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>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

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

Documentation/core-api/index.rst

@@ -16,6 +16,7 @@ Core utilities
    cpu_hotplug
    local_ops
    workqueue
+   flexible-arrays
 
 Interfaces for kernel debugging
 ===============================