linux-kernel - Re: [PATCH] ipc,shm: increase default size for shmmax

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Message-Id: <20140331170546.3b3e72f0.akpm@linux-foundation.org>
Date:	Mon, 31 Mar 2014 17:05:46 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Davidlohr Bueso <davidlohr@...com>
Cc:	Manfred Spraul <manfred@...orfullife.com>, aswin@...com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] ipc,shm: increase default size for shmmax

On Mon, 31 Mar 2014 16:25:32 -0700 Davidlohr Bueso <davidlohr@...com> wrote:

> On Mon, 2014-03-31 at 16:13 -0700, Andrew Morton wrote:
> > On Mon, 31 Mar 2014 15:59:33 -0700 Davidlohr Bueso <davidlohr@...com> wrote:
> > 
> > > > 
> > > > - Shouldn't there be a way to alter this namespace's shm_ctlmax?
> > > 
> > > Unfortunately this would also add the complexity I previously mentioned.
> > 
> > But if the current namespace's shm_ctlmax is too small, you're screwed.
> > Have to shut down the namespace all the way back to init_ns and start
> > again.
> > 
> > > > - What happens if we just nuke the limit altogether and fall back to
> > > >   the next check, which presumably is the rlimit bounds?
> > > 
> > > afaik we only have rlimit for msgqueues. But in any case, while I like
> > > that simplicity, it's too late. Too many workloads (specially DBs) rely
> > > heavily on shmmax. Removing it and relying on something else would thus
> > > cause a lot of things to break.
> > 
> > It would permit larger shm segments - how could that break things?  It
> > would make most or all of these issues go away?
> > 
> 
> So sysadmins wouldn't be very happy, per man shmget(2):
> 
> 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?


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?
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