linux/Documentation/flexible-arrays.txt
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   1===================================
   2Using flexible arrays in the kernel
   3===================================
   4
   5:Updated: Last updated for 2.6.32
   6:Author: Jonathan Corbet <corbet@lwn.net>
   7
   8Large contiguous memory allocations can be unreliable in the Linux kernel.
   9Kernel programmers will sometimes respond to this problem by allocating
  10pages with vmalloc().  This solution not ideal, though.  On 32-bit systems,
  11memory from vmalloc() must be mapped into a relatively small address space;
  12it's easy to run out.  On SMP systems, the page table changes required by
  13vmalloc() allocations can require expensive cross-processor interrupts on
  14all CPUs.  And, on all systems, use of space in the vmalloc() range
  15increases pressure on the translation lookaside buffer (TLB), reducing the
  16performance of the system.
  17
  18In many cases, the need for memory from vmalloc() can be eliminated by
  19piecing together an array from smaller parts; the flexible array library
  20exists to make this task easier.
  21
  22A flexible array holds an arbitrary (within limits) number of fixed-sized
  23objects, accessed via an integer index.  Sparse arrays are handled
  24reasonably well.  Only single-page allocations are made, so memory
  25allocation failures should be relatively rare.  The down sides are that the
  26arrays cannot be indexed directly, individual object size cannot exceed the
  27system page size, and putting data into a flexible array requires a copy
  28operation.  It's also worth noting that flexible arrays do no internal
  29locking at all; if concurrent access to an array is possible, then the
  30caller must arrange for appropriate mutual exclusion.
  31
  32The creation of a flexible array is done with::
  33
  34    #include <linux/flex_array.h>
  35
  36    struct flex_array *flex_array_alloc(int element_size,
  37                                        unsigned int total,
  38                                        gfp_t flags);
  39
  40The individual object size is provided by element_size, while total is the
  41maximum number of objects which can be stored in the array.  The flags
  42argument is passed directly to the internal memory allocation calls.  With
  43the current code, using flags to ask for high memory is likely to lead to
  44notably unpleasant side effects.
  45
  46It is also possible to define flexible arrays at compile time with::
  47
  48    DEFINE_FLEX_ARRAY(name, element_size, total);
  49
  50This macro will result in a definition of an array with the given name; the
  51element size and total will be checked for validity at compile time.
  52
  53Storing data into a flexible array is accomplished with a call to::
  54
  55    int flex_array_put(struct flex_array *array, unsigned int element_nr,
  56                       void *src, gfp_t flags);
  57
  58This call will copy the data from src into the array, in the position
  59indicated by element_nr (which must be less than the maximum specified when
  60the array was created).  If any memory allocations must be performed, flags
  61will be used.  The return value is zero on success, a negative error code
  62otherwise.
  63
  64There might possibly be a need to store data into a flexible array while
  65running in some sort of atomic context; in this situation, sleeping in the
  66memory allocator would be a bad thing.  That can be avoided by using
  67GFP_ATOMIC for the flags value, but, often, there is a better way.  The
  68trick is to ensure that any needed memory allocations are done before
  69entering atomic context, using::
  70
  71    int flex_array_prealloc(struct flex_array *array, unsigned int start,
  72                            unsigned int nr_elements, gfp_t flags);
  73
  74This function will ensure that memory for the elements indexed in the range
  75defined by start and nr_elements has been allocated.  Thereafter, a
  76flex_array_put() call on an element in that range is guaranteed not to
  77block.
  78
  79Getting data back out of the array is done with::
  80
  81    void *flex_array_get(struct flex_array *fa, unsigned int element_nr);
  82
  83The return value is a pointer to the data element, or NULL if that
  84particular element has never been allocated.
  85
  86Note that it is possible to get back a valid pointer for an element which
  87has never been stored in the array.  Memory for array elements is allocated
  88one page at a time; a single allocation could provide memory for several
  89adjacent elements.  Flexible array elements are normally initialized to the
  90value FLEX_ARRAY_FREE (defined as 0x6c in <linux/poison.h>), so errors
  91involving that number probably result from use of unstored array entries.
  92Note that, if array elements are allocated with __GFP_ZERO, they will be
  93initialized to zero and this poisoning will not happen.
  94
  95Individual elements in the array can be cleared with::
  96
  97    int flex_array_clear(struct flex_array *array, unsigned int element_nr);
  98
  99This function will set the given element to FLEX_ARRAY_FREE and return
 100zero.  If storage for the indicated element is not allocated for the array,
 101flex_array_clear() will return -EINVAL instead.  Note that clearing an
 102element does not release the storage associated with it; to reduce the
 103allocated size of an array, call::
 104
 105    int flex_array_shrink(struct flex_array *array);
 106
 107The return value will be the number of pages of memory actually freed.
 108This function works by scanning the array for pages containing nothing but
 109FLEX_ARRAY_FREE bytes, so (1) it can be expensive, and (2) it will not work
 110if the array's pages are allocated with __GFP_ZERO.
 111
 112It is possible to remove all elements of an array with a call to::
 113
 114    void flex_array_free_parts(struct flex_array *array);
 115
 116This call frees all elements, but leaves the array itself in place.
 117Freeing the entire array is done with::
 118
 119    void flex_array_free(struct flex_array *array);
 120
 121As of this writing, there are no users of flexible arrays in the mainline
 122kernel.  The functions described here are also not exported to modules;
 123that will probably be fixed when somebody comes up with a need for it.
 124