Linux Kernel Architecture

Chapter3:MemoryManagement

Interfaceto the Kernel

Allocating Memory

The kernel provides a variety of functions for reserving memory during initialization. The following

functions are available on UMA systems:

❑ alloc_bootmem(size)andalloc_bootmem_pages(size)reserve memory whose size is

expressed bysizeinZONE_NORMAL; data are aligned so that memory begins either at an ideal

position for the L1 cache or on a page boundary.

Even though the namealloc_bootmem_pagessuggests that the required size is

specified in page units,_pagesrefers only to thealignmentof the data.

❑ alloc_bootmem_lowandalloc_bootmem_low_pagesoperate in the same ways as the above

functions but take the areaZONE_DMAthat is suitable for DMA operations as their source. Con-

sequently, the functions should only be used when DMA memory is required.

Basically the same API applies for NUMA systems, but the suffix_nodeis appended to the function

name. As compared with the UMA functions, an additional parameter is required to specify which node

is used for memory reservation.

Thesefunctionsareallfrontendsfor__alloc_bootmem, which delegates the real work to

__alloc_bootmem_nopanic. Since more than one bootmem allocator can be registered (recall

that they are all kept in a global list),__alloc_bootmem_coreiterates over all of them until one

succeeds.

On NUMA systems,__alloc_bootmem_nodeis used to implement the API functions. First, work is passed

on to__alloc_bootmem_coreto try the allocation on the specific bootmem allocator of the node. If this

fails, the function falls back to__alloc_bootmem, which tries all nodes.

mm/bootmem.c

void * __init __alloc_bootmem(unsigned long size, unsigned long align,

unsigned long goal)

__alloc_bootmemrequires three parameters to describe a request:sizeis the size of the desired memory

area,alignindicates the alignment of the data, andgoalspecifies the start address at which the search

for a suitable free area is to begin. The front ends use the function as follows:

<bootmem.h>

#define alloc_bootmem(x) \

__alloc_bootmem((x), SMP_CACHE_BYTES, __pa(MAX_DMA_ADDRESS))

#define alloc_bootmem_low(x) \

__alloc_bootmem((x), SMP_CACHE_BYTES, 0)

#define alloc_bootmem_pages(x) \

__alloc_bootmem((x), PAGE_SIZE, __pa(MAX_DMA_ADDRESS))

#define alloc_bootmem_low_pages(x) \

__alloc_bootmem((x), PAGE_SIZE, 0)