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Embedded Software Lab.
Sungkyunkwan University
Embedded Software Lab.
Dongkun Shin
전공핵심실습1:운영체제론Chapter 8. Kernel Memory Management
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Contents
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• Physical Address– Used to address memory cells in memory chips.
• Linear Address (Virtual Address)– Single 32-bit unsigned integer used to address up to 4GB.
• Logical Address– The address that embodies 80x86 segmented architecture
Linux Memory Address
Physical Memory Cells
Paging Unit
Pages
Segmentation Unit
SegmentsAbstraction
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• Paging Unit– Translate Linear Address Physical Address
– Check access privilege of Linear Address
• Kernel or User mode, write or read privilege
• Page– Linear addresses are grouped in fixed-length intervals
• Page Frame– Corresponding memory cells to the page
• Page Table– The data structures that map linear to physical addresses
– Stored in main memory
Paging Terminology
Typical 4KB in 32-bit machine
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• Q) Why 2-level?– A) To save memory
Regular Paging
Each entry has same format
• Present Flag• 20bit page frame addr• Accessed Flag• Dirty Flag• Read/Write Flag• User/Supervisor Flag• Page Size Flag• Global Flag
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• Extended Paging– Is used to translate large contiguous linear address ranges
into corresponding physical ones.
– Page Size flag = 1
– Page size : 4MB
Extended Paging
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• Page table entry cache (Linear Physical address)
Translation Lookaside Buffer
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• Compatible with both 32bit and 64bit paging
• 4-level paging model
Paging in Linux
# of bit Depending on Arch
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• Defined in <asm/page.h>
Paging Macro in Linux
ex) alpha
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• Kernel must build a physical addresses map during the initialization
• Specifies which physical address ranges are usable and which are unavailable– Some address range are used by I/O shared memory or BIOS
data
• Reserved Memory Space– Unavailable address range
– Page frame containing kernel’s code and initialized data structures
• Never be dynamically assigned or swapped to disk.
Physical Memory Layout
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• Each Process has different Global Directory Table
• But ¼ of Global Directory entry is same
Process Page Table
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1. Create Mapping– In initial phase work with Real Mode (16bit)
– To access both Real and Protect mode mapping
– Initialize with : Linear A. 0x00000000~0x007FFFFF Physical A. 0x00000000~0x007FFFFF
– Linear A. 0xC0000000~0xC07FFFFFPhysical A. 0x00000000~0x007FFFFF
Procedure for kernel to create page table
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1. Create Mapping
2. Kernel creates the desired mapping by filling all the swapper_pg_dir entries with zeroes.
3. startup_32() : Enable paging unit
4. paging_init() : Initialize Master Kernel page Global Directory
Procedure for kernel to create page table
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• How the kernel allocates dynamic memory– Contiguous memory area
• Page Frame Management
• Memory Area Management
– Non-contiguous memory area
• Non-contiguous Memory Area Management
Memory Management
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• Describe current status of the page frame
Page Descriptor
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• Documented/documentation/vm/pagemap.txt
Page Flag
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• Node Descriptor– Each node has a descriptor of type pg_data_t
– All node descriptors are stored in a singly linked list, whose first implement is pointed by the pgdat_list variable
– node_zones: Array of zone descriptors of the node
– node_start_pfn: Index of the first page frame in the node
– node_id: Identifier of the node
Node
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• DMA ( Direct Memory Access ) processor can access to only first 16MB of RAM: zone_dma
• Some memory region is linearly mapped:– zone_normal
• Some memory region cannot be reached by kernel directly– zone_highmem (Kernel reaches anywhere in 64bit)
• Zone descriptor: struct_zone(/include/linux/mmzone.h)
Zone
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Zoned page frame allocator
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• Flag used in request for page frames
GFP flag
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• All free page frames are grouped into 11 lists of blocks that contain groups of 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, and 1024 contiguous page frames.(1024 page frames = 4MB) Method to solve the external fragmentation problem.
Buddy Algorithm
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• Prevent from internal framgenetation
• Used for allocation of little memory like few bytes or few hundreds bytes.
Slab Allocator
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• Create slab – cache_grow()
– Request for new object &&
– No free object in cache
• Release slab – cache_destroy()
– Too many free object ||
– Reclamation by timer function
Slab Allocator (cont.)
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• Allocate physically contiguous memory.
• Used for small memory allocation.
kmalloc()
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• Allocate virtually contiguous memory.– Non-contiguous memory area management
• Non-contiguous memory area management– The main advantage is to avoid external fragmentation.
– The disadvantage is to fiddle with the kernel page tables.
• Cannot reach outside of processor– Each page frames allocated by vmalloc() must be accessed by
referencing to MMU MMU is in-processor unit.
vmalloc()
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• vmalloc() only allocates vm_struct
Descriptor of non-contiguous memory areas
vmlist vm_struct
next
vm_struct
next
vm_struct
next
/include/linuxextern variable
– addr
• Linear address of the first memory cell of the area
– nr_pages
• Number of pages
– struct page **
• pointer of array of pages
– next
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Buddy Allcator
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• 간이 Buddy Allocator의 구현
• 전체 Page Frame수 : 16
• 초기 Free List 및 메모리 상태
실습
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• 실습내용– 다음 각각의 커맨드 이후의 Free List 상태 출력
– alloc(5p), alloc(1p), alloc(2p), alloc(2p), alloc(1p), alloc(1p), free(10), free(12), free(14), free(0)
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FreeAllocated
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실습
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alloc(5p)
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FreeAllocated
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실습
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alloc(5p), alloc(1p)
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FreeAllocated
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실습
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alloc(5p), alloc(1p), alloc(2p)
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FreeAllocated
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실습
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alloc(5p), alloc(1p), alloc(2p), alloc(2p)
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FreeAllocated
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실습
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alloc(5p), alloc(1p), alloc(2p), alloc(2p), alloc(1p)
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FreeAllocated
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실습
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alloc(5p), alloc(1p), alloc(2p), alloc(2p), alloc(1p), alloc(1p)
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FreeAllocated
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실습
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alloc(5p), alloc(1p), alloc(2p), alloc(2p), alloc(1p), alloc(1p), free(10)
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FreeAllocated
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실습
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alloc(5p), alloc(1p), alloc(2p), alloc(2p), alloc(1p), alloc(1p), free(10), free(12)
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FreeAllocated
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실습
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alloc(5p), alloc(1p), alloc(2p), alloc(2p), alloc(1p), alloc(1p), free(10), free(12), free(14)
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FreeAllocated
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실습
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alloc(5p), alloc(1p), alloc(2p), alloc(2p), alloc(1p), alloc(1p), free(10), free(12), free(14), free(0)
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FreeAllocated
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