linux-kernel - Re: [PATCH] ipc/shm: Increase the defaults for SHMALL, SHMMAX to infinity

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Date:	Sat, 19 Apr 2014 11:17:53 +0200
From:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:	Manfred Spraul <manfred@...orfullife.com>,
	Davidlohr Bueso <davidlohr@...com>
CC:	mtk.manpages@...il.com, Andrew Morton <akpm@...ux-foundation.org>,
	Davidlohr Bueso <davidlohr.bueso@...com>,
	LKML <linux-kernel@...r.kernel.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	gthelen@...gle.com, aswin@...com, linux-mm@...ck.org
Subject: Re: [PATCH] ipc/shm: Increase the defaults for SHMALL, SHMMAX to
 infinity

On 04/19/2014 10:37 AM, Manfred Spraul wrote:
> On 04/19/2014 08:55 AM, Davidlohr Bueso wrote:
>> On Fri, 2014-04-18 at 11:18 +0200, Manfred Spraul wrote:
>>> - ULONG_MAX is not really infinity, but 18 Exabyte segment size and
>>>    75 Zettabyte total size. This should be enough for the next few weeks.
>>>    (assuming a 64-bit system with 4k pages)
> Note: I found three integer overflows, none of them critical.
> I will send patches, I just must get a 32-bit test setup first.
>>> Risks:
>>> - The patch breaks installations that use "take current value and increase
>>>    it a bit". [seems to exist, http://marc.info/?l=linux-mm&m=139638334330127]
>> This really scares me. The probability of occurrence is now much higher,
>> and not just theoretical. It would legitimately break userspace.
> That's why I mentioned it.
> For shmmax, there is a simple answer: Use TASK_SIZE instead of ULONG_MAX.
> - sufficiently far away from overflow.
> - values beyond TASK_SIZE are useless anyway, you can't map such segments.
> 
> I don't have a good answer for shmall. 1L<<(BITS_PER_LONG-1) is too ugly.
> Any proposals?

If shmmax is TASK_SIZE, would not the existing
#define SHMALL (SHMMAX/getpagesize()*(SHMMNI/16))
suffice?


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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