Re: [PATCH v11 2/3] ipc: Conserve sequence numbers in ipcmni_extend mode

[Waiman Long <longman@redhat.com>]

On 11/10/2018 02:41 AM, Matthew Wilcox wrote:
> On Fri, Nov 09, 2018 at 03:11:31PM -0500, Waiman Long wrote:
>> The mixing in of a sequence number into the IPC IDs is probably to
>> avoid ID reuse in userspace as much as possible. With ipcmni_extend
>> mode, the number of usable sequence numbers is greatly reduced leading
>> to higher chance of ID reuse.
>>
>> To address this issue, we need to conserve the sequence number space
>> as much as possible. Right now, the sequence number is incremented
>> for every new ID created. In reality, we only need to increment the
>> sequence number when one or more IDs have been removed previously to
>> make sure that those IDs will not be reused when a new one is built.
>> This is being done irrespective of the ipcmni mode.
> That's not what I said.  Increment the sequence ID when the cursor wraps,
> not when there's been a deletion.

With non-cyclic idr allocation, the cursor will never wraps back to 0.
It is to the lowest available integer. I can do that with cyclic idr
allocation.

Cheers,
Longman