Linux Device Drivers, Second Edition

get_free_page and Friends

If a module needs to allocate big chunks of memory, it is usually 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_zeroed_page

Returns a pointer to a new page and fills the page with zeros.

__get_free_page

Similar to get_zeroed_page, but doesn’t clear the page.

__get_free_pages

Allocates and returns a pointer to the first byte of a memory area that is several (physically contiguous) pages long, but doesn’t zero the area.

__get_dma_pages

Similar to get_free_pages, but guarantees that the allocated memory is DMA capable. If you use version 2.2 or later of the kernel, you can simply use __get_free_pages and pass the __GFP_DMA flag; if you want backward compatibility with 2.0, you need to call this function instead.

The prototypes for the functions follow:

unsigned long get_zeroed_page(int flags);
unsigned long __get_free_page(int flags);
unsigned long __get_free_pages(int flags, unsigned long order);
unsigned long __get_dma_pages(int flags, unsigned long order);

The flags argument works in the same way as with kmalloc; usually either GFP_KERNEL or GFP_ATOMIC is used, perhaps with the addition of the __GFP_DMA flag (for memory that can be used for direct memory access operations) or __GFP_HIGHMEM when high memory can be used. order is the base-two logarithm of the number of pages ...