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

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Date:   Mon, 14 Feb 2022 19:26:49 +0000
From:   Chris Mason <clm@...com>
To:     "Paul E. McKenney" <paulmck@...nel.org>,
        Giuseppe Scrivano <gscrivan@...hat.com>
CC:     "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 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.

-chris