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

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Message-ID: <CAHGf_=r03QWxw3Jg7BE3z37k4omgo_HRE9qCGw80ngtUD_iEeA@mail.gmail.com>
Date:	Tue, 1 Apr 2014 16:15:14 -0400
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Davidlohr Bueso <davidlohr@...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

>> 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

That's why many open source software continue the silly game. But
again, I don't have knowledge about Fujitsu middleware. I'm waiting
kamezawa-san's answer.
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