Specifications

This is the Title of the Book, eMatter Edition

The mmap Device Operation

431

map the physical page located at address 64 KB—instead, we see a page full of zeros

(the host computer in this example is a PC, but the result would be the same on

other platforms):

morgana.root# ./mapper /dev/mem 0x10000 0x1000 | od -Ax -t x1

mapped "/dev/mem" from 65536 to 69632

000000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

001000

The inability of remap_pfn_range to deal with RAM suggests that memory-based

devices like scull can’t easily implement mmap, because its device memory is conven-

tional RAM, not I/O memory. Fortunately, a relatively easy workaround is available

to any driver that needs to map RAM into user space; it uses the nopage method that

we have seen earlier.

Remapping RAM with the nopage method

The way to map real RAM to user space is to use vm_ops->nopage to deal with page

faults one at a time. A sample implementation is part of the scullp module, intro-

duced in Chapter 8.

scullp is a page-oriented char device. Because it is page oriented, it can implement

mmap on its memory. The code implementing memory mapping uses some of the

concepts introduced in the section “Memory Management in Linux.”

Before examining the code, let’s look at the design choices that affect the mmap

implementation in scullp:

• scullp doesn’t release device memory as long as the device is mapped. This is a

matter of policy rather than a requirement, and it is different from the behavior

of scull and similar devices, which are truncated to a length of

0 when opened for

writing. Refusing to free a mapped scullp device allows a process to overwrite

regions actively mapped by another process, so you can test and see how pro-

cesses and device memory interact. To avoid releasing a mapped device, the

driver must keep a count of active mappings; the

vmas field in the device struc-

ture is used for this purpose.

• Memory mapping is performed only when the scullp

order parameter (set at mod-

ule load time) is

0. The parameter controls how __get_free_pages is invoked (see

the section “get_free_page and Friends” in Chapter 8). The zero-order limitation

(which forces pages to be allocated one at a time, rather than in larger groups) is

dictated by the internals of __get_free_pages, the allocation function used by

scullp. To maximize allocation performance, the Linux kernel maintains a list of

free pages for each allocation order, and only the reference count of the first page

in a cluster is incremented by get_free_pages and decremented by free_pages. The

mmap method is disabled for a scullp device if the allocation order is greater than

zero, because nopage deals with single pages rather than clusters of pages. scullp

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