Which Allocation Method Should I Use?

If you need contiguous physical pages, use one of the low-level page allocators or kmalloc(). This is the standard manner of allocating memory from within the kernel, and most likely, how you will allocate most of your memory. Recall that the two most common flags given to these functions are GFP_ATOMIC and GFP_KERNEL. Specify the GFP_ATOMIC flag to perform a high priority allocation that will not sleep. This is a requirement of interrupt handlers and other pieces of code that cannot sleep. Code that can sleep, such as process context code that does not hold a spin lock, should use GFP_KERNEL. This flag specifies an allocation that can sleep, if needed, to obtain the requested memory.

If you want to allocate from high memory, use alloc_pages(). The alloc_pages() function returns a struct page, and not a pointer to a logical address. Because high memory might not be mapped, the only way to access it might be via the corresponding struct page structure. To obtain an actual pointer, use kmap() to map the high memory into the kernel's logical address space.

If you do not need physically contiguous pagesonly virtually contiguoususe vmalloc(), although bear in mind the slight performance hit taken with vmalloc() over kmalloc(). The vmalloc() function allocates kernel memory that is virtually contiguous but not, per se, physically contiguous. It performs this feat much as user-space allocations do, by mapping chunks of physical memory into a contiguous logical address space.

If you are creating and destroying many large data structures, consider setting up a slab cache. The slab layer maintains a per-processor object cache (a free list), which might greatly enhance object allocation and deallocation performance. Rather than frequently allocate and free memory, the slab layer stores a cache of already allocated objects for you. When you need a new chunk of memory to hold your data structure, the slab layer often does not need to allocate more memory and instead simply can return an object from the cache.