pid: delete RESERVED_PIDS

PATCH New Alexey Dobriyan 1 participant 2 messages Sep 9, 2017 Namespaces:

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Alexey Dobriyan

author

2017-09-09 20:26 UTC #0 lore raw

PATCH 1/2 pid: delete RESERVED_PIDS

RESERVED_PIDS had a noble goal: to protect root from PID exhaustion since at least ~2.5.40 except it never did that because there was no capability or uid checks.

Allow small pids to be allocated after rollover, there is nothing sacred about them.

Resource exhaustion should be handled by rlimits and/or kernel memory accounting.

Signed-off-by Alexey Dobriyan <[email protected]>

---

 kernel/pid.c |   10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

kernel/pid.c +4 -6

@@ -48,9 +48,7 @@ struct pid init_struct_pid = INIT_STRUCT_PID;
 
 int pid_max = PID_MAX_DEFAULT;
 
-#define RESERVED_PIDS		300
-
-int pid_max_min = RESERVED_PIDS + 1;
+int pid_max_min = 301;
 int pid_max_max = PID_MAX_LIMIT;
 
 static inline int mk_pid(struct pid_namespace *pid_ns,

@@ -157,13 +155,13 @@ static int alloc_pidmap(struct pid_namespace *pid_ns)
 
 	pid = last + 1;
 	if (pid >= pid_max)
-		pid = RESERVED_PIDS;
+		pid = 1;
 	offset = pid & BITS_PER_PAGE_MASK;
 	map = &pid_ns->pidmap[pid/BITS_PER_PAGE];
 	/*
 	 * If last_pid points into the middle of the map->page we
 	 * want to scan this bitmap block twice, the second time
-	 * we start with offset == 0 (or RESERVED_PIDS).
+	 * we start with offset == 0.
 	 */
 	max_scan = DIV_ROUND_UP(pid_max, BITS_PER_PAGE) - !offset;
 	for (i = 0; i <= max_scan; ++i) {

@@ -203,7 +201,7 @@ static int alloc_pidmap(struct pid_namespace *pid_ns)
 			offset = 0;
 		} else {
 			map = &pid_ns->pidmap[0];
-			offset = RESERVED_PIDS;
+			offset = 1;
 			if (unlikely(last == offset))
 				break;
 		}

Alexey Dobriyan

author

2017-09-09 20:36 UTC #1 lore raw

PATCH 2/2 pid: delete struct pidmap::nr_free

There is a check in pid allocation code to skip a full page:

	if (likely(atomic_read(&map->nr_free))) {
		...

In practice it doesn't do anything. To skip a pidmap page one has to have 32K consecutive pids allocated at the same time which doesn't happen.

Currently the price is _every_ fork/exit on every system being slower than necessary.

Signed-off-by Alexey Dobriyan <[email protected]>

---

 include/linux/pid_namespace.h |    1 -
 kernel/pid.c                  |   28 ++++++++++------------------
 kernel/pid_namespace.c        |    6 ------
 3 files changed, 10 insertions(+), 25 deletions(-)

include/linux/pid_namespace.h +0 -1

@@ -11,7 +11,6 @@
 #include <linux/ns_common.h>
 
 struct pidmap {
-       atomic_t nr_free;
        void *page;
 };

kernel/pid.c +10 ~~-18~~

@@ -68,9 +68,6 @@ static inline int mk_pid(struct pid_namespace *pid_ns,
  */
 struct pid_namespace init_pid_ns = {
 	.kref = KREF_INIT(2),
-	.pidmap = {
-		[ 0 ... PIDMAP_ENTRIES-1] = { ATOMIC_INIT(BITS_PER_PAGE), NULL }
-	},
 	.last_pid = 0,
 	.nr_hashed = PIDNS_HASH_ADDING,
 	.level = 0,

@@ -106,7 +103,6 @@ static void free_pidmap(struct upid *upid)
 	int offset = nr & BITS_PER_PAGE_MASK;
 
 	clear_bit(offset, map->page);
-	atomic_inc(&map->nr_free);
 }
 
 /*

@@ -181,20 +177,17 @@ static int alloc_pidmap(struct pid_namespace *pid_ns)
 			if (unlikely(!map->page))
 				return -ENOMEM;
 		}
-		if (likely(atomic_read(&map->nr_free))) {
-			for ( ; ; ) {
-				if (!test_and_set_bit(offset, map->page)) {
-					atomic_dec(&map->nr_free);
-					set_last_pid(pid_ns, last, pid);
-					return pid;
-				}
-				offset = find_next_offset(map, offset);
-				if (offset >= BITS_PER_PAGE)
-					break;
-				pid = mk_pid(pid_ns, map, offset);
-				if (pid >= pid_max)
-					break;
+		for (;;) {
+			if (!test_and_set_bit(offset, map->page)) {
+				set_last_pid(pid_ns, last, pid);
+				return pid;
 			}
+			offset = find_next_offset(map, offset);
+			if (offset >= BITS_PER_PAGE)
+				break;
+			pid = mk_pid(pid_ns, map, offset);
+			if (pid >= pid_max)
+				break;
 		}
 		if (map < &pid_ns->pidmap[(pid_max-1)/BITS_PER_PAGE]) {
 			++map;

@@ -591,7 +584,6 @@ void __init pidmap_init(void)
 	init_pid_ns.pidmap[0].page = kzalloc(PAGE_SIZE, GFP_KERNEL);
 	/* Reserve PID 0. We never call free_pidmap(0) */
 	set_bit(0, init_pid_ns.pidmap[0].page);
-	atomic_dec(&init_pid_ns.pidmap[0].nr_free);
 
 	init_pid_ns.pid_cachep = KMEM_CACHE(pid,
 			SLAB_HWCACHE_ALIGN | SLAB_PANIC | SLAB_ACCOUNT);

kernel/pid_namespace.c +0 -6

@@ -98,7 +98,6 @@ static struct pid_namespace *create_pid_namespace(struct user_namespace *user_ns
 	struct pid_namespace *ns;
 	unsigned int level = parent_pid_ns->level + 1;
 	struct ucounts *ucounts;
-	int i;
 	int err;
 
 	err = -ENOSPC;

@@ -135,11 +134,6 @@ static struct pid_namespace *create_pid_namespace(struct user_namespace *user_ns
 	INIT_WORK(&ns->proc_work, proc_cleanup_work);
 
 	set_bit(0, ns->pidmap[0].page);
-	atomic_set(&ns->pidmap[0].nr_free, BITS_PER_PAGE - 1);
-
-	for (i = 1; i < PIDMAP_ENTRIES; i++)
-		atomic_set(&ns->pidmap[i].nr_free, BITS_PER_PAGE);
-
 	return ns;
 
 out_free_map: