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Update flex_arrays.txt

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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>
This commit is contained in:
Jonathan Corbet 2009-10-14 12:43:22 -06:00
parent 161291396e
commit 1243ba98e3
1 changed files with 31 additions and 10 deletions
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41
Documentation/flexible-arrays.txt
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@ -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.
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.
There is no way to remove a single element from the array. It is possible,
though, to remove all elements with a call to:
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);