linux-kernel - Re: [PATCH 3/3] kernel/sysctl.c: threads-max observe limits

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Message-ID: <alpine.DEB.2.10.1502241315250.3855@chino.kir.corp.google.com>
Date:	Tue, 24 Feb 2015 13:17:03 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Heinrich Schuchardt <xypron.glpk@....de>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Aaron Tomlin <atomlin@...hat.com>,
	Andy Lutomirski <luto@...capital.net>,
	Davidlohr Bueso <dave@...olabs.net>,
	"David S. Miller" <davem@...emloft.net>,
	Fabian Frederick <fabf@...net.be>,
	Guenter Roeck <linux@...ck-us.net>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>,
	Jens Axboe <axboe@...com>, Joe Perches <joe@...ches.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Kees Cook <keescook@...omium.org>,
	Michael Marineau <mike@...ineau.org>,
	Oleg Nesterov <oleg@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Prarit Bhargava <prarit@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Vladimir Davydov <vdavydov@...allels.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] kernel/sysctl.c: threads-max observe limits

On Tue, 24 Feb 2015, Heinrich Schuchardt wrote:

> Users can change the maximum number of threads by writing to
> /proc/sys/kernel/threads-max.
> 
> With the patch the value entered is checked against the same
> limits that apply when fork_init is called.
> 

Correct me if I'm wrong, but this is a change in functionality (without 
update to Documentation/sysctl/kernel.txt which describes threads-max) 
since it does not allow the value to be lowered as before from the 
calculation involving totalram_pages.  The value passed to 
set_max_threads() only caps the value.

> Signed-off-by: Heinrich Schuchardt <xypron.glpk@....de>
> ---
>  include/linux/sysctl.h |  3 +++
>  kernel/fork.c          | 24 ++++++++++++++++++++++++
>  kernel/sysctl.c        |  6 ++----
>  3 files changed, 29 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/sysctl.h b/include/linux/sysctl.h
> index b7361f8..795d5fe 100644
> --- a/include/linux/sysctl.h
> +++ b/include/linux/sysctl.h
> @@ -212,4 +212,7 @@ static inline void setup_sysctl_set(struct ctl_table_set *p,
>  
>  #endif /* CONFIG_SYSCTL */
>  
> +int sysctl_max_threads(struct ctl_table *table, int write,
> +		       void __user *buffer, size_t *lenp, loff_t *ppos);
> +
>  #endif /* _LINUX_SYSCTL_H */
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 880c78d..52a07c7 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -74,6 +74,7 @@
>  #include <linux/uprobes.h>
>  #include <linux/aio.h>
>  #include <linux/compiler.h>
> +#include <linux/sysctl.h>
>  
>  #include <asm/pgtable.h>
>  #include <asm/pgalloc.h>
> @@ -2024,3 +2025,26 @@ int unshare_files(struct files_struct **displaced)
>  	task_unlock(task);
>  	return 0;
>  }
> +
> +int sysctl_max_threads(struct ctl_table *table, int write,
> +		       void __user *buffer, size_t *lenp, loff_t *ppos)
> +{
> +	struct ctl_table t;
> +	int ret;
> +	int threads = max_threads;
> +	int min = MIN_THREADS;
> +	int max = MAX_THREADS;
> +
> +	t = *table;
> +	t.data = &threads;
> +	t.extra1 = &min;
> +	t.extra2 = &max;
> +
> +	ret = proc_dointvec_minmax(&t, write, buffer, lenp, ppos);
> +	if (ret || !write)
> +		return ret;
> +
> +	set_max_threads(threads);
> +
> +	return 0;
> +}
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index 137c7f6..2e195ae 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -92,11 +92,9 @@
>  #include <linux/nmi.h>
>  #endif
>  
> -
>  #if defined(CONFIG_SYSCTL)
>  
>  /* External variables not in a header file. */
> -extern int max_threads;
>  extern int suid_dumpable;
>  #ifdef CONFIG_COREDUMP
>  extern int core_uses_pid;
> @@ -709,10 +707,10 @@ static struct ctl_table kern_table[] = {
>  #endif
>  	{
>  		.procname	= "threads-max",
> -		.data		= &max_threads,
> +		.data		= NULL,
>  		.maxlen		= sizeof(int),
>  		.mode		= 0644,
> -		.proc_handler	= proc_dointvec,
> +		.proc_handler	= sysctl_max_threads,
>  	},
>  	{
>  		.procname	= "random",
--
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