The following commit has been merged in the linux branch: commit 1243ba98e3abecd12e9e90dd390801b95ef070f2 Author: Jonathan Corbet corbet@lwn.net Date: Wed Oct 14 12:43:22 2009 -0600

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

diff --git a/Documentation/flexible-arrays.txt b/Documentation/flexible-arrays.txt index 84eb268..cb8a3a0 100644 --- a/Documentation/flexible-arrays.txt +++ b/Documentation/flexible-arrays.txt @@ -1,5 +1,5 @@ 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);