Based on kernel version 4.16.1. Page generated on 2018-04-09 11:52 EST.
1 2 =================================== 3 Using flexible arrays in the kernel 4 =================================== 5 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 14 of the system. 15 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 18 this task easier. 19 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. 29 30 The creation of a flexible array is done with :c:func:`flex_array_alloc()`:: 31 32 #include <linux/flex_array.h> 33 34 struct flex_array *flex_array_alloc(int element_size, 35 unsigned int total, 36 gfp_t flags); 37 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. 43 44 It is also possible to define flexible arrays at compile time with:: 45 46 DEFINE_FLEX_ARRAY(name, element_size, total); 47 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. 50 51 Storing data into a flexible array is accomplished with a call to 52 :c:func:`flex_array_put()`:: 53 54 int flex_array_put(struct flex_array *array, unsigned int element_nr, 55 void *src, gfp_t flags); 56 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 61 otherwise. 62 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()`:: 69 70 int flex_array_prealloc(struct flex_array *array, unsigned int start, 71 unsigned int nr_elements, gfp_t flags); 72 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 76 block. 77 78 Getting data back out of the array is done with :c:func:`flex_array_get()`:: 79 80 void *flex_array_get(struct flex_array *fa, unsigned int element_nr); 81 82 The return value is a pointer to the data element, or NULL if that 83 particular element has never been allocated. 84 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. 93 94 Individual elements in the array can be cleared with 95 :c:func:`flex_array_clear()`:: 96 97 int flex_array_clear(struct flex_array *array, unsigned int element_nr); 98 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()`:: 104 105 int flex_array_shrink(struct flex_array *array); 106 107 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``. 111 112 It is possible to remove all elements of an array with a call to 113 :c:func:`flex_array_free_parts()`:: 114 115 void flex_array_free_parts(struct flex_array *array); 116 117 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()`:: 119 120 void flex_array_free(struct flex_array *array); 121 122 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. 125 126 127 Flexible array functions 128 ------------------------ 129 130 .. kernel-doc:: include/linux/flex_array.h