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3 Using flexible arrays in the kernel
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6 Large contiguous memory allocations can be unreliable in the Linux kernel.
7 Kernel programmers will sometimes respond to this problem by allocating
8 pages with :c:func:`vmalloc()`. This solution not ideal, though. On 32-bit
9 systems, memory from vmalloc() must be mapped into a relatively small address
10 space; it's easy to run out. On SMP systems, the page table changes required
11 by vmalloc() allocations can require expensive cross-processor interrupts on
12 all CPUs. And, on all systems, use of space in the vmalloc() range increases
13 pressure on the translation lookaside buffer (TLB), reducing the performance
16 In many cases, the need for memory from vmalloc() can be eliminated by piecing
17 together an array from smaller parts; the flexible array library exists to make
20 A flexible array holds an arbitrary (within limits) number of fixed-sized
21 objects, accessed via an integer index. Sparse arrays are handled
22 reasonably well. Only single-page allocations are made, so memory
23 allocation failures should be relatively rare. The down sides are that the
24 arrays cannot be indexed directly, individual object size cannot exceed the
25 system page size, and putting data into a flexible array requires a copy
26 operation. It's also worth noting that flexible arrays do no internal
27 locking at all; if concurrent access to an array is possible, then the
28 caller must arrange for appropriate mutual exclusion.
30 The creation of a flexible array is done with :c:func:`flex_array_alloc()`::
32 #include <linux/flex_array.h>
34 struct flex_array *flex_array_alloc(int element_size,
38 The individual object size is provided by ``element_size``, while total is the
39 maximum number of objects which can be stored in the array. The flags
40 argument is passed directly to the internal memory allocation calls. With
41 the current code, using flags to ask for high memory is likely to lead to
42 notably unpleasant side effects.
44 It is also possible to define flexible arrays at compile time with::
46 DEFINE_FLEX_ARRAY(name, element_size, total);
48 This macro will result in a definition of an array with the given name; the
49 element size and total will be checked for validity at compile time.
51 Storing data into a flexible array is accomplished with a call to
52 :c:func:`flex_array_put()`::
54 int flex_array_put(struct flex_array *array, unsigned int element_nr,
55 void *src, gfp_t flags);
57 This call will copy the data from src into the array, in the position
58 indicated by ``element_nr`` (which must be less than the maximum specified when
59 the array was created). If any memory allocations must be performed, flags
60 will be used. The return value is zero on success, a negative error code
63 There might possibly be a need to store data into a flexible array while
64 running in some sort of atomic context; in this situation, sleeping in the
65 memory allocator would be a bad thing. That can be avoided by using
66 ``GFP_ATOMIC`` for the flags value, but, often, there is a better way. The
67 trick is to ensure that any needed memory allocations are done before
68 entering atomic context, using :c:func:`flex_array_prealloc()`::
70 int flex_array_prealloc(struct flex_array *array, unsigned int start,
71 unsigned int nr_elements, gfp_t flags);
73 This function will ensure that memory for the elements indexed in the range
74 defined by ``start`` and ``nr_elements`` has been allocated. Thereafter, a
75 ``flex_array_put()`` call on an element in that range is guaranteed not to
78 Getting data back out of the array is done with :c:func:`flex_array_get()`::
80 void *flex_array_get(struct flex_array *fa, unsigned int element_nr);
82 The return value is a pointer to the data element, or NULL if that
83 particular element has never been allocated.
85 Note that it is possible to get back a valid pointer for an element which
86 has never been stored in the array. Memory for array elements is allocated
87 one page at a time; a single allocation could provide memory for several
88 adjacent elements. Flexible array elements are normally initialized to the
89 value ``FLEX_ARRAY_FREE`` (defined as 0x6c in <linux/poison.h>), so errors
90 involving that number probably result from use of unstored array entries.
91 Note that, if array elements are allocated with ``__GFP_ZERO``, they will be
92 initialized to zero and this poisoning will not happen.
94 Individual elements in the array can be cleared with
95 :c:func:`flex_array_clear()`::
97 int flex_array_clear(struct flex_array *array, unsigned int element_nr);
99 This function will set the given element to ``FLEX_ARRAY_FREE`` and return
100 zero. If storage for the indicated element is not allocated for the array,
101 ``flex_array_clear()`` will return ``-EINVAL`` instead. Note that clearing an
102 element does not release the storage associated with it; to reduce the
103 allocated size of an array, call :c:func:`flex_array_shrink()`::
105 int flex_array_shrink(struct flex_array *array);
107 The return value will be the number of pages of memory actually freed.
108 This function works by scanning the array for pages containing nothing but
109 ``FLEX_ARRAY_FREE`` bytes, so (1) it can be expensive, and (2) it will not work
110 if the array's pages are allocated with ``__GFP_ZERO``.
112 It is possible to remove all elements of an array with a call to
113 :c:func:`flex_array_free_parts()`::
115 void flex_array_free_parts(struct flex_array *array);
117 This call frees all elements, but leaves the array itself in place.
118 Freeing the entire array is done with :c:func:`flex_array_free()`::
120 void flex_array_free(struct flex_array *array);
122 As of this writing, there are no users of flexible arrays in the mainline
123 kernel. The functions described here are also not exported to modules;
124 that will probably be fixed when somebody comes up with a need for it.
127 Flexible array functions
128 ------------------------
130 .. kernel-doc:: include/linux/flex_array.h