Date: Tue, 1 Apr 2014 12:26:23 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Davidlohr Bueso <davidlohr@hp.com>
Cc: Manfred Spraul <manfred@colorfullife.com>, aswin@hp.com,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH] ipc,shm: increase default size for shmmax
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On Tue, 01 Apr 2014 10:01:39 -0700 Davidlohr Bueso <davidlohr@hp.com> wrote:

> > > EINVAL A new segment was to be created and size < SHMMIN or size >
> > > SHMMAX, or no new segment was to be created, a segment with given key
> > > existed, but size is greater than the size of that segment.
> > 
> > So their system will act as if they had set SHMMAX=enormous.  What
> > problems could that cause?
> 
> So, just like any sysctl configurable, only privileged users can change
> this value. If we remove this option, users can theoretically create
> huge segments, thus ignoring any custom limit previously set. This is
> what I fear.

What's wrong with that?  Waht are we actually ptoecting the system
from?  tmpfs exhaustion?

> Think of it kind of like mlock's rlimit. And for that
> matter, why does sysctl exist at all, the same would go for the rest of
> the limits.

These things exist to protect the system from intentional or accidental
service denials.  What are the service denials in this case?

> > Look.  The 32M thing is causing problems.  Arbitrarily increasing the
> > arbitrary 32M to an arbitrary 128M won't fix anything - we still have
> > the problem.  Think bigger, please: how can we make this problem go
> > away for ever?
> 
> That's the thing, I don't think we can make it go away without breaking
> userspace.

Still waiting for details!

> I'm not saying that my 4x increase is the correct value, I
> don't think any default value is really correct, as with any other
> hardcoded limits there are pros and cons. That's really why we give
> users the option to change it to the "correct" one via sysctl. All I'm
> saying is that 32mb is just too small for default in today's systems,
> and increasing it is just making a bad situation a tiny bit better.

Let's understand what's preventing us from making it a great deal better.
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