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

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Message-ID: <02ebafee-a295-58cf-044e-e2df0e878e0d@colorfullife.com>
Date:   Tue, 2 Oct 2018 18:32:14 +0200
From:   Manfred Spraul <manfred@...orfullife.com>
To:     Waiman Long <longman@...hat.com>,
        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>
Subject: Re: [PATCH v8 3/5] ipc: Allow boot time extension of IPCMNI from 32k
 to 2M

Hello together,

On 8/18/18 3:15 AM, Waiman Long wrote:
> 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.

- I think we should use the same numbers as Solaris.
Otherwise we later have to touch it again.

- What is the performance when using shmget() with already 10M segments 
present?

- I like the new logic for updating the sequence counter.

Is there a reason why you only enable it for extended mode?

You create a rarely used codepath, and I don't understand what speaks 
against switching to the 'deleted' approach for all systems.


--

     Manfred