+ pid-optional-first-fit-pid-allocation.patch added to mm-nonmm-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

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The patch titled
     Subject: pid: Optional first-fit pid allocation
has been added to the -mm mm-nonmm-unstable branch.  Its filename is
     pid-optional-first-fit-pid-allocation.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/pid-optional-first-fit-pid-allocation.patch

This patch will later appear in the mm-nonmm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: =?UTF-8?q?Michal=20Koutn=C3=BD?= <mkoutny@suse.com>
Subject: pid: Optional first-fit pid allocation
Date: Fri, 21 Feb 2025 18:02:49 +0100

Noone would need to use this allocation strategy (it's slower, pid
numbers collide sooner). Its primary purpose are pid namespaces in
conjunction with pids.max cgroup limit which keeps (virtual) pid numbers
below the given limit. This is for 32-bit userspace programs that may
not work well with pid numbers above 65536.

Link: https://lore.kernel.org/r/20241122132459.135120-1-aleksandr.mikhalitsyn@canonical.com/
Link: https://lkml.kernel.org/r/20250221170249.890014-3-mkoutny@suse.com
Signed-off-by: Michal Koutný <mkoutny@suse.com>
Cc: Alexander Mikhalitsyn <alexander@mihalicyn.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Kees Cook <kees@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 Documentation/admin-guide/sysctl/kernel.rst |    2 +
 include/linux/pid_namespace.h               |    3 +
 kernel/pid.c                                |   12 ++++++-
 kernel/pid_namespace.c                      |   28 +++++++++++++-----
 4 files changed, 36 insertions(+), 9 deletions(-)

--- a/Documentation/admin-guide/sysctl/kernel.rst~pid-optional-first-fit-pid-allocation
+++ a/Documentation/admin-guide/sysctl/kernel.rst
@@ -1043,6 +1043,8 @@ The last pid allocated in the current (t
 lives in) pid namespace. When selecting a pid for a next task on fork
 kernel tries to allocate a number starting from this one.
 
+When set to -1, first-fit pid numbering is used instead of the next-fit.
+
 
 powersave-nap (PPC only)
 ========================
--- a/include/linux/pid_namespace.h~pid-optional-first-fit-pid-allocation
+++ a/include/linux/pid_namespace.h
@@ -41,6 +41,9 @@ struct pid_namespace {
 #if defined(CONFIG_SYSCTL) && defined(CONFIG_MEMFD_CREATE)
 	int memfd_noexec_scope;
 #endif
+#ifdef CONFIG_IA32_EMULATION
+	bool pid_noncyclic;
+#endif
 } __randomize_layout;
 
 extern struct pid_namespace init_pid_ns;
--- a/kernel/pid.c~pid-optional-first-fit-pid-allocation
+++ a/kernel/pid.c
@@ -191,6 +191,10 @@ struct pid *alloc_pid(struct pid_namespa
 
 	for (i = ns->level; i >= 0; i--) {
 		int tid = 0;
+		bool pid_noncyclic = 0;
+#ifdef CONFIG_IA32_EMULATION
+		pid_noncyclic = READ_ONCE(tmp->pid_noncyclic);
+#endif
 
 		if (set_tid_size) {
 			tid = set_tid[ns->level - i];
@@ -235,8 +239,12 @@ struct pid *alloc_pid(struct pid_namespa
 			 * Store a null pointer so find_pid_ns does not find
 			 * a partially initialized PID (see below).
 			 */
-			nr = idr_alloc_cyclic(&tmp->idr, NULL, pid_min,
-					      pid_max, GFP_ATOMIC);
+			if (likely(!pid_noncyclic))
+				nr = idr_alloc_cyclic(&tmp->idr, NULL, pid_min,
+						      pid_max, GFP_ATOMIC);
+			else
+				nr = idr_alloc(&tmp->idr, NULL, pid_min,
+						      pid_max, GFP_ATOMIC);
 		}
 		spin_unlock_irq(&pidmap_lock);
 		idr_preload_end();
--- a/kernel/pid_namespace.c~pid-optional-first-fit-pid-allocation
+++ a/kernel/pid_namespace.c
@@ -114,6 +114,9 @@ static struct pid_namespace *create_pid_
 #if defined(CONFIG_SYSCTL) && defined(CONFIG_MEMFD_CREATE)
 	ns->memfd_noexec_scope = pidns_memfd_noexec_scope(parent_pid_ns);
 #endif
+#ifdef CONFIG_IA32_EMULATION
+	ns->pid_noncyclic = READ_ONCE(parent_pid_ns->pid_noncyclic);
+#endif
 	return ns;
 
 out_free_idr:
@@ -260,7 +263,7 @@ void zap_pid_ns_processes(struct pid_nam
 	return;
 }
 
-#ifdef CONFIG_CHECKPOINT_RESTORE
+#if defined(CONFIG_CHECKPOINT_RESTORE) || defined(CONFIG_IA32_EMULATION)
 static int pid_ns_ctl_handler(const struct ctl_table *table, int write,
 		void *buffer, size_t *lenp, loff_t *ppos)
 {
@@ -271,12 +274,23 @@ static int pid_ns_ctl_handler(const stru
 	if (write && !checkpoint_restore_ns_capable(pid_ns->user_ns))
 		return -EPERM;
 
-	next = idr_get_cursor(&pid_ns->idr) - 1;
+	next = -1;
+#ifdef CONFIG_IA32_EMULATION
+	if (!pid_ns->pid_noncyclic)
+#endif
+		next += idr_get_cursor(&pid_ns->idr);
 
 	tmp.data = &next;
 	ret = proc_dointvec_minmax(&tmp, write, buffer, lenp, ppos);
-	if (!ret && write)
-		idr_set_cursor(&pid_ns->idr, next + 1);
+	if (!ret && write) {
+		if (next > -1)
+			idr_set_cursor(&pid_ns->idr, next + 1);
+		else if (!IS_ENABLED(CONFIG_IA32_EMULATION))
+			ret = -EINVAL;
+#ifdef CONFIG_IA32_EMULATION
+		WRITE_ONCE(pid_ns->pid_noncyclic, next == -1);
+#endif
+	}
 
 	return ret;
 }
@@ -288,11 +302,11 @@ static const struct ctl_table pid_ns_ctl
 		.maxlen = sizeof(int),
 		.mode = 0666, /* permissions are checked in the handler */
 		.proc_handler = pid_ns_ctl_handler,
-		.extra1 = SYSCTL_ZERO,
+		.extra1 = SYSCTL_NEG_ONE,
 		.extra2 = &pid_max,
 	},
 };
-#endif	/* CONFIG_CHECKPOINT_RESTORE */
+#endif	/* CONFIG_CHECKPOINT_RESTORE || CONFIG_IA32_EMULATION */
 
 int reboot_pid_ns(struct pid_namespace *pid_ns, int cmd)
 {
@@ -449,7 +463,7 @@ static __init int pid_namespaces_init(vo
 {
 	pid_ns_cachep = KMEM_CACHE(pid_namespace, SLAB_PANIC | SLAB_ACCOUNT);
 
-#ifdef CONFIG_CHECKPOINT_RESTORE
+#if defined(CONFIG_CHECKPOINT_RESTORE) || defined(CONFIG_IA32_EMULATION)
 	register_sysctl_init("kernel", pid_ns_ctl_table);
 #endif
 
_

Patches currently in -mm which might be from mkoutny@suse.com are

revert-pid-allow-pid_max-to-be-set-per-pid-namespace.patch
pid-optional-first-fit-pid-allocation.patch