8.3. 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 are introduced in Chapter 15.

To allocate pages, the following functions are available:

get_zeroed_page(unsigned int flags);

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

_ _get_free_page(unsigned int flags);

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

_ _get_free_pages(unsigned int flags, unsigned int order);

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

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 ISA direct-memory-access operations) or _ _GFP_HIGHMEM when high memory can be used.^[2] order is the base-two logarithm of the number of pages you are requesting or freeing (i.e., log₂N). For example, order is 0 if you want one page and 3 if you request eight pages. If order is too big (no contiguous area of that size is available), the page allocation fails. The get_order function, which takes an integer argument, can be used to extract the order from a size (that must be a power of two) for the hosting platform. The maximum allowed value for order is 10 or 11 (corresponding to 1024 or 2048 pages), depending on the architecture. The chances of an order-10 allocation succeeding on anything other than a freshly booted system with a lot of memory are small, however.

^[2] Although alloc_pages (described shortly) should really be used for allocating high-memory pages, for reasons we can't really get into until Chapter 15.

If you are curious, /proc/buddyinfo tells you how many blocks of each order are available for each memory zone on the system.

When a program is done with the pages, it can free them with 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 try to free a different number of pages from what you allocated, the memory map becomes corrupted, and the system gets in trouble at a later time.

It's worth stressing that _ _get_free_pages and the other functions can be called at any time, subject to the same rules we saw for kmalloc. The functions can fail to allocate memory in certain circumstances, particularly when GFP_ATOMIC is used. Therefore, the program calling these allocation functions must be prepared to handle an allocation failure.

Although kmalloc(GFP_KERNEL) sometimes fails when there is no available memory, the kernel does its best to fulfill allocation requests. Therefore, it's easy to degrade system responsiveness by allocating too much memory. For example, you can bring the computer down by pushing too much data into a scull device; the system starts crawling while it tries to swap out as much as possible in order to fulfill the kmalloc request. Since every resource is being sucked up by the growing device, the computer is soon rendered unusable; at that point, you can no longer even start a new process to try to deal with the problem. We don't address this issue in scull, since it is just a sample module and not a real tool to put into a multiuser system. As a programmer, you must be careful nonetheless, because a module is privileged code and can open new security holes in the system (the most likely is a denial-of-service hole like the one just outlined).

8.3.1. A scull Using Whole Pages: scullp

In order to test page allocation for real, we have released the scullp module together with other sample code. It is a reduced scull, just like scullc introduced earlier.

Memory quanta allocated by scullp are whole pages or page sets: the scullp_order variable defaults to 0 but can be changed at either compile or load time.

The following lines show how it allocates memory:

/* Here's the allocation of a single quantum */
if (!dptr->data[s_pos]) {
    dptr->data[s_pos] =
        (void *)_ _get_free_pages(GFP_KERNEL, dptr->order);
    if (!dptr->data[s_pos])
        goto nomem;
    memset(dptr->data[s_pos], 0, PAGE_SIZE << dptr->order);
}

The code to deallocate memory in scullp looks like this:

/* This code frees a whole quantum-set */
for (i = 0; i < qset; i++)
    if (dptr->data[i])
        free_pages((unsigned long)(dptr->data[i]),
                dptr->order);

At the user level, the perceived difference is primarily a speed improvement and better memory use, because there is no internal fragmentation of memory. We ran some tests copying 4 MB from scull0 to scull1 and then from scullp0 to scullp1; the results showed a slight improvement in kernel-space processor usage.

The performance improvement is not dramatic, because kmalloc is designed to be fast. The main advantage of page-level allocation isn't actually speed, but rather more efficient memory usage. Allocating by pages wastes no memory, whereas using kmalloc wastes an unpredictable amount of memory because of allocation granularity.

But the biggest advantage of the _ _get_free_page functions is that the pages obtained are completely yours, and you could, in theory, assemble the pages into a linear area by appropriate tweaking of the page tables. For example, you can allow a user process to mmap memory areas obtained as single unrelated pages. We discuss this kind of operation in Chapter 15, where we show how scullp offers memory mapping, something that scull cannot offer.

8.3.2. The alloc_pages Interface

For completeness, we introduce another interface for memory allocation, even though we will not be prepared to use it until after Chapter 15. For now, suffice it to say that struct page is an internal kernel structure that describes a page of memory. As we will see, there are many places in the kernel where it is necessary to work with page structures; they are especially useful in any situation where you might be dealing with high memory, which does not have a constant address in kernel space.

The real core of the Linux page allocator is a function called alloc_pages_node:

struct page *alloc_pages_node(int nid, unsigned int flags, 
                              unsigned int order);

This function also has two variants (which are simply macros); these are the versions that you will most likely use:

struct page *alloc_pages(unsigned int flags, unsigned int order);
struct page *alloc_page(unsigned int flags);

The core function, alloc_pages_node, takes three arguments. nid is the NUMA node ID^[3] whose memory should be allocated, flags is the usual GFP_ allocation flags, and order is the size of the allocation. The return value is a pointer to the first of (possibly many) page structures describing the allocated memory, or, as usual, NULL on failure.

^[3] NUMA (nonuniform memory access) computers are multiprocessor systems where memory is "local" to specific groups of processors ("nodes"). Access to local memory is faster than access to nonlocal memory. On such systems, allocating memory on the correct node is important. Driver authors do not normally have to worry about NUMA issues, however.

alloc_pages simplifies the situation by allocating the memory on the current NUMA node (it calls alloc_pages_node with the return value from numa_node_id as the nid parameter). And, of course, alloc_page omits the order parameter and allocates a single page.

To release pages allocated in this manner, you should use one of the following:

void _ _free_page(struct page *page);
void _ _free_pages(struct page *page, unsigned int order);
void free_hot_page(struct page *page);
void free_cold_page(struct page *page);

If you have specific knowledge of whether a single page's contents are likely to be resident in the processor cache, you should communicate that to the kernel with free_hot_page (for cache-resident pages) or free_cold_page. This information helps the memory allocator optimize its use of memory across the system.