Lab2 Memory Management

[Preface] A Hope That My Crude Remarks May Draw Forth By Abler People

[前言]拋磚引玉

• Outline:
• Lab2 has 3 parts:
1. Physical page allocator.
2. Virtual page management.
3. Kernel page mapping installation.
• Preparation:
• Intel 80386 reference manual: Chap5, Chap6.
• xv6: Chap 1.
• JOS: test cases code: check_*()
• Understand xv6 how to create temporary memory management to touch memory after 1MB.
• Build the map about Segment and Page translation on x86 in mind.
• Part 1: Physical page allocator
• boot_alloc():
• Divide physical memory into each 4KB pages.
• It should account for booting memory overhead from NVRAM's status until now. 
• kernel image.
• allocation data structure.
• mem_init():
• Here, just allocate the kernel page directory and set its attributes.
• physical page allocation
• page_init():
• Calculate the free list.
• Set up data structure.
• Don't touch the reserved memory! But the reserve memory should be accounted.
• page_alloc():
• Just allocate one physical page.
• ALLOC_ZERO handling.
• page_free:
• Just free one physical page.
• Read code carefully to sync the thought between Judge code and lab code
• check_page_free_list()
• check_page_alloc()
• GitHub Link
• Physical page allocator.
• Part 2: Virtual page management.
• pgdir_walk()
• Ask yourself: If i am a x86 cpu, how could I find the corresponding page?
• If the new pte doesn't belong to any page table, then just allocate the new page table.
• Return virtual address of PTE slot!
• Consult the xv6 code.
• virtual page allocation
• page_insert()
• Use pgdir_walk to find corresponding pte.
• One physical page to map N virtual pages
• pp->ref  is N.
• Remember permission assignment.
• Return page table entry.
• page_remove()
• Decrease the count. 
• Be zero, then free the physical page by page_free.
• Clear the pte!
• page_lookup()
• Return the corresponding virtual page's page structure.
• by pgdir_walk and pa2page.
• Read code carefully to sync the thought between Judge code and lab code
• check_page()
• GitHub Link:
• check_page success
• Part 3: kernel page mapping and installation.
• boot_map_region()
• Map the memory block into pages.
• some extern symbols are already the virtual address in C language!
• PADDR  usage.
• bootstack ,bootstacktop
• Naming like virtual address, but it is extren variable in C to be Virtual address.
• Read code carefully to sync the thought between Judge code and lab code
• check_kern_pgdir()
• check_page_installed_pgdir()
• GitHub Link:
• pass all
• Discussion: 
• If  __ALL_COUNT__ defined
• When each PTE referenced, then It increases corresponding page table/directory's count.
• Sample: __ALL_COUNT__ defined
• page_table 3
• page 1 1
• page 2 1
• page 3 1
• Sample  __ALL_COUNT__  not defined
  • page_table 1
  • page 1 1
  • page 2 1
  • page 3 1
• GitHub Link
• __ALL_COUNT__ compile option