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Subject[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 <adobriyan@gmail.com>
---

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

--- a/include/linux/pid_namespace.h
+++ b/include/linux/pid_namespace.h
@@ -11,7 +11,6 @@
#include <linux/ns_common.h>

struct pidmap {
- atomic_t nr_free;
void *page;
};

--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -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);
--- a/kernel/pid_namespace.c
+++ b/kernel/pid_namespace.c
@@ -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:
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