xfs: set a default CoW extent size of 32 blocks

If the admin doesn't set a CoW extent size or a regular extent size
hint, default to creating CoW reservations 32 blocks long to reduce
fragmentation.
Signed-off-by: DarricK J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>

fs/xfs/xfs_inode.c

@@ -80,7 +80,8 @@ xfs_get_extsz_hint(
 /*
  * Helper function to extract CoW extent size hint from inode.
  * Between the extent size hint and the CoW extent size hint, we
- * return the greater of the two.
+ * return the greater of the two.  If the value is zero (automatic),
+ * use the default size.
  */
 xfs_extlen_t
 xfs_get_cowextsz_hint(
@@ -93,9 +94,10 @@ xfs_get_cowextsz_hint(
 		a = ip->i_d.di_cowextsize;
 	b = xfs_get_extsz_hint(ip);
 
-	if (a > b)
-		return a;
-	return b;
+	a = max(a, b);
+	if (a == 0)
+		return XFS_DEFAULT_COWEXTSZ_HINT;
+	return a;
 }
 
 /*

fs/xfs/xfs_inode.h

@@ -490,4 +490,7 @@ do { \
 
 extern struct kmem_zone	*xfs_inode_zone;
 
+/* The default CoW extent size hint. */
+#define XFS_DEFAULT_COWEXTSZ_HINT 32
+
 #endif	/* __XFS_INODE_H__ */