Allocating Memory

Chapter 8

Story of kmalloc

  • Similar to malloc
  • Allocates contiguous memory

The Flags Argument

void *kmalloc(size_t size, int flags);

    • Allocate memory outside of a process context. Never sleeps
    • Normal allocation of kernel memory
    • Used to allocate memory for user-space pages. It may sleep

Memory zones

  • DMA-capable memory
    • Used for DMA access
  • Normal memory
    • Normal allocation
  • High memory
    • Used to allow access to large amounts of memory on 32 bit platforms
  • __GFP_DMA (only the dma zone is searched)
  • If no flags present, DMA and and normal memory are searched
  • __GFP_HIGMEM (all 3 zones are searched)

Size Argument

  • Kernel memory available page sized chunks
  • Kernel creates set pools of memory objects of fixed size
  • Allocation is handled by returning a memory chunk from a sufficiently large pool back to the requester
  • memory allocated is fixed size byte arrays and requester will likely get back more memory than asked up to twice as much

Lookaside Caches

  • Special memory pool for high volume objects
  • kmem_cache_t *kmem_cache_create(...)
    • creates a new cache object that can host any number of memory areas all of the same size, specified by the size argument
  • void *kmem_cache_alloc(kmem_cache_t *cache, flags)
    • Allocate objects from cache object
    • cache ( cache created by kmem_cache_create )
  • void kmem_cache_free(kmem_cache_t *cache, void *obj)
    • Free object memory into a cache
  • kmem_cache_destroy(kmem_cache_t *cache)
    • Frees cache data structure; no longer usable

Memory pools

Form of lookaside cache in kernel where memory allocation is not allowed to fail

get_free_page and friends

Used to allocate memory in page sized chunks

  • for when driver needs big chunks of memory
  • void *get_zeroed_page(unsigned int flags);
    • Returns pointer to a new page and fills the page with zeros
  • __get_free_page
    • Similar to get_zeroed_page, but doesn’t clear the page
  • Flags argument works similarly to kmalloc
    • Usually GFP_KERNEL or GFP_ATOMIC is used
  • free_page(...), free_pages(...)


Allocates contiguous memory in virtual address space

  • Pages not consecutive. Each page retrieved by a call to alloc_page
  • Address range used by vmalloc is synthetic. Sets up page tables
  • void *vamalloc(unsigned long size);
  • void free(void * addr);

Per-CPU Variables

  • When a per-cpu variable is created, each processor gets a copy
  • Used in networking subsystem. Require no locking and are fast
  • DEFINE_PER_CPU(type, name)
  • Dynamically allocated
    • void *alloc_percpu(type);
    • void * __alloc_percpu(size_t size, size_t align);

Obtaining Large Buffers

  • Allocations of large contiguous memory buffers are prone to failure
  • Best way of performing large I/O operations is through scatter/gather operations (chapter 15)

Acquiring Dedicated Buffer at Boot Time

  • If you need a huge buffer of physical contiguous memory, request memory at boot time.
  • Dirty technique that bypasses memory management policies
  • Modules can’t allocate memory at boot time, only drivers directly linked to the kernel