Date:   Wed, 15 Jun 2022 17:34:02 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Brian Foster <bfoster@...hat.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        ikent@...hat.com, onestero@...hat.com
Subject: Re: [PATCH 1/3] radix-tree: propagate all tags in idr tree

On Wed, Jun 15, 2022 at 10:43:33AM -0400, Brian Foster wrote:
> Interesting, thanks. I'll have to dig more into this to grok the current
> state of the radix-tree interface vs. the underlying data structure. If
> I follow correctly, you're saying the radix-tree api is essentially
> already a translation layer to the xarray these days, and we just need
> to move legacy users off the radix-tree api so we can eventually kill it
> off...

If only it were that easy ... the XArray has a whole bunch of debugging
asserts to make sure the users are actually using it correctly, and a
lot of radix tree users don't (they're probably not buggy, but they
don't use the XArray's embedded lock).

Anyway, here's a first cut at converting the PID allocator from the IDR
to the XArray API.  It boots, but I haven't tried to do anything tricky
with PID namespaces or CRIU.


diff --git a/fs/proc/loadavg.c b/fs/proc/loadavg.c
index f32878d9a39f..cec85a07184a 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);
+		task_active_pid_ns(current)->cursor - 1);
 	return 0;
 }
 
diff --git a/include/linux/pid_namespace.h b/include/linux/pid_namespace.h
index 07481bb87d4e..68aaaf78491b 100644
--- a/include/linux/pid_namespace.h
+++ b/include/linux/pid_namespace.h
@@ -9,7 +9,7 @@
 #include <linux/threads.h>
 #include <linux/nsproxy.h>
 #include <linux/ns_common.h>
-#include <linux/idr.h>
+#include <linux/xarray.h>
 
 /* MAX_PID_NS_LEVEL is needed for limiting size of 'struct pid' */
 #define MAX_PID_NS_LEVEL 32
@@ -17,8 +17,9 @@
 struct fs_pin;
 
 struct pid_namespace {
-	struct idr idr;
+	struct xarray xa;
 	struct rcu_head rcu;
+	unsigned int cursor;
 	unsigned int pid_allocated;
 	struct task_struct *child_reaper;
 	struct kmem_cache *pid_cachep;
@@ -37,6 +38,20 @@ extern struct pid_namespace init_pid_ns;
 
 #define PIDNS_ADDING (1U << 31)
 
+/*
+ * Note: disable interrupts while the xarray lock is held as an
+ * interrupt might come in and do read_lock(&tasklist_lock).
+ *
+ * If we don't disable interrupts there is a nasty deadlock between
+ * detach_pid()->free_pid() and another cpu that locks the PIDs
+ * followed by an interrupt routine that does read_lock(&tasklist_lock);
+ *
+ * After we clean up the tasklist_lock and know there are no
+ * irq handlers that take it we can leave the interrupts enabled.
+ * For now it is easier to be safe than to prove it can't happen.
+ */
+#define PID_XA_FLAGS	(XA_FLAGS_TRACK_FREE | XA_FLAGS_LOCK_IRQ)
+
 #ifdef CONFIG_PID_NS
 static inline struct pid_namespace *get_pid_ns(struct pid_namespace *ns)
 {
@@ -84,7 +99,7 @@ static inline int reboot_pid_ns(struct pid_namespace *pid_ns, int cmd)
 
 extern struct pid_namespace *task_active_pid_ns(struct task_struct *tsk);
 void pidhash_init(void);
-void pid_idr_init(void);
+void pid_init(void);
 
 static inline bool task_is_in_init_pid_ns(struct task_struct *tsk)
 {
diff --git a/include/linux/threads.h b/include/linux/threads.h
index c34173e6c5f1..37e4391ee89f 100644
--- a/include/linux/threads.h
+++ b/include/linux/threads.h
@@ -38,7 +38,7 @@
  * Define a minimum number of pids per cpu.  Heuristically based
  * on original pid max of 32k for 32 cpus.  Also, increase the
  * minimum settable value for pid_max on the running system based
- * on similar defaults.  See kernel/pid.c:pid_idr_init() for details.
+ * on similar defaults.  See kernel/pid.c:pid_init() for details.
  */
 #define PIDS_PER_CPU_DEFAULT	1024
 #define PIDS_PER_CPU_MIN	8
diff --git a/init/main.c b/init/main.c
index 0ee39cdcfcac..3944dcd10c09 100644
--- a/init/main.c
+++ b/init/main.c
@@ -73,7 +73,6 @@
 #include <linux/sched.h>
 #include <linux/sched/init.h>
 #include <linux/signal.h>
-#include <linux/idr.h>
 #include <linux/kgdb.h>
 #include <linux/ftrace.h>
 #include <linux/async.h>
@@ -1100,7 +1099,7 @@ asmlinkage __visible void __init __no_sanitize_address start_kernel(void)
 		late_time_init();
 	sched_clock_init();
 	calibrate_delay();
-	pid_idr_init();
+	pid_init();
 	anon_vma_init();
 #ifdef CONFIG_X86
 	if (efi_enabled(EFI_RUNTIME_SERVICES))
diff --git a/kernel/pid.c b/kernel/pid.c
index 2fc0a16ec77b..de0b4614bdb8 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -41,7 +41,7 @@
 #include <linux/anon_inodes.h>
 #include <linux/sched/signal.h>
 #include <linux/sched/task.h>
-#include <linux/idr.h>
+#include <linux/xarray.h>
 #include <net/sock.h>
 #include <uapi/linux/pidfd.h>
 
@@ -66,15 +66,10 @@ int pid_max = PID_MAX_DEFAULT;
 int pid_max_min = RESERVED_PIDS + 1;
 int pid_max_max = PID_MAX_LIMIT;
 
-/*
- * PID-map pages start out as NULL, they get allocated upon
- * first use and are never deallocated. This way a low pid_max
- * value does not cause lots of bitmaps to be allocated, but
- * the scheme scales to up to 4 million PIDs, runtime.
- */
 struct pid_namespace init_pid_ns = {
 	.ns.count = REFCOUNT_INIT(2),
-	.idr = IDR_INIT(init_pid_ns.idr),
+	.xa = XARRAY_INIT(init_pid_ns.xa, PID_XA_FLAGS),
+	.cursor = 1,
 	.pid_allocated = PIDNS_ADDING,
 	.level = 0,
 	.child_reaper = &init_task,
@@ -86,22 +81,6 @@ struct pid_namespace init_pid_ns = {
 };
 EXPORT_SYMBOL_GPL(init_pid_ns);
 
-/*
- * Note: disable interrupts while the pidmap_lock is held as an
- * interrupt might come in and do read_lock(&tasklist_lock).
- *
- * If we don't disable interrupts there is a nasty deadlock between
- * detach_pid()->free_pid() and another cpu that does
- * spin_lock(&pidmap_lock) followed by an interrupt routine that does
- * read_lock(&tasklist_lock);
- *
- * After we clean up the tasklist_lock and know there are no
- * irq handlers that take it we can leave the interrupts enabled.
- * For now it is easier to be safe than to prove it can't happen.
- */
-
-static  __cacheline_aligned_in_smp DEFINE_SPINLOCK(pidmap_lock);
-
 void put_pid(struct pid *pid)
 {
 	struct pid_namespace *ns;
@@ -129,10 +108,11 @@ void free_pid(struct pid *pid)
 	int i;
 	unsigned long flags;
 
-	spin_lock_irqsave(&pidmap_lock, flags);
 	for (i = 0; i <= pid->level; i++) {
 		struct upid *upid = pid->numbers + i;
 		struct pid_namespace *ns = upid->ns;
+
+		xa_lock_irqsave(&ns->xa, flags);
 		switch (--ns->pid_allocated) {
 		case 2:
 		case 1:
@@ -149,9 +129,9 @@ void free_pid(struct pid *pid)
 			break;
 		}
 
-		idr_remove(&ns->idr, upid->nr);
+		__xa_erase(&ns->xa, upid->nr);
+		xa_unlock_irqrestore(&ns->xa, flags);
 	}
-	spin_unlock_irqrestore(&pidmap_lock, flags);
 
 	call_rcu(&pid->rcu, delayed_put_pid);
 }
@@ -161,7 +141,7 @@ struct pid *alloc_pid(struct pid_namespace *ns, pid_t *set_tid,
 {
 	struct pid *pid;
 	enum pid_type type;
-	int i, nr;
+	int i;
 	struct pid_namespace *tmp;
 	struct upid *upid;
 	int retval = -ENOMEM;
@@ -205,43 +185,42 @@ struct pid *alloc_pid(struct pid_namespace *ns, pid_t *set_tid,
 			set_tid_size--;
 		}
 
-		idr_preload(GFP_KERNEL);
-		spin_lock_irq(&pidmap_lock);
-
 		if (tid) {
-			nr = idr_alloc(&tmp->idr, NULL, tid,
-				       tid + 1, GFP_ATOMIC);
+			retval = xa_insert_irq(&tmp->xa, tid, NULL, GFP_KERNEL);
+
 			/*
-			 * If ENOSPC is returned it means that the PID is
+			 * If EBUSY is returned it means that the PID is
 			 * alreay in use. Return EEXIST in that case.
 			 */
-			if (nr == -ENOSPC)
-				nr = -EEXIST;
+			if (retval == -EBUSY)
+				retval = -EEXIST;
 		} else {
 			int pid_min = 1;
+
+			xa_lock_irq(&tmp->xa);
 			/*
 			 * 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->cursor > RESERVED_PIDS)
 				pid_min = RESERVED_PIDS;
 
 			/*
 			 * 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);
+			retval = __xa_alloc_cyclic(&tmp->xa, &tid, NULL,
+					XA_LIMIT(pid_min, pid_max),
+					&tmp->cursor, GFP_KERNEL);
+			xa_unlock_irq(&tmp->xa);
+			if (retval == -EBUSY)
+				retval = -EAGAIN;
 		}
-		spin_unlock_irq(&pidmap_lock);
-		idr_preload_end();
 
-		if (nr < 0) {
-			retval = (nr == -ENOSPC) ? -EAGAIN : nr;
+		if (retval < 0)
 			goto out_free;
-		}
 
-		pid->numbers[i].nr = nr;
+		pid->numbers[i].nr = tid;
 		pid->numbers[i].ns = tmp;
 		tmp = tmp->parent;
 	}
@@ -266,34 +245,35 @@ struct pid *alloc_pid(struct pid_namespace *ns, pid_t *set_tid,
 	INIT_HLIST_HEAD(&pid->inodes);
 
 	upid = pid->numbers + ns->level;
-	spin_lock_irq(&pidmap_lock);
+	xa_lock_irq(&ns->xa);
 	if (!(ns->pid_allocated & PIDNS_ADDING))
 		goto out_unlock;
 	for ( ; upid >= pid->numbers; --upid) {
 		/* Make the PID visible to find_pid_ns. */
-		idr_replace(&upid->ns->idr, pid, upid->nr);
+		if (upid->ns != ns)
+			xa_lock_irq(&ns->xa);
+		__xa_store(&upid->ns->xa, upid->nr, pid, 0);
 		upid->ns->pid_allocated++;
+		xa_unlock_irq(&ns->xa);
 	}
-	spin_unlock_irq(&pidmap_lock);
 
 	return pid;
 
 out_unlock:
-	spin_unlock_irq(&pidmap_lock);
+	xa_unlock_irq(&ns->xa);
 	put_pid_ns(ns);
 
 out_free:
-	spin_lock_irq(&pidmap_lock);
 	while (++i <= ns->level) {
 		upid = pid->numbers + i;
-		idr_remove(&upid->ns->idr, upid->nr);
+		xa_erase_irq(&upid->ns->xa, upid->nr);
 	}
 
+	xa_lock_irq(&ns->xa);
 	/* On failure to allocate the first pid, reset the state */
 	if (ns->pid_allocated == PIDNS_ADDING)
-		idr_set_cursor(&ns->idr, 0);
-
-	spin_unlock_irq(&pidmap_lock);
+		ns->cursor = 0;
+	xa_unlock_irq(&ns->xa);
 
 	kmem_cache_free(ns->pid_cachep, pid);
 	return ERR_PTR(retval);
@@ -301,14 +281,14 @@ struct pid *alloc_pid(struct pid_namespace *ns, pid_t *set_tid,
 
 void disable_pid_allocation(struct pid_namespace *ns)
 {
-	spin_lock_irq(&pidmap_lock);
+	xa_lock_irq(&ns->xa);
 	ns->pid_allocated &= ~PIDNS_ADDING;
-	spin_unlock_irq(&pidmap_lock);
+	xa_unlock_irq(&ns->xa);
 }
 
 struct pid *find_pid_ns(int nr, struct pid_namespace *ns)
 {
-	return idr_find(&ns->idr, nr);
+	return xa_load(&ns->xa, nr);
 }
 EXPORT_SYMBOL_GPL(find_pid_ns);
 
@@ -517,7 +497,9 @@ EXPORT_SYMBOL_GPL(task_active_pid_ns);
  */
 struct pid *find_ge_pid(int nr, struct pid_namespace *ns)
 {
-	return idr_get_next(&ns->idr, &nr);
+	unsigned long index = nr;
+
+	return xa_find(&ns->xa, &index, ULONG_MAX, XA_PRESENT);
 }
 
 struct pid *pidfd_get_pid(unsigned int fd, unsigned int *flags)
@@ -646,7 +628,7 @@ SYSCALL_DEFINE2(pidfd_open, pid_t, pid, unsigned int, flags)
 	return fd;
 }
 
-void __init pid_idr_init(void)
+void __init pid_init(void)
 {
 	/* Verify no one has done anything silly: */
 	BUILD_BUG_ON(PID_MAX_LIMIT >= PIDNS_ADDING);
@@ -658,8 +640,6 @@ void __init pid_idr_init(void)
 				PIDS_PER_CPU_MIN * num_possible_cpus());
 	pr_info("pid_max: default: %u minimum: %u\n", pid_max, pid_max_min);
 
-	idr_init(&init_pid_ns.idr);
-
 	init_pid_ns.pid_cachep = KMEM_CACHE(pid,
 			SLAB_HWCACHE_ALIGN | SLAB_PANIC | SLAB_ACCOUNT);
 }
diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
index f4f8cb0435b4..947e25fb8546 100644
--- a/kernel/pid_namespace.c
+++ b/kernel/pid_namespace.c
@@ -22,7 +22,7 @@
 #include <linux/export.h>
 #include <linux/sched/task.h>
 #include <linux/sched/signal.h>
-#include <linux/idr.h>
+#include <linux/xarray.h>
 
 static DEFINE_MUTEX(pid_caches_mutex);
 static struct kmem_cache *pid_ns_cachep;
@@ -92,15 +92,15 @@ static struct pid_namespace *create_pid_namespace(struct user_namespace *user_ns
 	if (ns == NULL)
 		goto out_dec;
 
-	idr_init(&ns->idr);
+	xa_init_flags(&ns->xa, PID_XA_FLAGS);
 
 	ns->pid_cachep = create_pid_cachep(level);
 	if (ns->pid_cachep == NULL)
-		goto out_free_idr;
+		goto out_free_xa;
 
 	err = ns_alloc_inum(&ns->ns);
 	if (err)
-		goto out_free_idr;
+		goto out_free_xa;
 	ns->ns.ops = &pidns_operations;
 
 	refcount_set(&ns->ns.count, 1);
@@ -112,8 +112,8 @@ static struct pid_namespace *create_pid_namespace(struct user_namespace *user_ns
 
 	return ns;
 
-out_free_idr:
-	idr_destroy(&ns->idr);
+out_free_xa:
+	xa_destroy(&ns->xa);
 	kmem_cache_free(pid_ns_cachep, ns);
 out_dec:
 	dec_pid_namespaces(ucounts);
@@ -135,7 +135,7 @@ static void destroy_pid_namespace(struct pid_namespace *ns)
 {
 	ns_free_inum(&ns->ns);
 
-	idr_destroy(&ns->idr);
+	xa_destroy(&ns->xa);
 	call_rcu(&ns->rcu, delayed_free_pidns);
 }
 
@@ -165,7 +165,7 @@ EXPORT_SYMBOL_GPL(put_pid_ns);
 
 void zap_pid_ns_processes(struct pid_namespace *pid_ns)
 {
-	int nr;
+	long nr;
 	int rc;
 	struct task_struct *task, *me = current;
 	int init_pids = thread_group_leader(me) ? 1 : 2;
@@ -198,8 +198,7 @@ void zap_pid_ns_processes(struct pid_namespace *pid_ns)
 	 */
 	rcu_read_lock();
 	read_lock(&tasklist_lock);
-	nr = 2;
-	idr_for_each_entry_continue(&pid_ns->idr, pid, nr) {
+	xa_for_each_range(&pid_ns->xa, nr, pid, 2, ULONG_MAX) {
 		task = pid_task(pid, PIDTYPE_PID);
 		if (task && !__fatal_signal_pending(task))
 			group_send_sig_info(SIGKILL, SEND_SIG_PRIV, task, PIDTYPE_MAX);
@@ -272,12 +271,12 @@ 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 = pid_ns->cursor - 1;
 
 	tmp.data = &next;
 	ret = proc_dointvec_minmax(&tmp, write, buffer, lenp, ppos);
 	if (!ret && write)
-		idr_set_cursor(&pid_ns->idr, next + 1);
+		pid_ns->cursor = next + 1;
 
 	return ret;
 }

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