Re: [PATCH 08/12] cgroups: Pull up res counter charge failure interpretation to caller

From: Andrew Morton <akpm@linux-foundation.org>
To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: LKML <linux-kernel@vger.kernel.org>,
	Paul Menage <paul@paulmenage.org>, Li Zefan <lizf@cn.fujitsu.com>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Aditya Kali <adityakali@google.com>,
	Oleg Nesterov <oleg@redhat.com>,
	Kay Sievers <kay.sievers@vrfy.org>,
	Tim Hockin <thockin@hockin.org>, Tejun Heo <tj@kernel.org>
Subject: Re: [PATCH 08/12] cgroups: Pull up res counter charge failure interpretation to caller
Date: Fri, 9 Sep 2011 08:17:11 -0700	[thread overview]
Message-ID: <20110909081711.d2420255.akpm@linux-foundation.org> (raw)
In-Reply-To: <20110909133316.GB14072@somewhere.redhat.com>

On Fri, 9 Sep 2011 15:33:20 +0200 Frederic Weisbecker <fweisbec@gmail.com> wrote:

> On Tue, Sep 06, 2011 at 03:26:50PM -0700, Andrew Morton wrote:
> > On Tue,  6 Sep 2011 02:13:02 +0200
> > Frederic Weisbecker <fweisbec@gmail.com> wrote:
> > 
> > > res_counter_charge() always returns -ENOMEM when the limit is reached
> > > and the charge thus can't happen.
> > > 
> > > However it's up to the caller to interpret this failure and return
> > > the appropriate error value. The task counter subsystem will need
> > > to report the user that a fork() has been cancelled because of some
> > > limit reached, not because we are too short on memory.
> > > 
> > > Fix this by returning -1 when res_counter_charge() fails.
> > > 
> > > Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
> > > Cc: Paul Menage <paul@paulmenage.org>
> > > Cc: Li Zefan <lizf@cn.fujitsu.com>
> > > Cc: Johannes Weiner <hannes@cmpxchg.org>
> > > Cc: Aditya Kali <adityakali@google.com>
> > > Cc: Oleg Nesterov <oleg@redhat.com>
> > > Cc: Andrew Morton <akpm@linux-foundation.org>
> > > Cc: Kay Sievers <kay.sievers@vrfy.org>
> > > Cc: Tim Hockin <thockin@hockin.org>
> > > Cc: Tejun Heo <tj@kernel.org>
> > > ---
> > >  kernel/res_counter.c |    2 +-
> > >  1 files changed, 1 insertions(+), 1 deletions(-)
> > > 
> > > diff --git a/kernel/res_counter.c b/kernel/res_counter.c
> > > index 4aaa790..45fa6fb 100644
> > > --- a/kernel/res_counter.c
> > > +++ b/kernel/res_counter.c
> > > @@ -26,7 +26,7 @@ int res_counter_charge_locked(struct res_counter *counter, unsigned long val)
> > >  {
> > >  	if (counter->usage + val > counter->limit) {
> > >  		counter->failcnt++;
> > > -		return -ENOMEM;
> > > +		return -1;
> > >  	}
> > >  
> > >  	counter->usage += val;
> > 
> > This also affects the return value of your new and undocumented
> > res_counter_charge_until().
> > 
> > That's a bit of a hand-grenade which could lead to system calls
> > returning -1 (ie: EPERM) to userspace.
> 
> Right. What about making it a boolean?

mmm, not sure.  0/-1 is a reasonable return value for a function which
either did or didn't succeed.  Adding appropriate interface
documentation is a way of reducing the opportunity for making this mistake.