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Subject[PATCH v2] Documentation: Add flexible-arrays.rst to the documentation tree
Add flexible-arrays.rst to Documentation/core-api. Add kernel-doc
comments to allow referencing.

Signed-off-by: sayli karnik <karniksayli1995@gmail.com>
---
Changes in v2:
-Removed default comment by sphinx and author's name
-Added :c:func:`vmalloc()`
-Added the parenthesis in :c:func:`func()`
-Edited spacing and line length
-Edited description for gfp flags and fa
-Corrected typo for 'retrieve'
-Added to lkml to cc

Documentation/core-api/flexible-arrays.rst | 130 +++++++++++++++++++++++++++++
Documentation/core-api/index.rst | 1 +
include/linux/flex_array.h | 67 +++++++++++++++
3 files changed, 198 insertions(+)
create mode 100644 Documentation/core-api/flexible-arrays.rst

diff --git a/Documentation/core-api/flexible-arrays.rst b/Documentation/core-api/flexible-arrays.rst
new file mode 100644
index 0000000..b6b85a1
--- /dev/null
+++ b/Documentation/core-api/flexible-arrays.rst
@@ -0,0 +1,130 @@
+
+===================================
+Using flexible arrays in the kernel
+===================================
+
+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
diff --git a/Documentation/core-api/index.rst b/Documentation/core-api/index.rst
index 0d93d80..2b908ac 100644
--- a/Documentation/core-api/index.rst
+++ b/Documentation/core-api/index.rst
@@ -16,6 +16,7 @@ Core utilities
cpu_hotplug
local_ops
workqueue
+ flexible-arrays

Interfaces for kernel debugging
===============================
diff --git a/include/linux/flex_array.h b/include/linux/flex_array.h
index b6efb0c..11366b3 100644
--- a/include/linux/flex_array.h
+++ b/include/linux/flex_array.h
@@ -61,16 +61,83 @@ struct flex_array {
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) \
--
2.7.4
\
 
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