Date: Tue, 1 Apr 2014 14:29:47 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: KOSAKI Motohiro <kosaki.motohiro@gmail.com>
Cc: Davidlohr Bueso <davidlohr@hp.com>,
        Manfred Spraul <manfred@colorfullife.com>, aswin@hp.com,
        LKML <linux-kernel@vger.kernel.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>
Subject: Re: [PATCH] ipc,shm: increase default size for shmmax
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On Tue, 1 Apr 2014 17:12:50 -0400 KOSAKI Motohiro <kosaki.motohiro@gmail.com> wrote:

> On Tue, Apr 1, 2014 at 5:01 PM, Davidlohr Bueso <davidlohr@hp.com> wrote:
> > On Tue, 2014-04-01 at 15:51 -0400, KOSAKI Motohiro wrote:
> >> >> So, I personally like 0 byte per default.
> >> >
> >> > If by this you mean 0 bytes == unlimited, then I agree. It's less harsh
> >> > then removing it entirely. So instead of removing the limit we can just
> >> > set it by default to 0, and in newseg() if shm_ctlmax == 0 then we don't
> >> > return EINVAL if the passed size is great (obviously), otherwise, if the
> >> > user _explicitly_ set it via sysctl then we respect that. Andrew, do you
> >> > agree with this? If so I'll send a patch.
> >>
> >> Yes, my 0 bytes mean unlimited. I totally agree we shouldn't remove the knob
> >> entirely.
> >
> > Hmmm so 0 won't really work because it could be weirdly used to disable
> > shm altogether... we cannot go to some negative value either since we're
> > dealing with unsigned, and cutting the range in half could also hurt
> > users that set the limit above that. So I was thinking of simply setting
> > SHMMAX to ULONG_MAX and be done with it. Users can then set it manually
> > if they want a smaller value.
> >
> > Makes sense?
> 
> I don't think people use 0 for disabling. but ULONG_MAX make sense to me too.

Distros could have set it to [U]LONG_MAX in initscripts ten years ago
- less phone calls, happier customers.  And they could do so today.

But they haven't.   What are the risks of doing this?
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