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

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Date:	Tue, 01 Apr 2014 13:26:29 -0700
From:	Davidlohr Bueso <davidlohr@...com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Manfred Spraul <manfred@...orfullife.com>, aswin@...com,
	LKML <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"Gotou, Yasunori" <y-goto@...fujitsu.com>,
	chenhanxiao <chenhanxiao@...fujitsu.com>,
	Gao feng <gaofeng@...fujitsu.com>
Subject: Re: [PATCH] ipc,shm: increase default size for shmmax

On Tue, 2014-04-01 at 16:15 -0400, KOSAKI Motohiro wrote:
> >> Our middleware engineers has been complaining about this sysctl limit.
> >> System administrator need to calculate required sysctl value by making sum
> >> of all planned middlewares, and middleware provider needs to write "please
> >> calculate systcl param by....." in their installation manuals.
> >
> > Why aren't people just setting the sysctl to a petabyte?  What problems
> > would that lead to?
> 
> I don't have much Fujitsu middleware knowledges. But I'd like to explain
> very funny bug I saw.
> 
> 1. middleware-A suggest to set SHMMAX to very large value (maybe
> LONG_MAX, but my memory was flushed)
> 2. middleware-B suggest to set SHMMAX to increase some dozen mega byte.
> 
> Finally, it was overflow and didn't work at all.
> 
> Let's demonstrate.
> 
> # echo 18446744073709551615 > /proc/sys/kernel/shmmax
> # cat /proc/sys/kernel/shmmax
> 18446744073709551615
> # echo 18446744073709551616 > /proc/sys/kernel/shmmax
> # cat /proc/sys/kernel/shmmax
> 0

hehe, what a nasty little tunable this is. Reminds me of this:
https://access.redhat.com/site/solutions/16333

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