flex_array: add flex_array_clear function

Add a new function to the flex_array API:

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

This function will zero the element at element_nr in the flex_array.

Although this is equivalent to using flex_array_put() and passing a
pointer to zero'd memory, flex_array_clear() does not require such a
pointer to memory that would most likely need to be allocated on the
caller's stack which could be significantly large depending on
element_size.

Signed-off-by: David Rientjes <rientjes@google.com>
Cc: Dave Hansen <dave@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/flex_array.h

@@ -44,6 +44,7 @@ void flex_array_free(struct flex_array *fa);
 void flex_array_free_parts(struct flex_array *fa);
 int flex_array_put(struct flex_array *fa, unsigned int element_nr, void *src,
 		gfp_t flags);
+int flex_array_clear(struct flex_array *fa, unsigned int element_nr);
 void *flex_array_get(struct flex_array *fa, unsigned int element_nr);
 
 #endif /* _FLEX_ARRAY_H */

lib/flex_array.c

@@ -206,6 +206,32 @@ int flex_array_put(struct flex_array *fa, unsigned int element_nr, void *src,
 	return 0;
 }
 
+/**
+ * flex_array_clear - clear element in array at @element_nr
+ * @element_nr:	index of the position to clear.
+ *
+ * Locking must be provided by the caller.
+ */
+int flex_array_clear(struct flex_array *fa, unsigned int element_nr)
+{
+	int part_nr = fa_element_to_part_nr(fa, element_nr);
+	struct flex_array_part *part;
+	void *dst;
+
+	if (element_nr >= fa->total_nr_elements)
+		return -ENOSPC;
+	if (elements_fit_in_base(fa))
+		part = (struct flex_array_part *)&fa->parts[0];
+	else {
+		part = fa->parts[part_nr];
+		if (!part)
+			return -EINVAL;
+	}
+	dst = &part->elements[index_inside_part(fa, element_nr)];
+	memset(dst, 0, fa->element_size);
+	return 0;
+}
+
 /**
  * flex_array_prealloc - guarantee that array space exists
  * @start:	index of first array element for which space is allocated