xarray: Extract entries from an XArray

The xa_extract function combines the functionality of
radix_tree_gang_lookup() and radix_tree_gang_lookup_tagged().
It extracts entries matching the specified filter into a normal array.
Signed-off-by: Matthew Wilcox <willy@infradead.org>

include/linux/xarray.h

@@ -284,6 +284,8 @@ void *xa_find(struct xarray *xa, unsigned long *index,
 		unsigned long max, xa_mark_t) __attribute__((nonnull(2)));
 void *xa_find_after(struct xarray *xa, unsigned long *index,
 		unsigned long max, xa_mark_t) __attribute__((nonnull(2)));
+unsigned int xa_extract(struct xarray *, void **dst, unsigned long start,
+		unsigned long max, unsigned int n, xa_mark_t);
 
 /**
  * xa_init() - Initialise an empty XArray.

lib/xarray.c

@@ -1444,6 +1444,86 @@ void *xa_find_after(struct xarray *xa, unsigned long *indexp,
 }
 EXPORT_SYMBOL(xa_find_after);
 
+static unsigned int xas_extract_present(struct xa_state *xas, void **dst,
+			unsigned long max, unsigned int n)
+{
+	void *entry;
+	unsigned int i = 0;
+
+	rcu_read_lock();
+	xas_for_each(xas, entry, max) {
+		if (xas_retry(xas, entry))
+			continue;
+		dst[i++] = entry;
+		if (i == n)
+			break;
+	}
+	rcu_read_unlock();
+
+	return i;
+}
+
+static unsigned int xas_extract_marked(struct xa_state *xas, void **dst,
+			unsigned long max, unsigned int n, xa_mark_t mark)
+{
+	void *entry;
+	unsigned int i = 0;
+
+	rcu_read_lock();
+	xas_for_each_marked(xas, entry, max, mark) {
+		if (xas_retry(xas, entry))
+			continue;
+		dst[i++] = entry;
+		if (i == n)
+			break;
+	}
+	rcu_read_unlock();
+
+	return i;
+}
+
+/**
+ * xa_extract() - Copy selected entries from the XArray into a normal array.
+ * @xa: The source XArray to copy from.
+ * @dst: The buffer to copy entries into.
+ * @start: The first index in the XArray eligible to be selected.
+ * @max: The last index in the XArray eligible to be selected.
+ * @n: The maximum number of entries to copy.
+ * @filter: Selection criterion.
+ *
+ * Copies up to @n entries that match @filter from the XArray.  The
+ * copied entries will have indices between @start and @max, inclusive.
+ *
+ * The @filter may be an XArray mark value, in which case entries which are
+ * marked with that mark will be copied.  It may also be %XA_PRESENT, in
+ * which case all entries which are not NULL will be copied.
+ *
+ * The entries returned may not represent a snapshot of the XArray at a
+ * moment in time.  For example, if another thread stores to index 5, then
+ * index 10, calling xa_extract() may return the old contents of index 5
+ * and the new contents of index 10.  Indices not modified while this
+ * function is running will not be skipped.
+ *
+ * If you need stronger guarantees, holding the xa_lock across calls to this
+ * function will prevent concurrent modification.
+ *
+ * Context: Any context.  Takes and releases the RCU lock.
+ * Return: The number of entries copied.
+ */
+unsigned int xa_extract(struct xarray *xa, void **dst, unsigned long start,
+			unsigned long max, unsigned int n, xa_mark_t filter)
+{
+	XA_STATE(xas, xa, start);
+
+	if (!n)
+		return 0;
+
+	if ((__force unsigned int)filter < XA_MAX_MARKS)
+		return xas_extract_marked(&xas, dst, max, n, filter);
+	return xas_extract_present(&xas, dst, max, n);
+}
+EXPORT_SYMBOL(xa_extract);
+
 #ifdef XA_DEBUG
 void xa_dump_node(const struct xa_node *node)
 {