Linux Device Drivers

vmalloc and Friends

The next memory allocation function that I’ll show you is vmalloc, which allocates a contiguous memory region in the virtual address space. Although the pages are not necessarily consecutive in physical memory (each page is retrieved with a separate call to __get_free_page), the kernel sees them as a contiguous range of addresses. The allocated space is mapped only to the kernel segments and is not visible from user space--not unlike the other allocation techniques. vmalloc returns 0 (the NULL address) if an error occurred, otherwise it returns a pointer to a linear memory area of size size.

The prototypes of the function, and its relatives, are the following:

void * vmalloc(unsigned long size);
void vfree(void * addr);
void * vremap(unsigned long offset, unsigned long size);

Note that vremap was renamed ioremap in version 2.1. Moreover, Linux 2.1 introduced a new header, <linux/vmalloc.h>, that must be included if you use vmalloc.

vmalloc is different from the other memory allocation functions because it returns ``high'' addresses--addresses that are higher than the top of physical memory. The processor is able to access the returned memory range because vmalloc arranged the processor’s page tables to access the allocated pages through consecutive ``high'' addresses. Kernel code can use addresses returned by vmalloc just like any other address, but the address used by the program is not the same as the one that appears on the electrical data bus.

Addresses ...