1 files changed, 35 insertions, 3 deletions

diff --git a/block/genhd.c b/block/genhd.c
index ee4b13520e59..67e5a59ced2a 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -299,6 +299,38 @@ EXPORT_SYMBOL(unregister_blkdev);
 static struct kobj_map *bdev_map;
 
 /**
+ * blk_mangle_minor - scatter minor numbers apart
+ * @minor: minor number to mangle
+ *
+ * Scatter consecutively allocated @minor number apart if MANGLE_DEVT
+ * is enabled.  Mangling twice gives the original value.
+ *
+ * RETURNS:
+ * Mangled value.
+ *
+ * CONTEXT:
+ * Don't care.
+ */
+static int blk_mangle_minor(int minor)
+{
+#ifdef CONFIG_DEBUG_BLOCK_EXT_DEVT
+        int i;
+
+        for (i = 0; i < MINORBITS / 2; i++) {
+                int low = minor & (1 << i);
+                int high = minor & (1 << (MINORBITS - 1 - i));
+                int distance = MINORBITS - 1 - 2 * i;
+
+                minor ^= low | high;    /* clear both bits */
+                low <<= distance;       /* swap the positions */
+                high >>= distance;
+                minor |= low | high;    /* and set */
+        }
+#endif
+        return minor;
+}
+
+/**
  * blk_alloc_devt - allocate a dev_t for a partition
  * @part: partition to allocate dev_t for
  * @gfp_mask: memory allocation flag
@@ -339,7 +371,7 @@ int blk_alloc_devt(struct hd_struct *part, dev_t *devt)
                 return -EBUSY;
         }
 
-        *devt = MKDEV(BLOCK_EXT_MAJOR, idx);
+        *devt = MKDEV(BLOCK_EXT_MAJOR, blk_mangle_minor(idx));
         return 0;
 }
 
@@ -361,7 +393,7 @@ void blk_free_devt(dev_t devt)
 
         if (MAJOR(devt) == BLOCK_EXT_MAJOR) {
                 mutex_lock(&ext_devt_mutex);
-                idr_remove(&ext_devt_idr, MINOR(devt));
+                idr_remove(&ext_devt_idr, blk_mangle_minor(MINOR(devt)));
                 mutex_unlock(&ext_devt_mutex);
         }
 }
@@ -473,7 +505,7 @@ struct gendisk *get_gendisk(dev_t devt, int *partno)
                 struct hd_struct *part;
 
                 mutex_lock(&ext_devt_mutex);
-                part = idr_find(&ext_devt_idr, MINOR(devt));
+                part = idr_find(&ext_devt_idr, blk_mangle_minor(MINOR(devt)));
                 if (part && get_disk(part_to_disk(part))) {
                         *partno = part->partno;
                         disk = part_to_disk(part);