crush: return CRUSH_ITEM_UNDEF for failed placements with indep

For firstn mode, if we fail to make a valid placement choice, we just
continue and return a short result to the caller.  For indep mode, however,
we need to make the position stable, and return an undefined value on
failed placements to avoid shifting later results to the left.

Reflects ceph.git commit b1d4dd4eb044875874a1d01c01c7d766db5d0a80.
Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>

include/linux/crush/crush.h

@@ -19,10 +19,11 @@
 
 #define CRUSH_MAGIC 0x00010000ul   /* for detecting algorithm revisions */
 
-
 #define CRUSH_MAX_DEPTH 10  /* max crush hierarchy depth */
 
 
+#define CRUSH_ITEM_UNDEF  0x7fffffff  /* undefined result */
+
 /*
  * CRUSH uses user-defined "rules" to describe how inputs should be
  * mapped to devices.  A rule consists of sequence of steps to perform

net/ceph/crush/mapper.c

@@ -455,8 +455,12 @@ static int crush_choose(const struct crush_map *map,
 		} while (retry_descent);
 
 		if (skip_rep) {
-			dprintk("skip rep\n");
-			continue;
+			if (firstn) {
+				dprintk("skip rep\n");
+				continue;
+			}
+			dprintk("undef rep, continuing\n");
+			item = CRUSH_ITEM_UNDEF;
 		}
 
 		dprintk("CHOOSE got %d\n", item);