On Mon, Feb 14, 2022 at 07:26:49PM +0000, Chris Mason wrote: > > > > On Feb 14, 2022, at 2:05 PM, Paul E. McKenney <paulmck@xxxxxxxxxx> wrote: > > > > Experimental. Not for inclusion. Yet, anyway. > > > > Freeing large numbers of namespaces in quick succession can result in > > a bottleneck on the synchronize_rcu() invoked from kern_unmount(). > > This patch applies the synchronize_rcu_expedited() hammer to allow > > further testing and fault isolation. > > > > Hey, at least there was no need to change the comment! ;-) > > > > I don’t think this will be fast enough. I think the problem is that commit e1eb26fa62d04ec0955432be1aa8722a97cb52e7 is putting all of the ipc namespace frees onto a list, and every free includes one call to synchronize_rcu() > > The end result is that we can create new namespaces much much faster than we can free them, and eventually we run out. I found this while debugging clone() returning ENOSPC because create_ipc_ns() was returning ENOSPC. Moving from synchronize_rcu() to synchronize_rcu_expedited() does buy you at least an order of magnitude. But yes, it should be possible to get rid of all but one call per batch, which would be better. Maybe a bit more complicated, but probably not that much. Let me see what I can come up with. If this is an emergency, I still suggest trying the patch as a short-term workaround. Thanx, Paul
