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Kernel Memory Allocator

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Presentation on theme: "Kernel Memory Allocator"— Presentation transcript:

1 Kernel Memory Allocator
Exploring memory allocation in Linux kernel

2 KMA Subsystem Goals Must be fast (this is crucial)
Should minimize memory waste Try to avoid memory fragmentation Cooperate with other kernel subsystems

3 ‘Layered’ software structure
At the lowest level, the kernel allocates and frees ‘blocks’ of contiguous pages of phyical memory: struct page * __alloc_pages( zonelist_t *zonelist, unsigned long order ); (The number of pages in a ‘block’ is a power of 2.)

4 The zoned buddy allocator
128 KB 64 KB 32 KB ‘splitting’ a free memory region 32 KB

5 block allocation sizes
Smallest block is 4 KB (i.e., one page) order = 0 Largest block is 128 KB (i.e., 32 pages) order = 5

6 Inefficiency of small requests
Many requests are for less than a full page Wasteful to allocate an entire page! So Linux uses a ‘slab allocator’ subsystem

7 Idea of a ‘slab cache’ kmem_cache_create()
manager The memory block contains several equal-sized ‘slabs’ (together with a data-structure used to ‘manage’ them)

8 Allocation Flags GFP_KERNEL (might sleep) GFP_ATOMIC (will not sleep)
__get_free_pages( flags, order ); GFP_KERNEL (might sleep) GFP_ATOMIC (will not sleep) GFP_USER (low priority) __GFP_DMA (below 16MB) __GFP_HIGHMEM (from high_memory)

9 Virtual memory allocations
Want to allocate a larger-sized block? Don’t need physically contiguous pages? You can use the ‘vmalloc()’ function

10 The VMALLOC address-region
gap gap VMALLOC_END VMALLOC_START vmlist Linked list of ‘struct vm_struct’ objects

11 ‘struct vm_struct’ struct vm_struct { unsigned long flags; void *addr;
unsigned long size; struct vm_struct *next; }; Defined in <include/linux/vmalloc.h>

12 The ‘vmlist’ variable Not a public kernel symbol:
$ grep vmlist /proc/ksyms So our modules cannot link to ‘vmlist’  Yet maybe we can find its address anyway

13 The ‘’ file When the kernel is compiled, a textfile gets created in the ‘source’ directory: /usr/src/linux/ Each line shows the name and address for a kernel symbol (function-name or data-object)

14 Sometimes file gets moved
Some Linux distributions copy (or move) the ‘’ file to ‘/boot’ directory Some Linux distributions rename the file (e.g., ‘/boot/ ’) This file will show where ‘vmlist’ is located (Can we find our ‘’ file?)

15 Another ‘solution’ We can ‘decompile’ our Linux kernel! 
The compiled kernel is written to the file: ‘vmlinux’ gcc puts file in the ‘/usr/src/linux’ directory Some distributions may move (or delete) it It is NOT the same as the file ‘vmlinuz’ ! Can use ‘objdump’ to get a list of symbols

16 ‘objdump’ Here’s how to find the ‘vmlist’ address:
$ objdump –t vmlinux > vmlinux.sym $ grep vmlist vmlinux.sym You can also get a code-disassembly: $ objdump –d vmlinux > vmlinux.asm

17 Looking at ‘vm_struct’ list
Let’s write a module (named ‘vmlist.c’) It will create a pseudo-file: ‘/proc/vmlist’ We can look at the current ‘vmlist’ objects: $ cat /proc/vmlist Similar to seeing list of process descriptors

18 ‘my_proc_read()’ struct vm_struct **vmlistp, *vm;
vmlistp = (struct vm_struct **)0xD64A5124; vm = *vmlistp; while ( vm ) { /* Display information in this vm_struct; */ vm = vm->next; // point to next vm_struct }

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