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Date:	Fri, 18 May 2007 18:05:11 +0200 (MEST)
From:	Andrea Righi <righiandr@...rs.sourceforge.net>
To:	Robin Holt <holt@....com>
Cc:	Rik van Riel <riel@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC] log out-of-virtual-memory events

Andrea Righi wrote:
> Robin Holt wrote:
>> On Fri, May 18, 2007 at 09:50:03AM +0200, Andrea Righi wrote:
>>> Rik van Riel wrote:
>>>> Andrea Righi wrote:
>>>>> I'm looking for a way to keep track of the processes that fail to
>>>>> allocate new
>>>>> virtual memory. What do you think about the following approach
>>>>> (untested)?
>>>> Looks like an easy way for users to spam syslogd over and
>>>> over and over again.
>>>>
>>>> At the very least, shouldn't this be dependant on print_fatal_signals?
>>>>
>>> Anyway, with print-fatal-signals enabled a user could spam syslogd too, simply
>>> with a (char *)0 = 0 program, but we could always identify the spam attempts
>>> logging the process uid...
>>>
>>> In any case, I agree, it should depend on that patch...
>>>
>>> What about adding a simple msleep_interruptible(SOME_MSECS) at the end of
>>> log_vm_enomem() or, at least, a might_sleep() to limit the potential spam/second
>>> rate?
>> An msleep will slow down this process, but do nothing about slowing
>> down the amount of logging.  Simply fork a few more processes and all
>> you are doing with msleep is polluting the pid space.
>>
> 
> Very true.
> 
>> What about a throttling similar to what ia64 does for floating point
>> assist faults (handle_fpu_swa()).  There is a thread flag to not log
>> the events at all.  It is rate throttled globally, but uses per cpu
>> variables for early exits.  This algorithm scaled well to a thousand
>> cpus.
> 
> Actually using printk_ratelimit() should be enough... BTW print_fatal_signals()
> should use it too.
> 

I mean, something like this...

---

Limit the rate of the printk()s in print_fatal_signal() to avoid potential DoS
problems.

Signed-off-by: Andrea Righi <a.righi@...eca.it>

diff -urpN linux-2.6.22-rc1-mm1/kernel/signal.c linux-2.6.22-rc1-mm1-limit-print_fatal_signals-rate/kernel/signal.c
--- linux-2.6.22-rc1-mm1/kernel/signal.c	2007-05-18 17:48:55.000000000 +0200
+++ linux-2.6.22-rc1-mm1-limit-print_fatal_signals-rate/kernel/signal.c	2007-05-18 17:58:13.000000000 +0200
@@ -790,6 +790,9 @@ static void print_vmas(void)
 
 static void print_fatal_signal(struct pt_regs *regs, int signr)
 {
+	if (unlikely(!printk_ratelimit()))
+		return;
+
 	printk("%s/%d: potentially unexpected fatal signal %d.\n",
 		current->comm, current->pid, signr);
 
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