Specifications

This is the Title of the Book, eMatter Edition

Memory Management in Linux

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the data structures used by the kernel to manage memory. Once the necessary back-

ground has been covered, we can get into working with these structures.

Address Types

Linux is, of course, a virtual memory system, meaning that the addresses seen by

user programs do not directly correspond to the physical addresses used by the hard-

ware. Virtual memory introduces a layer of indirection that allows a number of nice

things. With virtual memory, programs running on the system can allocate far more

memory than is physically available; indeed, even a single process can have a virtual

address space larger than the system’s physical memory. Virtual memory also allows

the program to play a number of tricks with the process’s address space, including

mapping the program’s memory to device memory.

Thus far, we have talked about virtual and physical addresses, but a number of the

details have been glossed over. The Linux system deals with several types of

addresses, each with its own semantics. Unfortunately, the kernel code is not always

very clear on exactly which type of address is being used in each situation, so the

programmer must be careful.

The following is a list of address types used in Linux. Figure 15-1 shows how these

address types relate to physical memory.

User virtual addresses

These are the regular addresses seen by user-space programs. User addresses are

either 32 or 64 bits in length, depending on the underlying hardware architec-

ture, and each process has its own virtual address space.

Physical addresses

The addresses used between the processor and the system’s memory. Physical

addresses are 32- or 64-bit quantities; even 32-bit systems can use larger physi-

cal addresses in some situations.

Bus addresses

The addresses used between peripheral buses and memory. Often, they are the

same as the physical addresses used by the processor, but that is not necessarily

the case. Some architectures can provide an I/O memory management unit

(IOMMU) that remaps addresses between a bus and main memory. An IOMMU

can make life easier in a number of ways (making a buffer scattered in memory

appear contiguous to the device, for example), but programming the IOMMU is

an extra step that must be performed when setting up DMA operations. Bus

addresses are highly architecture dependent, of course.

Kernel logical addresses

These make up the normal address space of the kernel. These addresses map

some portion (perhaps all) of main memory and are often treated as if they were

physical addresses. On most architectures, logical addresses and their associated

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