ipc: do cyclic id allocation for the ipc object.

For ipcmni_extend mode, the sequence number space is only 7 bits.  So
the chance of id reuse is relatively high compared with the non-extended
mode.

To alleviate this id reuse problem, this patch enables cyclic allocation
for the index to the radix tree (idx).  The disadvantage is that this
can cause a slight slow-down of the fast path, as the radix tree could
be higher than necessary.

To limit the radix tree height, I have chosen the following limits:
 1) The cycling is done over in_use*1.5.
 2) At least, the cycling is done over
   "normal" ipcnmi mode: RADIX_TREE_MAP_SIZE elements
   "ipcmni_extended": 4096 elements

Result:
- for normal mode:
	No change for <= 42 active ipc elements. With more than 42
	active ipc elements, a 2nd level would be added to the radix
	tree.
	Without cyclic allocation, a 2nd level would be added only with
	more than 63 active elements.

- for extended mode:
	Cycling creates always at least a 2-level radix tree.
	With more t...

ipc/ipc_sysctl.c

@@ -122,6 +122,7 @@ static int one = 1;
 static int int_max = INT_MAX;
 int ipc_mni = IPCMNI;
 int ipc_mni_shift = IPCMNI_SHIFT;
+int ipc_min_cycle = RADIX_TREE_MAP_SIZE;
 
 static struct ctl_table ipc_kern_table[] = {
 	{
@@ -252,6 +253,7 @@ static int __init ipc_mni_extend(char *str)
 {
 	ipc_mni = IPCMNI_EXTEND;
 	ipc_mni_shift = IPCMNI_EXTEND_SHIFT;
+	ipc_min_cycle = IPCMNI_EXTEND_MIN_CYCLE;
 	pr_info("IPCMNI extended to %d.\n", ipc_mni);
 	return 0;
 }

ipc/util.c

@@ -220,9 +220,14 @@ static inline int ipc_idr_alloc(struct ipc_ids *ids, struct kern_ipc_perm *new)
 	 */
 
 	if (next_id < 0) { /* !CHECKPOINT_RESTORE or next_id is unset */
+		int max_idx;
+
+		max_idx = max(ids->in_use*3/2, ipc_min_cycle);
+		max_idx = min(max_idx, ipc_mni);
 
 		/* allocate the idx, with a NULL struct kern_ipc_perm */
-		idx = idr_alloc(&ids->ipcs_idr, NULL, 0, 0, GFP_NOWAIT);
+		idx = idr_alloc_cyclic(&ids->ipcs_idr, NULL, 0, max_idx,
+					GFP_NOWAIT);
 
 		if (idx >= 0) {
 			/*

ipc/util.h

@@ -27,12 +27,14 @@
  */
 #define IPCMNI_SHIFT		15
 #define IPCMNI_EXTEND_SHIFT	24
+#define IPCMNI_EXTEND_MIN_CYCLE	(RADIX_TREE_MAP_SIZE * RADIX_TREE_MAP_SIZE)
 #define IPCMNI			(1 << IPCMNI_SHIFT)
 #define IPCMNI_EXTEND		(1 << IPCMNI_EXTEND_SHIFT)
 
 #ifdef CONFIG_SYSVIPC_SYSCTL
 extern int ipc_mni;
 extern int ipc_mni_shift;
+extern int ipc_min_cycle;
 
 #define ipcmni_seq_shift()	ipc_mni_shift
 #define IPCMNI_IDX_MASK		((1 << ipc_mni_shift) - 1)
@@ -40,6 +42,7 @@ extern int ipc_mni_shift;
 #else /* CONFIG_SYSVIPC_SYSCTL */
 
 #define ipc_mni			IPCMNI
+#define ipc_min_cycle		((int)RADIX_TREE_MAP_SIZE)
 #define ipcmni_seq_shift()	IPCMNI_SHIFT
 #define IPCMNI_IDX_MASK		((1 << IPCMNI_SHIFT) - 1)
 #endif /* CONFIG_SYSVIPC_SYSCTL */