Date: Tue, 29 Mar 2011 12:15:41 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Daniel Kiper <dkiper@net-space.pl>
Cc: ian.campbell@citrix.com, andi.kleen@intel.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, rientjes@google.com,
        xen-devel@lists.xensource.com, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
Subject: Re: [PATCH 3/3] mm: Extend memory hotplug API to allow memory
 hotplug in virtual machines
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On Tue, 29 Mar 2011 20:59:13 +0200
Daniel Kiper <dkiper@net-space.pl> wrote:

> > This is a bit strange.  Normally we'll use a notifier chain to tell
> > listeners "hey, X just happened".  But this code is different - it
> > instead uses a notifier chain to tell handlers "hey, do X".  Where in
> > this case, X is "free a page".
> >
> > And this (ab)use of notifiers is not a good fit!  Because we have the
> > obvious problem that if there are three registered noftifiers, we don't
> > want to be freeing the page three times.  Hence the tricks with
> > notifier callout return values.
> >
> > If there are multiple independent notifier handlers, how do we manage
> > their priorities?  And what are the effects of the ordering of the
> > registration calls?
> >
> > And when one callback overrides an existing one, is there any point in
> > leaving the original one installed at all?
> >
> > I dunno, it's all a bit confusing and strange.  Perhaps it would help
> > if you were to explain exactly what behaviour you want here, and we can
> > look to see if there is a more idiomatic way of doing it.
> 
> OK. I am looking for simple generic mechanism which allow runtime
> registration/unregistration of generic or module specific (in that
> case Xen) page onlining function. Dave Hansen sugested compile time
> solution (https://lkml.org/lkml/2011/2/8/235), however, it does not
> fit well in my new project on which I am working on (I am going post
> details at the end of April).

Well, without a complete description of what you're trying to do and
without any indication of what "does not fit well" means, I'm at a bit
of a loss to suggest anything.

If we are assured that only one callback will ever be registered at a
time then a simple

typdef void (*callback_t)(struct page *);

static callback_t g_callback;

int register_callback(callback_t callback)
{
	int ret = -EINVAL;

	lock(some_lock);
	if (g_callback == NULL) {
		g_callback = callback;
		ret = 0;
	}
	unlock(some_lock)
	return ret;
}

would suffice.  That's rather nasty because calls to (*g_callback)
require some_lock.  Use RCU.

> > Also...  I don't think we need (the undocumented)
> > OP_DO_NOT_INCREMENT_TOTAL_COUNTERS and OP_INCREMENT_TOTAL_COUNTERS.
> > Just do
> >
> > void __online_page_increment_counters(struct page *page,
> > 					bool inc_total_counters);
> >
> > and pass it "true" or false".
> 
> What do you think about __online_page_increment_counters()
> (totalram_pages and totalhigh_pages) and
> __online_page_set_limits() (num_physpages and max_mapnr) ???

I don't understand the proposal.
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