My confusion about kernel and corresponding clarification.

My confusion about kernel and corresponding clarification.

There are many questions coming into my mind when I read the Linux kernel book and source code. As time goes by, I become more knowledgeable than before and can address those questions by myself, here is the first question addressed by myself.

 

Q: why kernel have to map the high memory in kernel space, why not just allocate the high memory and only map it in user process.

A: Because kernel also need to access the high memory before it returned the allocated memory to user process. For example, kernel must zero the page or initialized the page for security reason. Please refer to linux device driver page 9.

Q: why not let the clib zero the page or initialize it, it saves the kernel's effort and simplifies the kernel.

A: besides Requesting memory through clib, user program can also request memory through direct System call, in this situation, the security is not guaranteed, the information in memory will be leaked.