linux-kernel - Re: [PATCH RFC fs/namespace] Make kern_unmount() use synchronize_rcu

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Date:   Mon, 14 Feb 2022 11:44:40 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Chris Mason <clm@...com>
Cc:     Giuseppe Scrivano <gscrivan@...hat.com>,
        "riel@...riel.com" <riel@...riel.com>,
        "viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH RFC fs/namespace] Make kern_unmount() use
 synchronize_rcu_expedited()

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@...nel.org> 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