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

[Manfred Spraul]

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Hi Matthew,

On 11/10/18 8:41 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.

Something like the attached patch?

Unfortunately, idr_alloc_cyclic cannot be used, this creates some 
copy&paste from lib/idr.c to ipc/util.c
[as potential replacement for patch 2 and 3 from the series]

--

ÂÂÂ Manfred