Diffstat (limited to 'Documentation/flexible-arrays.txt')
1 files changed, 32 insertions, 11 deletions
diff --git a/Documentation/flexible-arrays.txt b/Documentation/flexible-arrays.txt
index 84eb26808dee..cb8a3a00cc92 100644
@@ -1,5 +1,5 @@
Using flexible arrays in the kernel
-Last updated for 2.6.31
+Last updated for 2.6.32
Jonathan Corbet <email@example.com>
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);