pidfs: convert rb-tree to rhashtable

Mateusz reported performance penalties [1] during task creation because
pidfs uses pidmap_lock to add elements into the rbtree. Switch to an
rhashtable to have separate fine-grained locking and to decouple from
pidmap_lock moving all heavy manipulations outside of it.

Convert the pidfs inode-to-pid mapping from an rb-tree with seqcount
protection to an rhashtable. This removes the global pidmap_lock
contention from pidfs_ino_get_pid() lookups and allows the hashtable
insert to happen outside the pidmap_lock.

pidfs_add_pid() is split. pidfs_prepare_pid() allocates inode number and
initializes pid fields and is called inside pidmap_lock. pidfs_add_pid()
inserts pid into rhashtable and is called outside pidmap_lock. Insertion
into the rhashtable can fail and memory allocation may happen so we need
to drop the spinlock.

To guard against accidently opening an already reaped task
pidfs_ino_get_pid() uses additional checks beyond pid_vnr(). If
pid->attr is PIDFS_PID_DEAD or NULL the pid either never had a pidfd or
it already went through pidfs_exit() aka the process as already reaped.
If pid->attr is valid check PIDFS_ATTR_BIT_EXIT to figure out whether
the task has exited.

This slightly changes visibility semantics: pidfd creation is denied
after pidfs_exit() runs, which is just before the pid number is removed
from the via free_pid(). That should not be an issue though.

Link: https://lore.kernel.org/20251206131955.780557-1-mjguzik@gmail.com [1]
Link: https://patch.msgid.link/20260120-work-pidfs-rhashtable-v2-1-d593c4d0f576@kernel.org
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/pidfs.c

@@ -21,6 +21,7 @@
 #include <linux/utsname.h>
 #include <net/net_namespace.h>
 #include <linux/coredump.h>
+#include <linux/rhashtable.h>
 #include <linux/xattr.h>
 
 #include "internal.h"
@@ -55,7 +56,14 @@ struct pidfs_attr {
 	__u32 coredump_signal;
 };
 
-static struct rb_root pidfs_ino_tree = RB_ROOT;
+static struct rhashtable pidfs_ino_ht;
+
+static const struct rhashtable_params pidfs_ino_ht_params = {
+	.key_offset		= offsetof(struct pid, ino),
+	.key_len		= sizeof(u64),
+	.head_offset		= offsetof(struct pid, pidfs_hash),
+	.automatic_shrinking	= true,
+};
 
 #if BITS_PER_LONG == 32
 static inline unsigned long pidfs_ino(u64 ino)
@@ -84,21 +92,11 @@ static inline u32 pidfs_gen(u64 ino)
 }
 #endif
 
-static int pidfs_ino_cmp(struct rb_node *a, const struct rb_node *b)
-{
-	struct pid *pid_a = rb_entry(a, struct pid, pidfs_node);
-	struct pid *pid_b = rb_entry(b, struct pid, pidfs_node);
-	u64 pid_ino_a = pid_a->ino;
-	u64 pid_ino_b = pid_b->ino;
-
-	if (pid_ino_a < pid_ino_b)
-		return -1;
-	if (pid_ino_a > pid_ino_b)
-		return 1;
-	return 0;
-}
-
-void pidfs_add_pid(struct pid *pid)
+/*
+ * Allocate inode number and initialize pidfs fields.
+ * Called with pidmap_lock held.
+ */
+void pidfs_prepare_pid(struct pid *pid)
 {
 	static u64 pidfs_ino_nr = 2;
 
@@ -131,20 +129,22 @@ void pidfs_add_pid(struct pid *pid)
 		pidfs_ino_nr += 2;
 
 	pid->ino = pidfs_ino_nr;
+	pid->pidfs_hash.next = NULL;
 	pid->stashed = NULL;
 	pid->attr = NULL;
 	pidfs_ino_nr++;
+}
 
-	write_seqcount_begin(&pidmap_lock_seq);
-	rb_find_add_rcu(&pid->pidfs_node, &pidfs_ino_tree, pidfs_ino_cmp);
-	write_seqcount_end(&pidmap_lock_seq);
+int pidfs_add_pid(struct pid *pid)
+{
+	return rhashtable_insert_fast(&pidfs_ino_ht, &pid->pidfs_hash,
+				      pidfs_ino_ht_params);
 }
 
 void pidfs_remove_pid(struct pid *pid)
 {
-	write_seqcount_begin(&pidmap_lock_seq);
-	rb_erase(&pid->pidfs_node, &pidfs_ino_tree);
-	write_seqcount_end(&pidmap_lock_seq);
+	rhashtable_remove_fast(&pidfs_ino_ht, &pid->pidfs_hash,
+			       pidfs_ino_ht_params);
 }
 
 void pidfs_free_pid(struct pid *pid)
@@ -773,42 +773,24 @@ static int pidfs_encode_fh(struct inode *inode, u32 *fh, int *max_len,
 	return FILEID_KERNFS;
 }
 
-static int pidfs_ino_find(const void *key, const struct rb_node *node)
-{
-	const u64 pid_ino = *(u64 *)key;
-	const struct pid *pid = rb_entry(node, struct pid, pidfs_node);
-
-	if (pid_ino < pid->ino)
-		return -1;
-	if (pid_ino > pid->ino)
-		return 1;
-	return 0;
-}
-
 /* Find a struct pid based on the inode number. */
 static struct pid *pidfs_ino_get_pid(u64 ino)
 {
 	struct pid *pid;
-	struct rb_node *node;
-	unsigned int seq;
+	struct pidfs_attr *attr;
 
 	guard(rcu)();
-	do {
-		seq = read_seqcount_begin(&pidmap_lock_seq);
-		node = rb_find_rcu(&ino, &pidfs_ino_tree, pidfs_ino_find);
-		if (node)
-			break;
-	} while (read_seqcount_retry(&pidmap_lock_seq, seq));
-
-	if (!node)
+	pid = rhashtable_lookup(&pidfs_ino_ht, &ino, pidfs_ino_ht_params);
+	if (!pid)
+		return NULL;
+	attr = READ_ONCE(pid->attr);
+	if (IS_ERR_OR_NULL(attr))
+		return NULL;
+	if (test_bit(PIDFS_ATTR_BIT_EXIT, &attr->attr_mask))
 		return NULL;
-
-	pid = rb_entry(node, struct pid, pidfs_node);
-
 	/* Within our pid namespace hierarchy? */
 	if (pid_vnr(pid) == 0)
 		return NULL;
-
 	return get_pid(pid);
 }
 
@@ -1086,6 +1068,9 @@ struct file *pidfs_alloc_file(struct pid *pid, unsigned int flags)
 
 void __init pidfs_init(void)
 {
+	if (rhashtable_init(&pidfs_ino_ht, &pidfs_ino_ht_params))
+		panic("Failed to initialize pidfs hashtable");
+
 	pidfs_attr_cachep = kmem_cache_create("pidfs_attr_cache", sizeof(struct pidfs_attr), 0,
 					 (SLAB_HWCACHE_ALIGN | SLAB_RECLAIM_ACCOUNT |
 					  SLAB_ACCOUNT | SLAB_PANIC), NULL);

include/linux/pid.h

@@ -6,6 +6,7 @@
 #include <linux/rculist.h>
 #include <linux/rcupdate.h>
 #include <linux/refcount.h>
+#include <linux/rhashtable-types.h>
 #include <linux/sched.h>
 #include <linux/wait.h>
 
@@ -60,7 +61,7 @@ struct pid {
 	spinlock_t lock;
 	struct {
 		u64 ino;
-		struct rb_node pidfs_node;
+		struct rhash_head pidfs_hash;
 		struct dentry *stashed;
 		struct pidfs_attr *attr;
 	};
@@ -73,7 +74,6 @@ struct pid {
 	struct upid numbers[];
 };
 
-extern seqcount_spinlock_t pidmap_lock_seq;
 extern struct pid init_struct_pid;
 
 struct file;

include/linux/pidfs.h

@@ -6,7 +6,8 @@ struct coredump_params;
 
 struct file *pidfs_alloc_file(struct pid *pid, unsigned int flags);
 void __init pidfs_init(void);
-void pidfs_add_pid(struct pid *pid);
+void pidfs_prepare_pid(struct pid *pid);
+int pidfs_add_pid(struct pid *pid);
 void pidfs_remove_pid(struct pid *pid);
 void pidfs_exit(struct task_struct *tsk);
 #ifdef CONFIG_COREDUMP

kernel/pid.c

@@ -43,7 +43,6 @@
 #include <linux/sched/task.h>
 #include <linux/idr.h>
 #include <linux/pidfs.h>
-#include <linux/seqlock.h>
 #include <net/sock.h>
 #include <uapi/linux/pidfd.h>
 
@@ -85,7 +84,6 @@ struct pid_namespace init_pid_ns = {
 EXPORT_SYMBOL_GPL(init_pid_ns);
 
 static  __cacheline_aligned_in_smp DEFINE_SPINLOCK(pidmap_lock);
-seqcount_spinlock_t pidmap_lock_seq = SEQCNT_SPINLOCK_ZERO(pidmap_lock_seq, &pidmap_lock);
 
 void put_pid(struct pid *pid)
 {
@@ -141,9 +139,9 @@ void free_pid(struct pid *pid)
 
 		idr_remove(&ns->idr, upid->nr);
 	}
-	pidfs_remove_pid(pid);
 	spin_unlock(&pidmap_lock);
 
+	pidfs_remove_pid(pid);
 	call_rcu(&pid->rcu, delayed_put_pid);
 }
 
@@ -316,7 +314,8 @@ struct pid *alloc_pid(struct pid_namespace *ns, pid_t *arg_set_tid,
 	retval = -ENOMEM;
 	if (unlikely(!(ns->pid_allocated & PIDNS_ADDING)))
 		goto out_free;
-	pidfs_add_pid(pid);
+	pidfs_prepare_pid(pid);
+
 	for (upid = pid->numbers + ns->level; upid >= pid->numbers; --upid) {
 		/* Make the PID visible to find_pid_ns. */
 		idr_replace(&upid->ns->idr, pid, upid->nr);
@@ -326,6 +325,12 @@ struct pid *alloc_pid(struct pid_namespace *ns, pid_t *arg_set_tid,
 	idr_preload_end();
 	ns_ref_active_get(ns);
 
+	retval = pidfs_add_pid(pid);
+	if (unlikely(retval)) {
+		free_pid(pid);
+		pid = ERR_PTR(-ENOMEM);
+	}
+
 	return pid;
 
 out_free: