Block layer provides an interface between the file system and the physical storage device, allowing the file system to read and write data to the device
Block devices are usually implemented with a File System meant for storing files. Now whenever an userspace application initiates a File O operation (read, write), the kernel in turn initiates a sequence of Block I/O operation through File System Manager. The struct bio keeps track of all Block I/O transactions (initiated by user app) that is to be processed. That’s what is mentioned here as flight/active regions.
Memory buffers are required by the kernel to hold data to/from Block device.
The block I/O layer also provides functions that allow the kernel to put data buffers in high memory and implement a “zero-copy” schema, where disk data is directly put in the User Mode address space without being copied to kernel memory first
