Linux Device Drivers

get_free_page and Friends

If a module needs to allocate big chunks of memory, it is better to use a page-oriented technique. Requesting whole pages also has other advantages, which will be introduced later, in Section 13.2 in Chapter 13.

To allocate pages, the following functions are available:

• get_free_page returns a pointer to a new page and zeros the page.

• __get_free_page is like get_free_page, but doesn’t clear the page.

• __get_free_pages returns a pointer to the first byte of a memory area that is several pages long, but doesn’t zero the area.

• __get_dma_pages returns the pointer to the first byte of a memory area several pages long; the pages are consecutive in physical memory and suitable for DMA.

The prototypes for the functions, as defined in Linux 2.0, follow:

unsigned long get_free_page(int priority);
unsigned long __get_free_page(int priority);
unsigned long __get_dma_pages(int priority,
                              unsigned long order);
unsigned long __get_free_pages(int priority,
                               unsigned long order, int dma);

As a matter of fact, all the functions except __get_free_pages are either macros or inline functions that ultimately map to __get_free_pages.

When a program is done with the pages, it can call one of the following functions. The first function is a macro that falls back on the second:

void free_page(unsigned long addr);
void free_pages(unsigned long addr, unsigned long order);

If you’re writing code to work with both 1.2 and 2.0, it’s better not to use __get_free_pages directly because ...