Date: Thu, 19 May 2011 17:04:46 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Daniel Kiper <dkiper@net-space.pl>
Cc: David Rientjes <rientjes@google.com>, ian.campbell@citrix.com,
        haicheng.li@linux.intel.com, fengguang.wu@intel.com, jeremy@goop.org,
        konrad.wilk@oracle.com, dan.magenheimer@oracle.com,
        v.tolstov@selfip.ru, pasik@iki.fi, dave@linux.vnet.ibm.com,
        wdauchy@gmail.com, xen-devel@lists.xensource.com,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH V3 2/2] mm: Extend memory hotplug API to allow memory
 hotplug in virtual machines
Message-Id: <20110519170446.477b39ba.akpm@linux-foundation.org>
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	<20110519204509.GD27202@router-fw-old.local.net-space.pl>
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	<20110519232520.GB28832@router-fw-old.local.net-space.pl>
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On Fri, 20 May 2011 01:25:20 +0200 Daniel Kiper <dkiper@net-space.pl> wrote:

> On Thu, May 19, 2011 at 04:01:43PM -0700, Andrew Morton wrote:
> > On Thu, 19 May 2011 22:45:09 +0200
> > Daniel Kiper <dkiper@net-space.pl> wrote:
> >
> > > On Wed, May 18, 2011 at 08:36:02PM -0700, David Rientjes wrote:
> > > > On Tue, 17 May 2011, Daniel Kiper wrote:
> > > >
> > > > > This patch contains online_page_callback and apropriate functions for
> > > > > setting/restoring online page callbacks. It allows to do some machine
> > > > > specific tasks during online page stage which is required to implement
> > > > > memory hotplug in virtual machines. Additionally, __online_page_set_limits(),
> > > > > __online_page_increment_counters() and __online_page_free() function
> > > > > was added to ease generic hotplug operation.
> > > >
> > > > There are several issues with this.
> > > >
> > > > First, this is completely racy and only allows one global callback to be
> > > > in use at a time without looping, which is probably why you had to pass an
> > >
> > > One callback is allowed by design. Currently I do not see
> > > any real usage for more than one callback.
> >
> > I'd suggest that you try using the notifier.h tools here and remove the
> > restriction.  Sure, we may never use the capability but I expect the
> > code will look nice and simple and once it's done, it's done.
> 
> Hmmm... I am a bit confused. Here https://lkml.org/lkml/2011/3/28/510 you
> was against (ab)using notifiers. Here https://lkml.org/lkml/2011/3/29/313
> you proposed currently implemented solution. Maybe I missed something...
> What should I do now ??? I agree that the code should look nice and simple
> and once it's done, it's done.

Oh, OK, the callback's role is to free a page, so there's no sens in
there ever being more than a single registered callback.

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