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+Using flexible arrays in the kernel
+Last updated for 2.6.32
+Jonathan Corbet <corbet@lwn.net>
+Large contiguous memory allocations can be unreliable in the Linux kernel.
+Kernel programmers will sometimes respond to this problem by allocating
+pages with vmalloc(). This solution not ideal, though. On 32-bit systems,
+memory from vmalloc() must be mapped into a relatively small address space;
+it's easy to run out. On SMP systems, the page table changes required by
+vmalloc() allocations can require expensive cross-processor interrupts on
+all CPUs. And, on all systems, use of space in the vmalloc() range
+increases pressure on the translation lookaside buffer (TLB), reducing the
+performance of the system.
+In many cases, the need for memory from vmalloc() can be eliminated by
+piecing together an array from smaller parts; the flexible array library
+exists to make this task easier.
+A flexible array holds an arbitrary (within limits) number of fixed-sized
+objects, accessed via an integer index. Sparse arrays are handled
+reasonably well. Only single-page allocations are made, so memory
+allocation failures should be relatively rare. The down sides are that the
+arrays cannot be indexed directly, individual object size cannot exceed the
+system page size, and putting data into a flexible array requires a copy
+operation. It's also worth noting that flexible arrays do no internal
+locking at all; if concurrent access to an array is possible, then the
+caller must arrange for appropriate mutual exclusion.
+The creation of a flexible array is done with:
+ #include <linux/flex_array.h>
+ struct flex_array *flex_array_alloc(int element_size,
+ unsigned int total,
+ gfp_t flags);
+The individual object size is provided by element_size, while total is the
+maximum number of objects which can be stored in the array. The flags
+argument is passed directly to the internal memory allocation calls. With
+the current code, using flags to ask for high memory is likely to lead to
+notably unpleasant side effects.
+It is also possible to define flexible arrays at compile time with:
+ DEFINE_FLEX_ARRAY(name, element_size, total);
+This macro will result in a definition of an array with the given name; the
+element size and total will be checked for validity at compile time.
+Storing data into a flexible array is accomplished with a call to:
+ int flex_array_put(struct flex_array *array, unsigned int element_nr,
+ void *src, gfp_t flags);
+This call will copy the data from src into the array, in the position
+indicated by element_nr (which must be less than the maximum specified when
+the array was created). If any memory allocations must be performed, flags
+will be used. The return value is zero on success, a negative error code
+There might possibly be a need to store data into a flexible array while
+running in some sort of atomic context; in this situation, sleeping in the
+memory allocator would be a bad thing. That can be avoided by using
+GFP_ATOMIC for the flags value, but, often, there is a better way. The
+trick is to ensure that any needed memory allocations are done before
+entering atomic context, using:
+ int flex_array_prealloc(struct flex_array *array, unsigned int start,
+ unsigned int nr_elements, gfp_t flags);
+This function will ensure that memory for the elements indexed in the range
+defined by start and nr_elements has been allocated. Thereafter, a
+flex_array_put() call on an element in that range is guaranteed not to
+Getting data back out of the array is done with:
+ void *flex_array_get(struct flex_array *fa, unsigned int element_nr);
+The return value is a pointer to the data element, or NULL if that
+particular element has never been allocated.
+Note that it is possible to get back a valid pointer for an element which
+has never been stored in the array. Memory for array elements is allocated
+one page at a time; a single allocation could provide memory for several
+adjacent elements. Flexible array elements are normally initialized to the
+value FLEX_ARRAY_FREE (defined as 0x6c in <linux/poison.h>), so errors
+involving that number probably result from use of unstored array entries.
+Note that, if array elements are allocated with __GFP_ZERO, they will be
+initialized to zero and this poisoning will not happen.
+Individual elements in the array can be cleared with:
+ int flex_array_clear(struct flex_array *array, unsigned int element_nr);
+This function will set the given element to FLEX_ARRAY_FREE and return
+zero. If storage for the indicated element is not allocated for the array,
+flex_array_clear() will return -EINVAL instead. Note that clearing an
+element does not release the storage associated with it; to reduce the
+allocated size of an array, call:
+ int flex_array_shrink(struct flex_array *array);
+The return value will be the number of pages of memory actually freed.
+This function works by scanning the array for pages containing nothing but
+FLEX_ARRAY_FREE bytes, so (1) it can be expensive, and (2) it will not work
+if the array's pages are allocated with __GFP_ZERO.
+It is possible to remove all elements of an array with a call to:
+ void flex_array_free_parts(struct flex_array *array);
+This call frees all elements, but leaves the array itself in place.
+Freeing the entire array is done with:
+ void flex_array_free(struct flex_array *array);
+As of this writing, there are no users of flexible arrays in the mainline
+kernel. The functions described here are also not exported to modules;
+that will probably be fixed when somebody comes up with a need for it.