[PATCH] pid: Do not set pid_max in new pid namespaces

[Michal Koutný]

It is already difficult for users to troubleshoot which of multiple pid
limits restricts their workload. The per-(hierarchical-)NS pid_max would
contribute to the confusion.
Also, the implementation copies the limit upon creation from
parent, this pattern showed cumbersome with some attributes in legacy
cgroup controllers -- it's subject to race condition between parent's
limit modification and children creation and once copied it must be
changed in the descendant.

Let's do what other places do (ucounts or cgroup limits) -- create new
pid namespaces without any limit at all. The global limit (actually any
ancestor's limit) is still effectively in place, we avoid the
set/unshare race and bumps of global (ancestral) limit have the desired
effect on pid namespace that do not care.

Link: https://lore.kernel.org/r/20240408145819.8787-1-mkoutny@xxxxxxxx/
Link: https://lore.kernel.org/r/20250221170249.890014-1-mkoutny@xxxxxxxx/
Fixes: 7863dcc72d0f4 ("pid: allow pid_max to be set per pid namespace")
Signed-off-by: Michal Koutný <mkoutny@xxxxxxxx>
---
 kernel/pid_namespace.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
index 8f6cfec87555a..7098ed44e717d 100644
--- a/kernel/pid_namespace.c
+++ b/kernel/pid_namespace.c
@@ -107,7 +107,7 @@ static struct pid_namespace *create_pid_namespace(struct user_namespace *user_ns
 		goto out_free_idr;
 	ns->ns.ops = &pidns_operations;
 
-	ns->pid_max = parent_pid_ns->pid_max;
+	ns->pid_max = PID_MAX_LIMIT;
 	err = register_pidns_sysctls(ns);
 	if (err)
 		goto out_free_inum;

base-commit: 48a5eed9ad584315c30ed35204510536235ce402
-- 
2.48.1