linux-kernel - Re: [Patch 2/8] ipc/mqueue: switch back to using non-max values on create

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Date:	Tue, 17 Apr 2012 18:32:25 -0400
From:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Doug Ledford <dledford@...hat.com>, linux-kernel@...r.kernel.org,
	kosaki.motohiro@...il.com
Subject: Re: [Patch 2/8] ipc/mqueue: switch back to using non-max values on
 create

(4/17/12 6:17 PM), Andrew Morton wrote:
> On Tue, 17 Apr 2012 11:46:19 -0400
> Doug Ledford<dledford@...hat.com>  wrote:
>
>> Commit b231cca4381ee15ec99afbfb244fbc0324869927 changed
>> how we create a queue that does not include an attr
>> struct passed to open so that it creates the queue
>> with whatever the maximum values are.  However, if the
>> admin has set the maximums to allow flexibility in
>> creating a queue (aka, both a large size and large queue
>> are allowed, but combined they create a queue too large
>> for the RLIMIT_MSGQUEUE of the user), then attempts to
>> create a queue without an attr struct will fail.  Switch
>> back to using acceptable defaults regardless of what
>> the maximums are.
>>
>> Note: so far, we only know of a few applications that rely
>> on this behavior (specifically, set the maximums in /proc,
>> then run the application which calls mq_open() without
>> passing in an attr struct, and the application expects the
>> newly created message queue to have the maximum sizes that
>> were set in /proc used on the mq_open() call, and all of
>> those applications that we know of are actually part of
>> regression test suites that were coded to do something
>> like this:
>>
>> for size in 4096 65536 $((1024 * 1024)) $((16 * 1024 * 1024)); do
>> 	echo $size>  /proc/sys/fs/mqueue/msgsize_max
>> 	mq_open || echo "Error opening mq with size $size"
>> done
>>
>> These test suites that depend on any behavior like this are
>> broken.  The concept that programs should rely upon the
>> system wide maximum in order to get their desired results
>> instead of simply using a attr struct to specify what they
>> want is fundamentally unfriendly programming practice for
>> any multi-tasking OS.
>>
>> Fixing this will break those few apps that we know of (and
>> those app authors recognize the brokenness of their code
>> and the need to fix it).  However, a future patch will allow
>> a workaround in the form of new knobs for the default
>> msg queue creation parameters for any software out there that
>> we don't already know about that might rely on this behavior
>> at the moment.
>
> Here the "future patch" is "mqueue: separate mqueue default value from
> maximum value v2" in this series, yes?
>
> So people who have applications which are broken by this patch will
> need to manually set /proc/sys/fs/mqueue/msg_default and/or
> /proc/sys/fs/mqueue/msgsize_default to get those apps working again?
>

Yes, it works.


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