Linux Memory Management

Today's Lecture

• Q&A
• Memory Management

Last Lecture

• Page Frame
• Memory Zone
• Page Allocation: Buddy System and Free List
• Memory Manage: Slab Cache
• Useful routines:
  • kmalloc() and kfree()
  • valloc() and vfree()

Swapping and Page Cache

• For Pages in a Process's User Space
  • Swap: Secondary Memory on the disk
  • Page Cache: Main Memory
• Data Structure: 3 sets of lists
  • Active pages, usually mapped by a process's PTE
    • active_list (in mm/page_alloc.c)
  • Inactive, unmapped, clean or dirty
    • inactive_dirty_list (in mm/page_alloc.c)
  • Clean pages, unmapped (one list per zone)
    • zone_t.inactive_clean_list (in include/linux/mmzone.h)

Kernel Swap Daemon

• Implemented as a kernel thread
  • kswapd() (in mm/vmscan.c)
  • Wake up periodically
  • Wake up more frequently if memory shortage
• Check memory and if memory is tight
  • Age pages that have not be used
  • Move pages to inactive lists
  • Write dirty pages to disk
  • Swap pages out if necessary

More kswapd()

• Call swap_out() to scan inactive page lists
  • Removes page reference from process's page table
  • Actual swapping is done independently by file I/O
• Call refill_inactive_scan() to
  • Scan the active_list to find unused page
  • Call age_page_down() to reduce page->age count
  • If page->age is zero, move to inactive_dirty_list
• Call page_launder() to clean dirty pages:
  • Scan inactive_dirty_list for dirty pages, write to disk
  • Move clean pages to the zone's inactive_clean_list

Life Cycle of a Page in Cache

• A page is read into memory from disk
  • Caused by page-fault, or read-ahead
  • Added to page cache: active_list
• Page is "dirty" if written by the process
• If not used, page->age count gradually reduced
  • If page->age is zero, moved to inactive_dirty_list
  • May be moved to an inactive_clean_list later
• Page can be recovered from an inactive list
• Inactive page can be released
  • reclaim_page() to free page for used by others

Process Address Space

• Linear Addresses in the Virtual Address Space

Address Space in ELF Format

• Compile-Time Arrangement:
  • Code: start_code (usually 0x0) to end_code
  • Data: start_data to end_data
  • BSS (Heap): start_brk to brk (growable)
• Run-Time Arrangement:
  • Stack: start_stack (growable)
  • Arguments: arg_start to arg_end
  • Environment: env_start to env_end (0xbfffffff)

Process Memory Descriptor

• One per process (in include/linux/sched.h)

  	struct mm_struct {
  		struct vm_area_struct * mmap;
  		...
  		pgd_t * pgd;
  		...
  		unsigned long start_code, end_code, start_data, end_data;
  		unsigned long start_brk, brk, start_stack;
  		unsigned long arg_start, arg_end, env_start, env_end;
  		...
  	};
  

VM Area

• Linear Address Interval (in include/linux/mm.h)

  	struct vm_area_struct {
          	struct mm_struct * vm_mm;
  		unsigned long vm_start;
  		unsigned long vm_end;
  		struct vm_area_struct *vm_next;
  		...
  	};
  

VM Area Handlers

• To Create VM Area and Do the Mapping
  • do_mmap() (in include/linux/mm.h)
  • Used in system calls execve(), brk()
• To Release a VM Area and Shrink the Address Space
  • do_munmap() (in mm/mmap.c)
  • Used in system calls brk()
• To Find a VM Area that includes a given address
  • find_vma() (in mm/mmap.c)

Page Fault

• Exception handler
  • Raised by address translation (hardware)
  • Call do_page_fault() to handle this interrupt
• do_page_fault()
  • Architecture-specific
  • i386: arch/i386/mm/fault.c
  • Find a page frame in the physical memory
  • Swap the missing page in
  • Update the page tables

do_page_fault()

	...
        vma = find_vma(mm, address);
        if (!vma)
                goto bad_area;
        if (vma->vm_start <= address)
                goto good_area;
        ...
good_area:
        ...
        handle_mm_fault(mm, vma, address, write);
        ...
bad_area:
        ...
        force_sig_info(SIGSEGV, &info, tsk);
        ...

Memory Management Summary

• Physical Page Management
  • Page Table Structure
  • Memory Zones and Page Allocation
• Kernel Memory Management
  • Slab and Allocation Routines
• Virtual Memory Management (Process Address Space)
  • Page Cache and Swapping
  • Memory Layout and VM Area
  • Page Fault

Next Lecture

• Process Management
• Time to review related topics in CS372
  • Process, thread
  • Context switch
  • Scheduling

More about Projects 4-8

• Choose 5 out of the following 8 projects
  • 1) KP4L: Exercise 6 (Shared Memory)
  • 2) KP4L: Exercise 7 (Virtual Memory)
  • 3) KP4L: Exercise 8 (Synchronization)
  • 4) KP4L: Exercise 9 (Scheduler)
  • 5) KP4L: Exercise 10 (Device Driver)
  • 6) inode undelete (File System) (see course web site)
  • 7) Packet Dropper (Networking)
  • 8) IPsec (Special Topic)

Choosing Projects

• Based on Your Interests
• Not to Guess which one is Harder