linux-kernel - Re: [PATCH v2 1/2] pid: Replace pid bitmap implementation with IDR API

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Date:   Wed, 27 Sep 2017 17:05:55 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Gargi Sharma <gs051095@...il.com>
Cc:     linux-kernel@...r.kernel.org, riel@...riel.com,
        julia.lawall@...6.fr, akpm@...ux-foundation.org, mingo@...nel.org,
        pasha.tatashin@...cle.com, ktkhai@...tuozzo.com
Subject: Re: [PATCH v2 1/2] pid: Replace pid bitmap implementation with IDR
 API

On 09/27, Gargi Sharma wrote:
>
> @@ -309,7 +168,22 @@ struct pid *alloc_pid(struct pid_namespace *ns)
>  	tmp = ns;
>  	pid->level = ns->level;
>  	for (i = ns->level; i >= 0; i--) {
> -		nr = alloc_pidmap(tmp);
> +		int pid_min = 1;
> +		idr_preload(GFP_KERNEL);
> +		spin_lock_irq(&pidmap_lock);
> +
> +		/*
> +		 * init really needs pid 1, but after reaching the maximum
> +		 * wrap back to RESERVED_PIDS
> +		 */
> +		if (tmp->idr.idr_next > RESERVED_PIDS)
> +			pid_min = RESERVED_PIDS;
> +
> +		nr = idr_alloc_cyclic(&tmp->idr, pid, pid_min,
> +				      pid_max, GFP_ATOMIC);
> +		spin_unlock_irq(&pidmap_lock);
> +		idr_preload_end();
> +
>  		if (nr < 0) {
>  			retval = nr;
>  			goto out_free;
> @@ -346,12 +220,14 @@ struct pid *alloc_pid(struct pid_namespace *ns)
>  	return pid;
>  
>  out_unlock:
> -	spin_unlock_irq(&pidmap_lock);
>  	put_pid_ns(ns);
> +	spin_unlock_irq(&pidmap_lock);

Why? No need to move put_pid_ns() under pidmap_lock, please remove this change...

>  void __init pidhash_init(void)
>  {
> +	unsigned int pidhash_size;
> +
>  	pid_hash = alloc_large_system_hash("PID", sizeof(*pid_hash), 0, 18,
>  					   HASH_EARLY | HASH_SMALL | HASH_ZERO,
>  					   &pidhash_shift, NULL,
>  					   0, 4096);
> +	pidhash_size = 1U << pidhash_shift;
>  }

Hmm, this change makes no sense. And the next patch kills pidhash_init()
altogether.

Oleg.