsigned long ios    [NR_STAT_GROUPS];
        unsigned long merges [NR_STAT_GROUPS];
        unsigned long io_ticks;
        local_t       in_flight[2];
};
As of release 6.10, NR_STAT_GROUPS is defined to 4 and corresponds to the four statistics groups defined in include/linux/blk_types.h (enum stat_group):

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