idr: add idr_layer->prefix

Add a field which carries the prefix of ID the idr_layer covers.  This
will be used to implement lookup hint.

This patch doesn't make use of the new field and doesn't introduce any
behavior difference.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 13 insertions, 0 deletions

diff --git a/lib/idr.c b/lib/idr.c
index d66e75bfc1a0..5cd602936645 100644
--- a/lib/idr.c
+++ b/lib/idr.c
@@ -60,6 +60,16 @@ static int idr_max(int layers)
         return (1 << bits) - 1;
 }
 
+/*
+ * Prefix mask for an idr_layer at @layer.  For layer 0, the prefix mask is
+ * all bits except for the lower IDR_BITS.  For layer 1, 2 * IDR_BITS, and
+ * so on.
+ */
+static int idr_layer_prefix_mask(int layer)
+{
+        return ~idr_max(layer + 1);
+}
+
 static struct idr_layer *get_from_free_list(struct idr *idp)
 {
         struct idr_layer *p;
@@ -272,6 +282,7 @@ static int sub_alloc(struct idr *idp, int *starting_id, struct idr_layer **pa,
                         if (!new)
                                 return -ENOMEM;
                         new->layer = l-1;
+                        new->prefix = id & idr_layer_prefix_mask(new->layer);
                         rcu_assign_pointer(p->ary[m], new);
                         p->count++;
                 }
@@ -313,6 +324,7 @@ build_up:
                          * upwards.
                          */
                         p->layer++;
+                        WARN_ON_ONCE(p->prefix);
                         continue;
                 }
                 if (!(new = idr_layer_alloc(gfp_mask, layer_idr))) {
@@ -334,6 +346,7 @@ build_up:
                 new->ary[0] = p;
                 new->count = 1;
                 new->layer = layers-1;
+                new->prefix = id & idr_layer_prefix_mask(new->layer);
                 if (bitmap_full(p->bitmap, IDR_SIZE))
                         __set_bit(0, new->bitmap);
                 p = new;