Documentation: Add flexible-arrays.rst to the documentation tree

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Using flexible arrays in the kernel

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Large contiguous memory allocations can be unreliable in the Linux kernel.

Kernel programmers will sometimes respond to this problem by allocating

pages with :c:func:`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 :c:func:`flex_array_alloc()`::

    #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

:c:func:`flex_array_put()`::

    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

otherwise.

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 :c:func:`flex_array_prealloc()`::

    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

block.

Getting data back out of the array is done with :c:func:`flex_array_get()`::

    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

:c:func:`flex_array_clear()`::

    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 :c:func:`flex_array_shrink()`::

    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

:c:func:`flex_array_free_parts()`::

    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 :c:func:`flex_array_free()`::

    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.

Flexible array functions

------------------------

.. kernel-doc:: include/linux/flex_array.h

   cpu_hotplug

   local_ops

   workqueue

   flexible-arrays

Interfaces for kernel debugging

===============================

			FLEX_ARRAY_ELEMENTS_PER_PART(__element_size));	\

	}

/**

 * flex_array_alloc() - Creates a flexible array.

 * @element_size:	individual object size.

 * @total:		maximum number of objects which can be stored.

 * @flags:		GFP flags

 *

 * Return:		Returns an object of structure flex_array.

 */

struct flex_array *flex_array_alloc(int element_size, unsigned int total,

		gfp_t flags);

/**

 * flex_array_prealloc() - Ensures that memory for the elements indexed in the

 * range defined by start and nr_elements has been allocated.

 * @fa:			array to allocate memory to.

 * @start:		start address

 * @nr_elements:	number of elements to be allocated.

 * @flags:		GFP flags

 *

 */

int flex_array_prealloc(struct flex_array *fa, unsigned int start,

		unsigned int nr_elements, gfp_t flags);

/**

 * flex_array_free() - Removes all elements of a flexible array.

 * @fa:		array to be freed.

 */

void flex_array_free(struct flex_array *fa);

/**

 * flex_array_free_parts() - Removes all elements of a flexible array, but

 * leaves the array itself in place.

 * @fa:		array to be emptied.

 */

void flex_array_free_parts(struct flex_array *fa);

/**

 * flex_array_put() - Stores data into a flexible array.

 * @fa:		array where element is to be stored.

 * @element_nr:	position to copy, must be less than the maximum specified when

 *		the array was created.

 * @src:	data source to be copied into the array.

 * @flags:	GFP flags

 *

 * Return:	Returns zero on success, a negative error code otherwise.

 */

int flex_array_put(struct flex_array *fa, unsigned int element_nr, void *src,

		gfp_t flags);

/**

 * flex_array_clear() - Clears an individual element in the array, sets the

 * given element to FLEX_ARRAY_FREE.

 * @element_nr:	element position to clear.

 * @fa:		array to which element to be cleared belongs.

 *

 * Return:	Returns zero on success, -EINVAL otherwise.

 */

int flex_array_clear(struct flex_array *fa, unsigned int element_nr);

/**

 * flex_array_get() - Retrieves data into a flexible array.

 *

 * @element_nr:	Element position to retrieve data from.

 * @fa:		array from which data is to be retrieved.

 *

 * Return:	Returns a pointer to the data element, or NULL if that

 *		particular element has never been allocated.

 */

void *flex_array_get(struct flex_array *fa, unsigned int element_nr);

/**

 * flex_array_shrink() - Reduces the allocated size of an array.

 * @fa:		array to shrink.

 *

 * Return:	Returns number of pages of memory actually freed.

 *

 */

int flex_array_shrink(struct flex_array *fa);

#define flex_array_put_ptr(fa, nr, src, gfp) \