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Based on kernel version 3.9. Page generated on 2013-05-02 23:07 EST.

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