# Re: \[RFC\]\[PATCH\] ipc: Remove IPCMNI

- From: Manfred Spraul \<manfred@colorfullife.com\>
- Date: 2018-03-29 08:47:48 
- List: linux-api
- Page: https://ratatoskr.run/linux-api/2018/03/133150
- Thread: https://ratatoskr.run/linux-fsdevel/2018/03/133154/t.md

```text
Hello together,

On 03/29/2018 04:14 AM, Davidlohr Bueso wrote:
> Cc'ing mtk, Manfred and linux-api.
>
> See below.
>
> On Thu, 15 Mar 2018, Waiman Long wrote:
>
>> On 03/15/2018 03:00 PM, Eric W. Biederman wrote:
>>> Waiman Long [ref] writes:
>>>
>>>> On 03/14/2018 08:49 PM, Eric W. Biederman wrote:
>>>>> The define IPCMNI was originally the size of a statically sized array in
>>>>> the kernel and that has long since been removed. Therefore there is no
>>>>> fundamental reason for IPCMNI.
>>>>>
>>>>> The only remaining use IPCMNI serves is as a convoluted way to format
>>>>> the ipc id to userspace.  It does not appear that anything except for
>>>>> the CHECKPOINT_RESTORE code even cares about this variety of assignment
>>>>> and the CHECKPOINT_RESTORE code only cares about this weirdness because
>>>>> it has to restore these peculiar ids.
>>>>>
My assumption is that if an array is recreated, it should get a different id.
    a=semget(1234,,);
    semctl(a,,IPC_RMID);
    b=semget(1234,,);
now a!=b.

Rational: semop() calls only refer to the array by the id.
If there is a stale process in the system that tries to access the "old" array and the new array has the same id, then the locking gets corrupted.
>>>>> Therefore make the assignment of ipc ids match the description in
>>>>> Advanced Programming in the Unix Environment and assign the next id
>>>>> until INT_MAX is hit then loop around to the lower ids.
>>>>>
Ok, sounds good.
That way we really cycle through INT_MAX, right now a==b would happen after 128k RMID calls.
>>>>> This can be implemented trivially with the current code using idr_alloc_cyclic.
>>>>>
Is there a performance impact?
Right now, the idr tree is only large if there are lots of objects.
What happens if we have only 1 object, with id=INT_MAX-1?

semop() that do not sleep are fairly fast.
The same applies for msgsnd/msgrcv, if the message is small enough.

@Davidlohr:
Do you know if there are application that frequently call semop() and it doesn't have to sleep?
From the scalability that was pushed into the kernel, I assume that this exists.

I have myself only checked postgresql, and postgresql always sleeps.
(and this was long ago)
>>>>> To make it possible to keep checkpoint/restore working I have renamed
>>>>> the sysctls from xxx_next_id to xxx_nextid.  That is enough change that
>>>>> a smart CRIU implementation can see that what is exported has changed,
>>>>> and act accordingly.  New kernels will be able to restore the old id's.
>>>>>
>>>>> This code still needs some real world testing to verify my assumptions.
>>>>> And some work with the CRIU implementations to actually add the code
>>>>> that deals with the new for of id assignment.
>>>>>
It means that all existing checkpoint/restore application will not work with a new kernel.
Everyone must first update the checkpoint/restore application, then update the kernel.

Is this acceptable?

--
    Manfred
```