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Date:   Sat, 19 Feb 2022 06:07:47 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Rik van Riel <riel@surriel.com>
Cc:     linux-kernel@vger.kernel.org, kernel-team@fb.com,
        linux-fsdevel@vger.kernel.org, paulmck@kernel.org,
        gscrivan@redhat.com, Eric Biederman <ebiederm@xmission.com>,
        Chris Mason <clm@fb.com>
Subject: Re: [PATCH 1/2] vfs: free vfsmount through rcu work from kern_unmount
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References: <20220218183114.2867528-1-riel@surriel.com>
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On Sat, Feb 19, 2022 at 05:58:13AM +0000, Al Viro wrote:
> On Sat, Feb 19, 2022 at 05:53:37AM +0000, Al Viro wrote:
> > On Fri, Feb 18, 2022 at 01:31:13PM -0500, Rik van Riel wrote:
> > 
> > >  struct super_block;
> > >  struct vfsmount;
> > > @@ -73,6 +74,7 @@ struct vfsmount {
> > >  	struct super_block *mnt_sb;	/* pointer to superblock */
> > >  	int mnt_flags;
> > >  	struct user_namespace *mnt_userns;
> > > +	struct rcu_work free_rwork;
> > >  } __randomize_layout;
> > 
> > Wait, what?  First of all, that has no business being in vfsmount -
> > everything that deeply internal belongs in struct mount, not in
> > its public part.  Moreover, there's already mount->mnt_rcu, so what's
> > the point duplicating that?
> 
> Argh... You need rcu_work there...
> 
> OK, so make that a member of the same union mnt_rcu is.  In struct mount,
> please.  And I'm not sure I like the idea of shoving that much into
> struct mount, TBH...

We might have a plenty of struct mount instances.  Very few of them will
ever be internal, in the first place.  Fewer yet - using kern_unmount_rcu().
And it's not small.  If anything, I would consider something like
	call_rcu(&m->mnt_rcu, callback)
with callback adding struct mount to llist and doing schedule_delayed_work()
on that.  With work consisting of doing mntput on everything in it.

I'll get some sleep and put together something along those lines tomorrow
morning.