linux-kernel - Re: [PATCH v8 3/5] ipc: Allow boot time extension of IPCMNI from 32k to 2M

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Date:   Fri, 17 Aug 2018 21:15:45 -0400
From:   Waiman Long <longman@...hat.com>
To:     Davidlohr Bueso <dave@...olabs.net>
Cc:     "Luis R. Rodriguez" <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jonathan Corbet <corbet@....net>, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-doc@...r.kernel.org,
        Al Viro <viro@...iv.linux.org.uk>,
        Matthew Wilcox <willy@...radead.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Takashi Iwai <tiwai@...e.de>, Davidlohr Bueso <dbueso@...e.de>,
        manfred@...orfullife.com
Subject: Re: [PATCH v8 3/5] ipc: Allow boot time extension of IPCMNI from 32k
 to 2M

On 08/17/2018 12:45 PM, Davidlohr Bueso wrote:
> Cc'ing Manfred.
>
> On Mon, 18 Jun 2018, Waiman Long wrote:
>
>> The maximum number of unique System V IPC identifiers was limited to
>> 32k.  That limit should be big enough for most use cases.
>>
>> However, there are some users out there requesting for more. To satisfy
>> the need of those users, a new boot time kernel option "ipcmni_extend"
>> is added to extend the IPCMNI value to 2M. This is a 64X increase which
>> hopefully is big enough for them.
>
> Could you please provide more info on the need of these users and how
> you came up with this new value (which just seems quite arbitrary)?
>
> Thanks,
> Davidlohr 

Red Hat has a customer that is migrating from Solaris to Linux. Some of
their applications just happen to use more than 32k of shared memory
segments. I think Solaris allows up to 16M unique ID.

Yes, the amount of increase is a bit arbitrary. I was trying to balance
how many bits should be left for sequence number. Maybe I should just
take 8 more bits for ID and leave 8 bits for sequence number to match
Solaris.

-Longman