XArray: Add xa_for_each_range

This function supports iterating over a range of an array.  Also add
documentation links for xa_for_each_start().

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

Documentation/core-api/xarray.rst

@@ -82,10 +82,10 @@ at that index is ``NULL``, you can use xa_insert() which
 returns ``-EBUSY`` if the entry is not empty.
 
 You can copy entries out of the XArray into a plain array by calling
-xa_extract().  Or you can iterate over the present entries in
+xa_extract().  Or you can iterate over the present entries in the XArray
-the XArray by calling xa_for_each().  You may prefer to use
+by calling xa_for_each(), xa_for_each_start() or xa_for_each_range().
-xa_find() or xa_find_after() to move to the next present
+You may prefer to use xa_find() or xa_find_after() to move to the next
-entry in the XArray.
+present entry in the XArray.
 
 Calling xa_store_range() stores the same entry in a range
 of indices.  If you do this, some of the other operations will behave
@@ -193,6 +193,8 @@ No lock needed:
 Takes RCU read lock:
  * xa_load()
  * xa_for_each()
+ * xa_for_each_start()
+ * xa_for_each_range()
  * xa_find()
  * xa_find_after()
  * xa_extract()

include/linux/xarray.h

@@ -416,6 +416,36 @@ static inline bool xa_marked(const struct xarray *xa, xa_mark_t mark)
 	return xa->xa_flags & XA_FLAGS_MARK(mark);
 }
 
+/**
+ * xa_for_each_range() - Iterate over a portion of an XArray.
+ * @xa: XArray.
+ * @index: Index of @entry.
+ * @entry: Entry retrieved from array.
+ * @start: First index to retrieve from array.
+ * @last: Last index to retrieve from array.
+ *
+ * During the iteration, @entry will have the value of the entry stored
+ * in @xa at @index.  You may modify @index during the iteration if you
+ * want to skip or reprocess indices.  It is safe to modify the array
+ * during the iteration.  At the end of the iteration, @entry will be set
+ * to NULL and @index will have a value less than or equal to max.
+ *
+ * xa_for_each_range() is O(n.log(n)) while xas_for_each() is O(n).  You have
+ * to handle your own locking with xas_for_each(), and if you have to unlock
+ * after each iteration, it will also end up being O(n.log(n)).
+ * xa_for_each_range() will spin if it hits a retry entry; if you intend to
+ * see retry entries, you should use the xas_for_each() iterator instead.
+ * The xas_for_each() iterator will expand into more inline code than
+ * xa_for_each_range().
+ *
+ * Context: Any context.  Takes and releases the RCU lock.
+ */
+#define xa_for_each_range(xa, index, entry, start, last)		\
+	for (index = start,						\
+	     entry = xa_find(xa, &index, last, XA_PRESENT);		\
+	     entry;							\
+	     entry = xa_find_after(xa, &index, last, XA_PRESENT))
+
 /**
  * xa_for_each_start() - Iterate over a portion of an XArray.
  * @xa: XArray.
@@ -439,11 +469,8 @@ static inline bool xa_marked(const struct xarray *xa, xa_mark_t mark)
  *
  * Context: Any context.  Takes and releases the RCU lock.
  */
-#define xa_for_each_start(xa, index, entry, start)			\
+#define xa_for_each_start(xa, index, entry, start) \
-	for (index = start,						\
+	xa_for_each_range(xa, index, entry, start, ULONG_MAX)
-	     entry = xa_find(xa, &index, ULONG_MAX, XA_PRESENT);	\
-	     entry;							\
-	     entry = xa_find_after(xa, &index, ULONG_MAX, XA_PRESENT))
 
 /**
  * xa_for_each() - Iterate over present entries in an XArray.