1. Linux –
Memory Management
D3 Tsung-Han “Johnny” Lin

2. 12/4/26 Linux - Memory Management 2

3. Outline
• Memory in Linux
– Dynamic Allocation
– Contiguous Allocation
•Memory Allocation
– Buddy System
– Zone
– SLAB Allocator
• Per-CPU data
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4. MMU - Simple History
Early Linux on x86
Linux 2.5.46
(MMU)
Embedded Developer?!
µClinux
(Linux for microconcroller)
Jeff Dionne et al.1998
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5. All about PAGE
• Physical page -> page frame
– Page descriptor, page
– All pages, mem_map
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6. Initialization
query or invoke Kernel
BIOS
machine_specific_memory_setup()
setup_memory_map()
struct page* mem_map
Physical Memory
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7. Memory
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8. How pages are allocated?
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9. Kernel API
• alloc_pages
• __get_free_pages
• kmalloc
• …
• But, seriously
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10. HOW?
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11. Hierarchy Overview
App w/ glibc heap
Zone Allocator
Slab Allocator
glibc
Buddy Allocator
MMU
Kernel Physical
Subsystems VM Subsystem Memory
kswapd
App w/ heap
bdflush Disk
Driver
User Space Kernel Space Disk
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12. Memory Zone
Physical Memory (mem_map)
0x37999999, 896MB
ZONE_DMA ZONE_NORMAL ZONE_HIGHMEM
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0x40000000, 1GB

13. Buddy System
zone descriptor
free_area[10] 1024 pages ……
………
………
………
free_area[11]
free_area[4]
free_area[3]
free_area[2] 4 pages ……
………
free_area[1] 2 pages 2 pages ……
free_area[0] 1 page 1 page 1 page ……
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14. Zone, again
• ZONE_DMA, 0 ~ 16 MB
896MB ?!
• ZONE_NORMAL, 16 MB ~ 896 MB
• ZONE_HIGHMEM, rest of them …
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15. Kernel View
Linear – 4 GB Physical
TASK_SIZE
0x0
3 GB
PAGE_OFFSET ZONE_HIGHMEM
0xc0000000 0x40000000, 1GB
Kernel
8 MB Image Reserved
0x37999999, 896MB
mem_map 128 MB
ZONE_NORMAL 1 GB
…
0x01000000, 16MB
896 MB ZONE_NORMAL ZONE_DMA
+
ZONE_DMA
0xffffffff
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16. Do we need this large?!
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17. SLAB Allocator
• First introduced in
大辞泉
SunOS 5.4
slab/slǽb/
• Implemented in Linux
名 詞
2.2 1 （石木材金属などの）広い厚板；（材木
の）背板.
• SLAB system 2 （食べ物などの）厚切り
– SLAB a thick slab of apple pie｜厚切りのアップル
パイ.
– SLOB, embedded system 3｟ 俗語｟ 〘野球〙ピッチャーズプレート.
– SLUB, efficient ver. 4｟ ～｟｟
the 俗語｟ （病院などの）死体置
き台
– SLQB, per-CPU scaling on the slab｜（死体となって）安置されて.
• kmalloc()!
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Cache !! 17

18. Design of SLAB
cache_chain
slab_full
……
page struct
cache
slab
slab
object Page 1
page
…
cache
slab_partial slab
(kmem_cache) object
page
…
slab
object Page 2
cache
page
slab_partial
……
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19. Per-CPU Allocation
• Request/Release single page
– Frequently
• Per-CPU
– Reducing cache invalidation
– Reducing locking
– A simple data array
• In 2.4 kernel
– get_cpu(), preemption disabled
– Access array directly
• In 2.6 kernel
– Wrapper
– percpu interface
– Cache alignment
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20. Review
App w/ glibc heap
Zone Allocator
Slab Allocator
glibc
Buddy Allocator
MMU
Kernel Physical
Subsystems VM Subsystem Memory
kswapd
App w/ heap
bdflush Disk
Driver
User Space Kernel Space Disk
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21. Reference
• Linux Kernel Development, 3rd
• Understanding the Linux Kernel, 3rd
– http://www.makelinux.net/books/ulk3/t
oc
• Understanding the Virtual Memory
Manager
– http://www.kernel.org/doc/gorman/htm
l/understand/index.html
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