Remap_vm_area

How to change a read only page to read_write or change a read_write page to read only?
	Given an address addr
	1. Find the corresponding vma by calling the find_vma(mm, addr), then change its vm_flags
	2. Walk the page table and use pte-apis to change the r/w bit
		pte_wrprotect, pte_mkdirty, pte_mkwrite etc.
	3. Then flush the tlb by calling flush_tlb_page(vma, addr)


Note:
	#1 vma will merge automatically every time mmap_region was called. So when calling the find_vma, it is neccessary to check the vm_start and vm_end of the vma, and split the vma by calling split_vma to extract the to-be-modified area. Otherwise, it will also change the vm_flags of the pages in the same vma.
	#2 tlb_flush costs a lot. SPECCPU2006(@h264ref:400%, @perlbench:200%) has a great overhead. So try other methods to achieve the same goal.