linux-kernel - Re: [PATCH V2 2/2] mm: Extend memory hotplug API to allow memory hotplug in virtual machines

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Date:	Tue, 3 May 2011 22:13:34 +0200
From:	Daniel Kiper <dkiper@...-space.pl>
To:	Dave Hansen <dave@...ux.vnet.ibm.com>
Cc:	Daniel Kiper <dkiper@...-space.pl>, ian.campbell@...rix.com,
	akpm@...ux-foundation.org, andi.kleen@...el.com,
	haicheng.li@...ux.intel.com, fengguang.wu@...el.com,
	jeremy@...p.org, konrad.wilk@...cle.com,
	dan.magenheimer@...cle.com, v.tolstov@...fip.ru, pasik@....fi,
	wdauchy@...il.com, rientjes@...gle.com,
	xen-devel@...ts.xensource.com, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH V2 2/2] mm: Extend memory hotplug API to allow memory hotplug in virtual machines

On Tue, May 03, 2011 at 09:25:52AM -0700, Dave Hansen wrote:
> On Mon, 2011-05-02 at 23:49 +0200, Daniel Kiper wrote:
> > +int register_online_page_callback(online_page_callback_t callback)
> > +{
> > +       int rc = -EPERM;
> > +
> > +       lock_memory_hotplug();
> > +
> > +       if (online_page_callback == generic_online_page) {
> > +               online_page_callback = callback;
> > +               rc = 0;
> > +       }
> > +
> > +       unlock_memory_hotplug();
> > +
> > +       return rc;
> > +}
> > +EXPORT_SYMBOL_GPL(register_online_page_callback);
> 
> -EPERM is a bit uninformative here.  How about -EEXIST, plus a printk?

EEXIST means File exists (POSIX.1). It could be misleading. That is why
I decided to use EPERM. I could not find any better choice. I think another
choice is EINVAL (not the best one in my opinion). Additionally, I am not
sure it should have printk. I think it is role of caller to notify (or not)
about possible errors.

> I also don't seen the real use behind having a "register" that can only
> take a single callback.  At worst, it should be
> "set_online_page_callback()" so it's more apparent that there can only
> be one of these.

OK.

> > +int unregister_online_page_callback(online_page_callback_t callback)
> > +{
> > +       int rc = -EPERM;
> > +
> > +       lock_memory_hotplug();
> > +
> > +       if (online_page_callback == callback) {
> > +               online_page_callback = generic_online_page;
> > +               rc = 0;
> > +       }
> > +
> > +       unlock_memory_hotplug();
> > +
> > +       return rc;
> > +}
> > +EXPORT_SYMBOL_GPL(unregister_online_page_callback); 
> 
> Again, -EPERM is a bad code here. -EEXIST, perhaps?  It also deserves a
> WARN_ON() or a printk on failure here.  

Please look above.

> Your changelog doesn't mention, but what ever happened to doing
> something dirt-simple like this?  I have a short memory.

Andrew Morton complained about (ab)use of notifiers. He suggested
to use callback machanism (I could not find any better solution
in Linux Kernel). He convinced me.

Daniel
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