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Subject: Re: [PATCH v8 3/5] ipc: Allow boot time extension of IPCMNI from 32k
 to 2M
To: Davidlohr Bueso <dave@stgolabs.net>
Cc: "Luis R. Rodriguez" <mcgrof@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Andrew Morton <akpm@linux-foundation.org>,
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        manfred@colorfullife.com
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 <20180817164548.GA32382@linux-r8p5>
From: Waiman Long <longman@redhat.com>
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Date: Fri, 17 Aug 2018 21:15:45 -0400
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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