The VFS knows which user/process has sent read/write request. Is the actual block device aware of these details.
Does 'bio' struct contain any details which point to the user space parameters? Using current_uid(), etc, give the UID as 0 which actually corresponds to kjournald which performs the read/write
No, the block device doesn't know what process or user caused a disk access. The fundamental reason why it doesn't know is that this concept is not well-defined. There is no direct relationship between a disk access and a file access. For example, if multiple processes read from the same location in the same file at practically the same time, then they may both have a pending read request which trickles down to the hardware. Another example is that a buffered write could take place after the process whose activity caused the write has died.
btrace (wrapper for blktrace) on Linux can tell you what process triggered a block device I/O operation (at least for some of them)