vmalloc()

The vmalloc() function works in a similar fashion to kmalloc(), except it allocates memory that is only virtually contiguous and not necessarily physically contiguous. This is how a user-space allocation function works: The pages returned by malloc() are contiguous within the virtual address space of the processor, but there is no guarantee that they are actually contiguous in physical RAM. The kmalloc() function guarantees that the pages are physically contiguous (and virtually contiguous). The vmalloc() function only ensures that the pages are contiguous within the virtual address space. It does this by allocating potentially noncontiguous chunks of physical memory and "fixing up" the page tables to map the memory into a contiguous chunk of the logical address space.

For the most part, only hardware devices require physically contiguous memory allocations. On many architectures, hardware devices live on the other side of the memory management unit and, thus, do not understand virtual addresses. Consequently, any regions of memory that hardware devices work with must exist as a physically contiguous block and not merely a virtually contiguous one. Blocks of memory used only by softwarefor example, process-related buffersare fine using memory that is only virtually contiguous. In your programming, you will never know the difference. All memory appears to the kernel as logically contiguous.

Despite the fact that physically contiguous memory is required in only certain cases, most kernel code uses kmalloc() and not vmalloc() to obtain memory. Primarily, this is for performance. The vmalloc() function, to make nonphysically contiguous pages contiguous in the virtual address space, must specifically set up the page table entries. Worse, pages obtained via vmalloc() must be mapped by their individual pages (because they are not physically contiguous), which results in much greater TLB^[4] thrashing than you see when directly mapped memory is used. Because of these concerns, vmalloc() is used only when absolutely necessarytypically, to obtain very large regions of memory. For example, when modules are dynamically inserted into the kernel, they are loaded into memory created via vmalloc().

^[4] The TLB (translation lookaside buffer) is a hardware cache used by most architectures to cache the mapping of virtual addresses to physical addresses. This greatly improves the performance of the system, because most memory access is done via virtual addressing.

The vmalloc() function is declared in <linux/vmalloc.h> and defined inmm/vmalloc.c. Usage is identical to user-space's malloc():

void * vmalloc(unsigned long size)

The function returns a pointer to at least size bytes of virtually contiguous memory. On error, the function returns NULL. The function might sleep, and thus cannot be called from interrupt context or other situations where blocking is not permissible.

To free an allocation obtained via vmalloc(), use

void vfree(void *addr)

This function frees the block of memory beginning at addr that was previously allocated via vmalloc(). The function can also sleep and, thus, cannot be called from interrupt context. It has no return value.

Usage of these functions is simple:

char *buf;

buf = vmalloc(16 * PAGE_SIZE); /* get 16 pages */
if (!buf)
        /* error! failed to allocate memory */

/*
 * buf now points to at least a 16*PAGE_SIZE bytes
 * of virtually contiguous block of memory
 */

After you are finished with the memory, make sure to free it by using

vfree(buf);