linux-kernel - [PATCH 2/3] pid: split cyclic id allocation cursor from idr

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Message-Id: <20220715113349.831370-3-bfoster@redhat.com>
Date:   Fri, 15 Jul 2022 07:33:48 -0400
From:   Brian Foster <bfoster@...hat.com>
To:     linux-kernel@...r.kernel.org, linux-mm@...ck.org
Cc:     Matthew Wilcox <willy@...radead.org>, ikent@...hat.com,
        oleg@...hat.com
Subject: [PATCH 2/3] pid: split cyclic id allocation cursor from idr

As a next step in separating pid allocation from the idr, split off
the cyclic pid allocation cursor from the idr. Lift the cursor value
into the sturct pid_namespace. Note that this involves temporarily
open-coding the cursor increment on allocation, but this is cleaned
up in the subsequent patch.

Signed-off-by: Matthew Wilcox <willy@...radead.org>
Signed-off-by: Brian Foster <bfoster@...hat.com>
---
 arch/powerpc/platforms/cell/spufs/sched.c | 2 +-
 fs/proc/loadavg.c                         | 2 +-
 include/linux/pid_namespace.h             | 1 +
 kernel/pid.c                              | 6 ++++--
 kernel/pid_namespace.c                    | 4 ++--
 5 files changed, 9 insertions(+), 6 deletions(-)

diff --git a/arch/powerpc/platforms/cell/spufs/sched.c b/arch/powerpc/platforms/cell/spufs/sched.c
index 99bd027a7f7c..a2ed928d7658 100644
--- a/arch/powerpc/platforms/cell/spufs/sched.c
+++ b/arch/powerpc/platforms/cell/spufs/sched.c
@@ -1072,7 +1072,7 @@ static int show_spu_loadavg(struct seq_file *s, void *private)
 		LOAD_INT(c), LOAD_FRAC(c),
 		count_active_contexts(),
 		atomic_read(&nr_spu_contexts),
-		idr_get_cursor(&task_active_pid_ns(current)->idr) - 1);
+		READ_ONCE(task_active_pid_ns(current)->pid_next) - 1);
 	return 0;
 }
 #endif
diff --git a/fs/proc/loadavg.c b/fs/proc/loadavg.c
index f32878d9a39f..62f89d549582 100644
--- a/fs/proc/loadavg.c
+++ b/fs/proc/loadavg.c
@@ -21,7 +21,7 @@ static int loadavg_proc_show(struct seq_file *m, void *v)
 		LOAD_INT(avnrun[1]), LOAD_FRAC(avnrun[1]),
 		LOAD_INT(avnrun[2]), LOAD_FRAC(avnrun[2]),
 		nr_running(), nr_threads,
-		idr_get_cursor(&task_active_pid_ns(current)->idr) - 1);
+		READ_ONCE(task_active_pid_ns(current)->pid_next) - 1);
 	return 0;
 }
 
diff --git a/include/linux/pid_namespace.h b/include/linux/pid_namespace.h
index 07481bb87d4e..82c72482019d 100644
--- a/include/linux/pid_namespace.h
+++ b/include/linux/pid_namespace.h
@@ -18,6 +18,7 @@ struct fs_pin;
 
 struct pid_namespace {
 	struct idr idr;
+	unsigned int pid_next;
 	struct rcu_head rcu;
 	unsigned int pid_allocated;
 	struct task_struct *child_reaper;
diff --git a/kernel/pid.c b/kernel/pid.c
index 72a6e9d0db81..409303ada383 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -75,6 +75,7 @@ int pid_max_max = PID_MAX_LIMIT;
 struct pid_namespace init_pid_ns = {
 	.ns.count = REFCOUNT_INIT(2),
 	.idr = IDR_INIT(init_pid_ns.idr),
+	.pid_next = 0,
 	.pid_allocated = PIDNS_ADDING,
 	.level = 0,
 	.child_reaper = &init_task,
@@ -208,7 +209,7 @@ struct pid *alloc_pid(struct pid_namespace *ns, pid_t *set_tid,
 			 * init really needs pid 1, but after reaching the
 			 * maximum wrap back to RESERVED_PIDS
 			 */
-			if (idr_get_cursor(&tmp->idr) > RESERVED_PIDS)
+			if (tmp->pid_next > RESERVED_PIDS)
 				pid_min = RESERVED_PIDS;
 
 			/*
@@ -217,6 +218,7 @@ struct pid *alloc_pid(struct pid_namespace *ns, pid_t *set_tid,
 			 */
 			nr = idr_alloc_cyclic(&tmp->idr, NULL, pid_min,
 					      pid_max, GFP_ATOMIC);
+			tmp->pid_next = nr + 1;
 		}
 		xa_unlock_irq(&tmp->idr.idr_rt);
 		idr_preload_end();
@@ -278,7 +280,7 @@ struct pid *alloc_pid(struct pid_namespace *ns, pid_t *set_tid,
 
 		/* On failure to allocate the first pid, reset the state */
 		if (tmp == ns && tmp->pid_allocated == PIDNS_ADDING)
-			idr_set_cursor(&ns->idr, 0);
+			ns->pid_next = 0;
 
 		idr_remove(&tmp->idr, upid->nr);
 		xa_unlock_irq(&tmp->idr.idr_rt);
diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
index f4f8cb0435b4..a53d20c5c85e 100644
--- a/kernel/pid_namespace.c
+++ b/kernel/pid_namespace.c
@@ -272,12 +272,12 @@ static int pid_ns_ctl_handler(struct ctl_table *table, int write,
 	 * it should synchronize its usage with external means.
 	 */
 
-	next = idr_get_cursor(&pid_ns->idr) - 1;
+	next = READ_ONCE(pid_ns->pid_next) - 1;
 
 	tmp.data = &next;
 	ret = proc_dointvec_minmax(&tmp, write, buffer, lenp, ppos);
 	if (!ret && write)
-		idr_set_cursor(&pid_ns->idr, next + 1);
+		WRITE_ONCE(pid_ns->pid_next, next + 1);
 
 	return ret;
 }
-- 
2.35.3