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

[Matthew Wilcox]

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.