Memory%20Management%20in%20Linux

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Memory Management in Linux

• David Chang
• OS Lab., NCTU,Taiwan

2
Agenda

• Initialization
• Page Fault Handling
• Page Cache and Swap Cache
• Swap Area Management
• Copying Virtual Address Spaces

3
Initialization

• paging_init()
• set up the page tables to map in all the
  available physical memory
• Kernel has the virtual address from 3G4G
• physical address virtual address - 3G (see
  figure)
• free_area_init()
• allocate and initialize the page structures
• Linux use a page structure for one physical page
• all page structures are marked as DMAable and
  RESERVED
• allocate and initialize the swap cache and free
  area list

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Address Mapping in Kernel
0 3G 4G
E.g. 16M
Virtual address Physical address
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Initialization

• mem_init()
• fixup the page structures and make all the
  available pages free.

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Page Fault Handling

• do_page_fault()
• this routine is a dispatcher that is invoked when
  page fault occurs
• Works
• find out the faulting address
• find out the reason for the fault and call the
  right routine
• protection fault do_wp_page()
• page-not-present fault do_no_page()

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Page Fault Handling

• do_wp_page()
• handle write protection fault
• Copy-On-Write if necessary
• do_no_page()
• used to page-in a page
• determine the source of the page
• Executable file? Swap area ? Or Anonymous page?

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Page Fault Handling

• Page-in from an executable file
• When an executable file is opened, Linux will
  attach the file operations for the file. The file
  operations include a mmap( ) function
• For UFS, this mmap() function points to
  generic_file_mmap( )
• After opening the file, Linux maps segments by
  do_mmap( )
• do_mmap() will actually call the mmap() file
  operation to attach the vm_operations for the vma
• The vm operation contains a nopage( ) which is
  called when page fault occurs( see
  filemap_nopage() )

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Page Fault Handling

• do_mmap()
• construct a vma
• insert the vma-gtvm_op by calling
  file-gtf_op-gtmmap()
• brings in pages if VM_LOCKED is specified
• filemap_nopage()
• look for the page from page cache first
• read-in the page if cache miss
• read-ahead if necessary

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Swap Area Management

• do_swap_page()
• determine if the vma that contains the faulting
  page has swapin() vm_operation
• call swap_in() if the vma doesnt have swapin()
  vm operation
• swap_in()
• the default swap in operation allocate a free
  page and get the content of the page from the
  swap area
• de-allocate the swap page in swap area!!!!!!!

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Swap Area Organization
swap_info
A swap area
1111111111111 1111111111111 111 11100000
000 00SWAP-SPACE
lockmap
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Swap Area Management

• swap_on()
• enable a swap area
• allocate pages for lockmap and swapmap
• insert the swap area into the swap list

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Copying Virtual Address Spaces

• When Linux create a new process by fork(), it
  will call copy_mm() to copy the address space
  from the father to the child.
• The child can share the same address space with
  its father if the CLONE_VM is specified
• copy_mm()
• new_page_tables( )
• allocate a page for PTEs and copy the kernel-part
  PTEs
• dup_mmap( )
• duplicate the mmap