Storing errors in the XArray

[Matthew Wilcox <willy@infradead.org>]

On Mon, Dec 11, 2017 at 03:10:22PM -0800, Randy Dunlap wrote:
> > +The XArray does not support storing :c:func:`IS_ERR` pointers; some
> > +conflict with data values and others conflict with entries the XArray
> > +uses for its own purposes.  If you need to store special values which
> > +cannot be confused with real kernel pointers, the values 4, 8, ... 4092
> > +are available.
> 
> or if I know that they values are errno-range values, I can just shift them
> left by 2 to store them and then shift them right by 2 to use them?

On further thought, I like this idea so much, it's worth writing helpers
for this usage.  And test-suite (also doubles as a demonstration of how
to use it).

diff --git a/include/linux/xarray.h b/include/linux/xarray.h
index c616e9319c7c..53aa251df57a 100644
--- a/include/linux/xarray.h
+++ b/include/linux/xarray.h
@@ -232,6 +232,39 @@ static inline bool xa_is_value(const void *entry)
 	return (unsigned long)entry & 1;
 }
 
+/**
+ * xa_mk_errno() - Create an XArray entry from an error number.
+ * @error: Error number to store in XArray.
+ *
+ * Return: An entry suitable for storing in the XArray.
+ */
+static inline void *xa_mk_errno(long error)
+{
+	return (void *)(error << 2);
+}
+
+/**
+ * xa_to_errno() - Get error number stored in an XArray entry.
+ * @entry: XArray entry.
+ *
+ * Return: The error number stored in the XArray entry.
+ */
+static inline unsigned long xa_to_errno(const void *entry)
+{
+	return (long)entry >> 2;
+}
+
+/**
+ * xa_is_errno() - Determine if an entry is an errno.
+ * @entry: XArray entry.
+ *
+ * Return: True if the entry is an errno, false if it is a pointer.
+ */
+static inline bool xa_is_errno(const void *entry)
+{
+	return (((unsigned long)entry & 3) == 0) && (entry > (void *)-4096);
+}
+
 /**
  * xa_is_internal() - Is the entry an internal entry?
  * @entry: Entry retrieved from the XArray
diff --git a/tools/testing/radix-tree/xarray-test.c b/tools/testing/radix-tree/xarray-test.c
index 43111786ebdd..b843cedf3988 100644
--- a/tools/testing/radix-tree/xarray-test.c
+++ b/tools/testing/radix-tree/xarray-test.c
@@ -29,7 +29,13 @@ void check_xa_err(struct xarray *xa)
 	assert(xa_err(xa_store(xa, 1, xa_mk_value(0), GFP_KERNEL)) == 0);
 	assert(xa_err(xa_store(xa, 1, NULL, 0)) == 0);
 // kills the test-suite :-(
-//     assert(xa_err(xa_store(xa, 0, xa_mk_internal(0), 0)) == -EINVAL);
+//	assert(xa_err(xa_store(xa, 0, xa_mk_internal(0), 0)) == -EINVAL);
+
+	assert(xa_err(xa_store(xa, 0, xa_mk_errno(-ENOMEM), GFP_KERNEL)) == 0);
+	assert(xa_err(xa_load(xa, 0)) == 0);
+	assert(xa_is_errno(xa_load(xa, 0)) == true);
+	assert(xa_to_errno(xa_load(xa, 0)) == -ENOMEM);
+	xa_erase(xa, 0);
 }
 
 void check_xa_tag(struct xarray *xa)