Re: [PATCH v3 3/5] pid: switch pid_namespace from idr to xarray

[Ian Kent <ikent@redhat.com>]

On 3/12/22 01:16, Brian Foster wrote:
> Switch struct pid[_namespace] management over to use the xarray api
> directly instead of the idr. The underlying data structures used by
> both interfaces is the same. The difference is that the idr api
> relies on the old, idr-custom radix-tree implementation for things
> like efficient tracking/allocation of free ids. The xarray already
> supports this, so most of this is a direct switchover from the old
> api to the new.
>
> Signed-off-by: Matthew Wilcox <willy@infradead.org>
> Signed-off-by: Brian Foster <bfoster@redhat.com>


Reviewed-by: Ian Kent <raven@themaw.net>

> ---
>   include/linux/pid_namespace.h |  8 ++--
>   include/linux/threads.h       |  2 +-
>   init/main.c                   |  3 +-
>   kernel/pid.c                  | 78 ++++++++++++++++-------------------
>   kernel/pid_namespace.c        | 19 ++++-----
>   5 files changed, 51 insertions(+), 59 deletions(-)
>
> diff --git a/include/linux/pid_namespace.h b/include/linux/pid_namespace.h
> index 82c72482019d..e4f5979b482b 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
> @@ -17,7 +17,7 @@
>   struct fs_pin;
>   
>   struct pid_namespace {
> -	struct idr idr;
> +	struct xarray xa;
>   	unsigned int pid_next;
>   	struct rcu_head rcu;
>   	unsigned int pid_allocated;
> @@ -38,6 +38,8 @@ extern struct pid_namespace init_pid_ns;
>   
>   #define PIDNS_ADDING (1U << 31)
>   
> +#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)
>   {
> @@ -85,7 +87,7 @@ 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 aa21add5f7c5..7dd8888036c7 100644
> --- a/init/main.c
> +++ b/init/main.c
> @@ -74,7 +74,6 @@
>   #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>
> @@ -1108,7 +1107,7 @@ 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 2e2d33273c8e..53db06f9882d 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 +66,9 @@ 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),
>   	.pid_next = 0,
>   	.pid_allocated = PIDNS_ADDING,
>   	.level = 0,
> @@ -118,7 +112,7 @@ void free_pid(struct pid *pid)
>   		struct upid *upid = pid->numbers + i;
>   		struct pid_namespace *ns = upid->ns;
>   
> -		xa_lock_irqsave(&ns->idr.idr_rt, flags);
> +		xa_lock_irqsave(&ns->xa, flags);
>   		switch (--ns->pid_allocated) {
>   		case 2:
>   		case 1:
> @@ -135,8 +129,8 @@ void free_pid(struct pid *pid)
>   			break;
>   		}
>   
> -		idr_remove(&ns->idr, upid->nr);
> -		xa_unlock_irqrestore(&ns->idr.idr_rt, flags);
> +		__xa_erase(&ns->xa, upid->nr);
> +		xa_unlock_irqrestore(&ns->xa, flags);
>   	}
>   
>   	call_rcu(&pid->rcu, delayed_put_pid);
> @@ -147,7 +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;
> @@ -191,18 +185,17 @@ struct pid *alloc_pid(struct pid_namespace *ns, pid_t *set_tid,
>   			set_tid_size--;
>   		}
>   
> -		idr_preload(GFP_KERNEL);
> -		xa_lock_irq(&tmp->idr.idr_rt);
> +		xa_lock_irq(&tmp->xa);
>   
>   		if (tid) {
> -			nr = idr_alloc(&tmp->idr, NULL, tid,
> -				       tid + 1, GFP_ATOMIC);
> +			retval = __xa_insert(&tmp->xa, tid, NULL, GFP_KERNEL);
> +
>   			/*
> -			 * If ENOSPC is returned it means that the PID is
> -			 * alreay in use. Return EEXIST in that case.
> +			 * If EBUSY is returned it means that the PID is already
> +			 * in use. Return EEXIST in that case.
>   			 */
> -			if (nr == -ENOSPC)
> -				nr = -EEXIST;
> +			if (retval == -EBUSY)
> +				retval = -EEXIST;
>   		} else {
>   			int pid_min = 1;
>   			/*
> @@ -216,19 +209,18 @@ struct pid *alloc_pid(struct pid_namespace *ns, pid_t *set_tid,
>   			 * 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);
> -			tmp->pid_next = nr + 1;
> +			retval = __xa_alloc_cyclic(&tmp->xa, &tid, NULL,
> +					XA_LIMIT(pid_min, pid_max),
> +					&tmp->pid_next, GFP_KERNEL);
> +			if (retval == -EBUSY)
> +				retval = -EAGAIN;
>   		}
> -		xa_unlock_irq(&tmp->idr.idr_rt);
> -		idr_preload_end();
> +		xa_unlock_irq(&tmp->xa);
>   
> -		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;
>   	}
> @@ -256,17 +248,17 @@ struct pid *alloc_pid(struct pid_namespace *ns, pid_t *set_tid,
>   	for ( ; upid >= pid->numbers; --upid) {
>   		tmp = upid->ns;
>   
> -		xa_lock_irq(&tmp->idr.idr_rt);
> +		xa_lock_irq(&tmp->xa);
>   		if (tmp == ns && !(tmp->pid_allocated & PIDNS_ADDING)) {
> -			xa_unlock_irq(&tmp->idr.idr_rt);
> +			xa_unlock_irq(&tmp->xa);
>   			put_pid_ns(ns);
>   			goto out_free;
>   		}
>   
>   		/* Make the PID visible to find_pid_ns. */
> -		idr_replace(&tmp->idr, pid, upid->nr);
> +		__xa_store(&tmp->xa, upid->nr, pid, 0);
>   		tmp->pid_allocated++;
> -		xa_unlock_irq(&tmp->idr.idr_rt);
> +		xa_unlock_irq(&tmp->xa);
>   	}
>   
>   	return pid;
> @@ -276,14 +268,14 @@ struct pid *alloc_pid(struct pid_namespace *ns, pid_t *set_tid,
>   		upid = pid->numbers + i;
>   		tmp = upid->ns;
>   
> -		xa_lock_irq(&tmp->idr.idr_rt);
> +		xa_lock_irq(&tmp->xa);
>   
>   		/* On failure to allocate the first pid, reset the state */
>   		if (tmp == ns && tmp->pid_allocated == PIDNS_ADDING)
>   			ns->pid_next = 0;
>   
> -		idr_remove(&tmp->idr, upid->nr);
> -		xa_unlock_irq(&tmp->idr.idr_rt);
> +		__xa_erase(&tmp->xa, upid->nr);
> +		xa_unlock_irq(&tmp->xa);
>   	}
>   
>   	kmem_cache_free(ns->pid_cachep, pid);
> @@ -292,14 +284,14 @@ struct pid *alloc_pid(struct pid_namespace *ns, pid_t *set_tid,
>   
>   void disable_pid_allocation(struct pid_namespace *ns)
>   {
> -	xa_lock_irq(&ns->idr.idr_rt);
> +	xa_lock_irq(&ns->xa);
>   	ns->pid_allocated &= ~PIDNS_ADDING;
> -	xa_unlock_irq(&ns->idr.idr_rt);
> +	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);
>   
> @@ -508,7 +500,9 @@ 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);
>   }
>   EXPORT_SYMBOL_GPL(find_ge_pid);
>   
> @@ -650,7 +644,7 @@ SYSCALL_DEFINE2(pidfd_open, pid_t, pid, unsigned int, flags)
>    * take it we can leave the interrupts enabled.  For now it is easier to be safe
>    * than to prove it can't happen.
>    */
> -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);
> @@ -662,8 +656,6 @@ 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 a53d20c5c85e..8561e01e2d01 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);