where is disk block access in kernel ?

Hello, I would like to monitor the write access to disk blocks (so that I can monitor the block index in some bitmap) I think this must be done in kernel (userspace have no such information) I have tried to search in kernel but did not found where is the API to access disk blocks. There is libata-core.c , but I can't find such routines there, and neither any documentation. Is there any idea ? Regards, Ran