linux-kernel - Re: [PATCH] vfs: Speed up deactivate

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Message-ID: <20120509135914.GD21152@linux.vnet.ibm.com>
Date:	Wed, 9 May 2012 06:59:14 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Nick Piggin <npiggin@...il.com>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Pavel Emelyanov <xemul@...allels.com>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	Louis Rilling <louis.rilling@...labs.com>,
	Mike Galbraith <efault@....de>,
	Christoph Hellwig <hch@...radead.org>,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] vfs: Speed up deactivate_super for non-modular
 filesystems

On Wed, May 09, 2012 at 05:55:57PM +1000, Nick Piggin wrote:
> On 8 May 2012 11:07, Eric W. Biederman <ebiederm@...ssion.com> wrote:
> > "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> writes:

[ . . . ]

> >> Is there anything in there for which synchronous operation is required?
> >> If not, one approach would be to drop the rcu_barrier() calls to a
> >> workqueue or something similar.
> >
> > We need to drain all of the rcu callbacks before we free the slab
> > and unload the module.
> >
> > This actually makes deactivate_locked_super the totally wrong place
> > for the rcu_barrier.  We want the rcu_barrier in the module exit
> > routine where we destroy the inode cache.
> >
> > What I see as the real need is the filesystem modules need to do:
> >        rcu_barrier()
> >        kmem_cache_destroy(cache);
> >
> > Perhaps we can add some helpers to make it easy.  But I think
> > I would be happy today with simply moving the rcu_barrier into
> > every filesystems module exit path, just before the file system
> > module destoryed it's inode cache.
> 
> No, because that's not the only requirement for the rcu_barrier.
> 
> Making it asynchronous is not something I wanted to do, because
> then we potentially have a process exiting from kernel space after
> releasing last reference on a mount, but the mount does not go
> away until "some time" later. Which is crazy.

In any case, I am looking into making concurrent calls to rcu_barrier()
share each others' work, so if asynchronous turns out to be needed,
it will be efficient.

						Thanx, Paul

> However. We are holding vfsmount_lock for read at the point
> where we ever actually do anything with an "rcu-referenced"
> dentry/inode. I wonder if we could use this to get i_sb pinned.
> 

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