Update flex_arrays.txt

The 2.6.32 merge window brought a number of changes to the flexible array
API; this patch updates the documentation to match the new state of
affairs.
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/flexible-arrays.txt

 Using flexible arrays in the kernel
-Last updated for 2.6.31
+Last updated for 2.6.32
 Jonathan Corbet <corbet@lwn.net>
 
 Large contiguous memory allocations can be unreliable in the Linux kernel.
@@ -40,6 +40,13 @@ 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,
@@ -76,16 +83,30 @@ 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.  The flexible array code does not know if a specific
-element has been written; it only knows if the associated memory is
-present.  So a flex_array_get() call on an element which was never stored
-in the array has the potential to return a pointer to random data.  If the
-caller does not have a separate way to know which elements were actually
-stored, it might be wise, at least, to add GFP_ZERO to the flags argument
-to ensure that all elements are zeroed.
-
-There is no way to remove a single element from the array.  It is possible,
-though, to remove all elements with a call to:
+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);