Kernel Korner - Allocating Memory in the Kernel

Unfortunately for kernel developers, allocating memory in the kernel is not as simple as allocating memory in user space. A number of factors contribute to the complication, among them:

Although easy access to an abundance of memory certainly is not a luxury to the kernel, a little understanding of the issues can go a long way toward making the process relatively painless.

The general interface for allocating memory inside of the kernel is kmalloc():

#include <linux/slab.h>

void * kmalloc(size_t size, int flags);

It should look familiar—it is pretty much the same as user space's malloc(), after all—except that it takes a second argument, flags. Let's ignore flags for a second and see what we recognize. First off, size is the same here as in malloc()'s—it specifies the size in bytes of the allocation. Upon successful return, kmalloc() returns a pointer to size bytes of memory. The alignment of the allocated memory is suitable for storage of and access to any type of object. As with malloc(), kmalloc() can fail, and you must check its return value against NULL. Let's look at an example:

struct falcon *p;

p = kmalloc(sizeof (struct falcon), GFP_KERNEL);
if (!p)
  /* the allocation failed - handle appropriately */

The flags field controls the behavior of memory allocation. We can divide flags into three groups: action modifiers, zone modifiers and types. Action modifiers tell the kernel how to allocate memory. They specify, for example, whether the kernel can sleep (that is, whether the call to kmalloc() can block) in order to satisfy the allocation. Zone modifiers, on the other hand, tell the kernel from where the request should be satisfied. For example, some requests may need to be satisfied from memory that hardware can access through direct memory access (DMA). Finally, type flags specify a type of allocation. They group together relevant action and zone modifiers into a single mnemonic. In general, instead of specifying multiple action and zone modifiers, you specify a single type flag.

Table 1 is a listing of the action modifiers, and Table 2 is a listing of the zone modifiers. Many different flags can be used; allocating memory in the kernel is nontrivial. It is possible to control many aspects of memory allocation in the kernel. Your code should use the type flags and not the individual action and zone modifiers. The two most common flags are GFP_ATOMIC and GFP_KERNEL. Nearly all of your kernel memory allocations should specify one of these two flags.

The GFP_ATOMIC flag instructs the memory allocator never to block. Use this flag in situations where it cannot sleep—where it must remain atomic—such as interrupt handlers, bottom halves and process context code that is holding a lock. Because the kernel cannot block the allocation and try to free up sufficient memory to satisfy the request, an allocation specifying GFP_ATOMIC has a lesser chance of succeeding than one that does not. Nonetheless, if your current context is incapable of sleeping, it is your only choice. Using GFP_ATOMIC is simple:

struct wolf *p;

p = kmalloc(sizeof (struct wolf), GFP_ATOMIC);
if (!p)
    /* error */

Conversely, the GFP_KERNEL flag specifies a normal kernel allocation. Use this flag in code executing in process context without any locks. A call to kmalloc() with this flag can sleep; thus, you must use this flag only when it is safe to do so. The kernel utilizes the ability to sleep in order to free memory, if needed. Therefore, allocations that specify this flag have a greater chance of succeeding. If insufficient memory is available, for example, the kernel can block the requesting code and swap some inactive pages to disk, shrink the in-memory caches, write out buffers and so on.

Sometimes, as when writing an ISA device driver, you need to ensure that the memory allocated is capable of undergoing DMA. For ISA devices, this is memory in the first 16MB of physical memory. To ensure that the kernel allocates from this specific memory, use the GFP_DMA flag. Generally, you would use this flag in conjunction with either GFP_ATOMIC or GFP_KERNEL; you can combine flags with a binary OR operation. For example, to instruct the kernel to allocate DMA-capable memory and to sleep if needed, do:

char *buf;

/* we want DMA-capable memory,
 * and we can sleep if needed */
buf = kmalloc(BUF_LEN, GFP_DMA | GFP_KERNEL);
if (!buf)
    /* error */

Table 3 is a listing of the type flags, and Table 4 shows to which type flag each action and zone modifier equates. The header <linux/gfp.h> defines all of the flags.