1 Using flexible arrays in the kernel 2 Last updated for 2.6.32 3 Jonathan Corbet <firstname.lastname@example.org> 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.