Re: [RFC][v8][PATCH 0/10] Implement clone3() system call

[ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org (Eric W. Biederman)]

Sukadev Bhattiprolu <sukadev-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org> writes:

> Eric W. Biederman [ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org] wrote:
> | > I will post a version of the patch outside this patchset with min
> | > and max parameters and we can see if it can be optimized/beautified.
> | 
> | Thanks,
> | Eric
>
> Here is the patch.  alloc_pid() calls alloc_pidmap() as follows:
>
> -		nr = alloc_pidmap(tmp);
> +		min = max = 0;
> +		if (target_pids) {
> +			min = target_pids[i];
> +			max = min + 1;
> +		}
> +		nr = alloc_pidmap(tmp, min, max);
>
> It does look like this patch executes a bit more code even in the common
> case but let me know if you think this is better.
> 	
> ---

Not what I was thinking.  The following untested patch is what I was
thinking.  It just exposes last, min, and max to the callers which pass
in different appropriate values.

Eric

diff --git a/kernel/pid.c b/kernel/pid.c
index d3f722d..d9b6f82 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -122,17 +122,17 @@ static void free_pidmap(struct upid *upid)
 	atomic_inc(&map->nr_free);
 }
 
-static int alloc_pidmap(struct pid_namespace *pid_ns)
+static int do_alloc_pidmap(struct pid_namespace *pid_ns, int last, int min, int max)
 {
-	int i, offset, max_scan, pid, last = pid_ns->last_pid;
+	int i, offset, max_scan, pid;
 	struct pidmap *map;
 
 	pid = last + 1;
-	if (pid >= pid_max)
-		pid = RESERVED_PIDS;
+	if (pid >= max)
+		pid = min;
 	offset = pid & BITS_PER_PAGE_MASK;
 	map = &pid_ns->pidmap[pid/BITS_PER_PAGE];
-	max_scan = (pid_max + BITS_PER_PAGE - 1)/BITS_PER_PAGE - !offset;
+	max_scan = (max + BITS_PER_PAGE - 1)/BITS_PER_PAGE - !offset;
 	for (i = 0; i <= max_scan; ++i) {
 		if (unlikely(!map->page)) {
 			void *page = kzalloc(PAGE_SIZE, GFP_KERNEL);
@@ -153,7 +153,6 @@ static int alloc_pidmap(struct pid_namespace *pid_ns)
 			do {
 				if (!test_and_set_bit(offset, map->page)) {
 					atomic_dec(&map->nr_free);
-					pid_ns->last_pid = pid;
 					return pid;
 				}
 				offset = find_next_offset(map, offset);
@@ -164,16 +163,16 @@ static int alloc_pidmap(struct pid_namespace *pid_ns)
 			 * bitmap block and the final block was the same
 			 * as the starting point, pid is before last_pid.
 			 */
-			} while (offset < BITS_PER_PAGE && pid < pid_max &&
+			} while (offset < BITS_PER_PAGE && pid < max &&
 					(i != max_scan || pid < last ||
 					    !((last+1) & BITS_PER_PAGE_MASK)));
 		}
-		if (map < &pid_ns->pidmap[(pid_max-1)/BITS_PER_PAGE]) {
+		if (map < &pid_ns->pidmap[(max-1)/BITS_PER_PAGE]) {
 			++map;
 			offset = 0;
 		} else {
 			map = &pid_ns->pidmap[0];
-			offset = RESERVED_PIDS;
+			offset = min;
 			if (unlikely(last == offset))
 				break;
 		}
@@ -182,6 +181,25 @@ static int alloc_pidmap(struct pid_namespace *pid_ns)
 	return -1;
 }
 
+static int alloc_pidmap(struct pid_namespace *pid_ns)
+{
+	int nr;
+
+	nr = do_alloc_pidmap(pid_ns, pid_ns->last, RESERVED_PIDS, pid_max);
+	if (nr >= 0)
+		pid_ns->last_pid = nr;
+	return nr;
+}
+
+static int set_pidmap(struct pid_namespace *pid_ns, int target)
+{
+	if (target >= pid_max)
+		return -1;
+	if (target < RESERVED_PIDS)
+		return -1;
+	return do_alloc_pidmap(pid_ns, target - 1, target, target + 1);
+}
+
 int next_pidmap(struct pid_namespace *pid_ns, int last)
 {
 	int offset;