linux-kernel - Re: [Patch] hugetlb: remove user_shm_lock() check from hugetlb_file

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Date:	Wed, 22 Jun 2011 22:32:30 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Américo Wang <xiyou.wangcong@...il.com>
cc:	LKML <linux-kernel@...r.kernel.org>,
	Ravikiran Thirumalai <kiran@...lex86.org>,
	William Irwin <wli@...omorphy.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mel@....ul.ie>
Subject: Re: [Patch] hugetlb: remove user_shm_lock() check from
 hugetlb_file_setup()

On Thu, 23 Jun 2011, Américo Wang wrote:

> > I know this is scheduled for removal and it is quite past due, but I think
> > we need to do some due diligence before just yanking the whole thing out.
> > A printk_once() about some mysterious application using SHM_HUGETLB
> > doesn't seem very helpful in migrating users to start using
> > /proc/sys/vm/hugetlb_shm_group, and suddenly returning -EPERM when
> > attempting it doesn't seem very responsible, despite what
> > Documentation/feature-removal-schedule.txt may say.
> >
> > For context, we just ran into this internally where this warning at one
> > point appeared to be WARN_ON_ONCE().  That seems more attention grabbing
> > because it at least allows users to start understanding what the issue is
> > and who needs to be fixed.
> 
> Well, we put printk_once with KERN_WARNING level, this is already an enough
> warning, the only difference with WARN_ON_ONCE() is this has no backtrace
> which is not important for this case.
> 

NACK.

As explained above, using printk_once() to identify that some unidentified 
application is using SHM_HUGETLB to charge against the mlock rlimit 
through mmap() isn't sufficient notice and _will_ result in userspace 
breakage.  Please convert this to a WARN_ON_ONCE() so the application is 
identified, wait 6-12 months, convert it to a WARN_ON(), wait 6-12 months, 
then remove it.

Thanks.