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

From: Manfred Spraul <manfred@colorfullife.com>
To: Waiman Long <longman@redhat.com>, Davidlohr Bueso <dave@stgolabs.net>
Cc: "Luis R. Rodriguez" <mcgrof@kernel.org>,
	Kees Cook <keescook@chromium.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Jonathan Corbet <corbet@lwn.net>,
	linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-doc@vger.kernel.org, Al Viro <viro@zeniv.linux.org.uk>,
	Matthew Wilcox <willy@infradead.org>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	Takashi Iwai <tiwai@suse.de>, Davidlohr Bueso <dbueso@suse.de>
Subject: Re: [PATCH v8 3/5] ipc: Allow boot time extension of IPCMNI from 32k to 2M
Date: Tue, 2 Oct 2018 18:32:14 +0200	[thread overview]
Message-ID: <02ebafee-a295-58cf-044e-e2df0e878e0d@colorfullife.com> (raw)
In-Reply-To: <5ddb67bb-872b-c8c1-7838-2622195ae1fc@redhat.com>

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