Date: Mon, 18 Feb 2008 15:50:01 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Jens Axboe <jens.axboe@oracle.com>
Cc: Paul Clements <paul.clements@steeleye.com>, randy.dunlap@oracle.com,
       linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/1] NBD: make nbd default to deadline I/O scheduler
Message-Id: <20080218155001.97bf5976.akpm@linux-foundation.org>
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On Mon, 18 Feb 2008 19:16:30 +0100 Jens Axboe <jens.axboe@oracle.com> wrote:

> On Tue, Feb 12 2008, Andrew Morton wrote:
> > On Sat, 09 Feb 2008 08:30:40 -0500
> > Paul Clements <paul.clements@steeleye.com> wrote:
> > 
> > > +		old_e = disk->queue->elevator;
> > > +		if (elevator_init(disk->queue, "deadline") == 0 ||
> > > +			elevator_init(disk->queue, "noop") == 0) {
> > > +				elevator_exit(old_e);
> > > +		}
> > >  	}
> > 
> > afacit elevator_init() will not trigger a request_module().  And you really
> > do want to trigger the request_module() here.  Perhaps the block layer
> > should provide a means of doing so?
> 
> Good point, I think elevator_get() should do that automatically. Does
> this look sane?
> 
> diff --git a/block/elevator.c b/block/elevator.c
> index bafbae0..88318c3 100644
> --- a/block/elevator.c
> +++ b/block/elevator.c
> @@ -134,6 +134,21 @@ static struct elevator_type *elevator_get(const char *name)
>  	spin_lock(&elv_list_lock);
>  
>  	e = elevator_find(name);
> +	if (!e) {
> +		char elv[ELV_NAME_MAX + strlen("-iosched")];
> +
> +		spin_unlock(&elv_list_lock);
> +
> +		if (!strcmp(name, "anticipatory"))
> +			sprintf(elv, "as-iosched");
> +		else
> +			sprintf(elv, "%s-iosched", name);
> +
> +		request_module(elv);
> +		spin_lock(&elv_list_lock);
> +		e = elevator_find(name);
> +	}
> +
>  	if (e && !try_module_get(e->elevator_owner))
>  		e = NULL;

Looks nice and simple.  There might be some of the usual ordering problems
when this is called during boot, maybe is-initramfs-available-yet problems,
etc.  But it's unlikely to make things regress from where they are now.

Should we emit a warning if the desired elevator wasn't available?
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