linux-kernel - Re: [PATCH] fs: limit maximum concurrent coredumps

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Date:	Wed, 23 Jun 2010 18:02:21 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Edward Allcutt <edward@...cutt.me.uk>
Cc:	Alexander Viro <viro@...iv.linux.org.uk>,
	Randy Dunlap <rdunlap@...otime.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jiri Kosina <jkosina@...e.cz>,
	Dave Young <hidave.darkstar@...il.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Neil Horman <nhorman@...driver.com>,
	Roland McGrath <roland@...hat.com>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] fs: limit maximum concurrent coredumps

On 06/23, Oleg Nesterov wrote:
>
> On 06/21, Edward Allcutt wrote:
> >
> > The ability to limit concurrent coredumps allows dumping core to be safely
> > enabled in these situations without affecting responsiveness of the system
> > as a whole.
>
> OK, but please note that the patch is not right,

OOPS, sorry, I was not exactly right too.

> > @@ -1844,6 +1845,7 @@ void do_coredump(long signr, int exit_code, struct pt_regs *regs)
> >  	int retval = 0;
> >  	int flag = 0;
> >  	int ispipe;
> > +	int dump_count = 0;
> >  	static atomic_t core_dump_count = ATOMIC_INIT(0);
> >  	struct coredump_params cprm = {
> >  		.signr = signr,
> > @@ -1865,6 +1867,14 @@ void do_coredump(long signr, int exit_code, struct pt_regs *regs)
> >  	if (!__get_dumpable(cprm.mm_flags))
> >  		goto fail;
> >
> > +	dump_count = atomic_inc_return(&core_dump_count);
> > +	if (core_max_concurrency && (core_max_concurrency < dump_count)) {
> > +		printk(KERN_WARNING "Pid %d(%s) over core_max_concurrency\n",
> > +		       task_tgid_vnr(current), current->comm);
> > +		printk(KERN_WARNING "Skipping core dump\n");
> > +		goto fail;
> > +	}
> > +
>
> We can't return here. We should kill other threads which share the same
> ->mm in any case.
>
> Suppose that core_dump_count > core_max_concurrency, and we send, say,
> SIGQUIT to the process. With this patch SIGQUIT suddenly starts to kill
> the single thread, this must not happen.

well, the caller does do_group_exit() after do_coredump(), this kills
sub-threads.

However, this doesn't kill other CLONE_VM tasks. Perhaps this is fine,
but I am not sure.

> If you change the patch to sleep until core_dump_count < core_max_concurrency,
> then, again, we should kill other threads first.

Yes, this is true. If we are going to sleep, we shouldn't allow other
threads to run.

Oleg.

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