Using flexible arrays in the kernel

  Last updated for 2.6.32

  Jonathan Corbet <corbet@lwn.net>
  
  Large contiguous memory allocations can be unreliable in the Linux kernel.
  Kernel programmers will sometimes respond to this problem by allocating
  pages with vmalloc().  This solution not ideal, though.  On 32-bit systems,
  memory from vmalloc() must be mapped into a relatively small address space;
  it's easy to run out.  On SMP systems, the page table changes required by
  vmalloc() allocations can require expensive cross-processor interrupts on
  all CPUs.  And, on all systems, use of space in the vmalloc() range
  increases pressure on the translation lookaside buffer (TLB), reducing the
  performance of the system.
  
  In many cases, the need for memory from vmalloc() can be eliminated by
  piecing together an array from smaller parts; the flexible array library
  exists to make this task easier.
  
  A flexible array holds an arbitrary (within limits) number of fixed-sized
  objects, accessed via an integer index.  Sparse arrays are handled
  reasonably well.  Only single-page allocations are made, so memory
  allocation failures should be relatively rare.  The down sides are that the
  arrays cannot be indexed directly, individual object size cannot exceed the
  system page size, and putting data into a flexible array requires a copy
  operation.  It's also worth noting that flexible arrays do no internal
  locking at all; if concurrent access to an array is possible, then the
  caller must arrange for appropriate mutual exclusion.
  
  The creation of a flexible array is done with:
  
      #include <linux/flex_array.h>
  
      struct flex_array *flex_array_alloc(int element_size,
  					unsigned int total,
  					gfp_t flags);
  
  The individual object size is provided by element_size, while total is the
  maximum number of objects which can be stored in the array.  The flags
  argument is passed directly to the internal memory allocation calls.  With
  the current code, using flags to ask for high memory is likely to lead to
  notably unpleasant side effects.

  It is also possible to define flexible arrays at compile time with:
  
      DEFINE_FLEX_ARRAY(name, element_size, total);
  
  This macro will result in a definition of an array with the given name; the
  element size and total will be checked for validity at compile time.

  Storing data into a flexible array is accomplished with a call to:
  
      int flex_array_put(struct flex_array *array, unsigned int element_nr,
      		       void *src, gfp_t flags);
  
  This call will copy the data from src into the array, in the position
  indicated by element_nr (which must be less than the maximum specified when
  the array was created).  If any memory allocations must be performed, flags
  will be used.  The return value is zero on success, a negative error code
  otherwise.
  
  There might possibly be a need to store data into a flexible array while
  running in some sort of atomic context; in this situation, sleeping in the
  memory allocator would be a bad thing.  That can be avoided by using
  GFP_ATOMIC for the flags value, but, often, there is a better way.  The
  trick is to ensure that any needed memory allocations are done before
  entering atomic context, using:
  
      int flex_array_prealloc(struct flex_array *array, unsigned int start,

  			    unsigned int nr_elements, gfp_t flags);

  
  This function will ensure that memory for the elements indexed in the range

  defined by start and nr_elements has been allocated.  Thereafter, a

  flex_array_put() call on an element in that range is guaranteed not to
  block.
  
  Getting data back out of the array is done with:
  
      void *flex_array_get(struct flex_array *fa, unsigned int element_nr);
  
  The return value is a pointer to the data element, or NULL if that
  particular element has never been allocated.
  
  Note that it is possible to get back a valid pointer for an element which
  has never been stored in the array.  Memory for array elements is allocated
  one page at a time; a single allocation could provide memory for several

  adjacent elements.  Flexible array elements are normally initialized to the
  value FLEX_ARRAY_FREE (defined as 0x6c in <linux/poison.h>), so errors
  involving that number probably result from use of unstored array entries.
  Note that, if array elements are allocated with __GFP_ZERO, they will be
  initialized to zero and this poisoning will not happen.
  
  Individual elements in the array can be cleared with:
  
      int flex_array_clear(struct flex_array *array, unsigned int element_nr);
  
  This function will set the given element to FLEX_ARRAY_FREE and return
  zero.  If storage for the indicated element is not allocated for the array,
  flex_array_clear() will return -EINVAL instead.  Note that clearing an
  element does not release the storage associated with it; to reduce the
  allocated size of an array, call:
  
      int flex_array_shrink(struct flex_array *array);
  
  The return value will be the number of pages of memory actually freed.
  This function works by scanning the array for pages containing nothing but
  FLEX_ARRAY_FREE bytes, so (1) it can be expensive, and (2) it will not work
  if the array's pages are allocated with __GFP_ZERO.
  
  It is possible to remove all elements of an array with a call to:

  
      void flex_array_free_parts(struct flex_array *array);
  
  This call frees all elements, but leaves the array itself in place.
  Freeing the entire array is done with:
  
      void flex_array_free(struct flex_array *array);
  
  As of this writing, there are no users of flexible arrays in the mainline
  kernel.  The functions described here are also not exported to modules;
  that will probably be fixed when somebody comes up with a need for it.