linux-kernel - Re: [PATCH R4 7/7] xen/balloon: Memory hotplug support for Xen balloon driver

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Date:	Thu, 10 Mar 2011 10:02:07 +0100
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 R4 7/7] xen/balloon: Memory hotplug support for Xen balloon driver

On Tue, Mar 08, 2011 at 04:02:19PM -0800, Dave Hansen wrote:
> On Tue, 2011-03-08 at 22:50 +0100, Daniel Kiper wrote:
> > +static int xen_online_page_notifier(struct notifier_block *nb, unsigned long val, void *v)
> > +{
> > +	struct page *page = v;
> > +	unsigned long pfn = page_to_pfn(page);
> > +
> > +	if (pfn >= num_physpages)
> > +		num_physpages = pfn + 1;
> > +
> > +	inc_totalhigh_pages();
> > +
> > +#ifdef CONFIG_FLATMEM
> > +	max_mapnr = max(pfn, max_mapnr);
> > +#endif
>
> I really don't like that this is a direct copy of online_page() up to
> this point.  They're already subtly different.  I'm also curious if this
> breaks on 32-bit kernels because of the unconditional
> inc_totalhigh_pages().
>
> If it's done this way, I'd almost guarantee that the first time someone
> fixes a bug or adds a generic feature in online_page() that Xen gets
> missed.

OK, I rewrite this part of code.

> > +	mutex_lock(&balloon_mutex);
> > +
> > +	__balloon_append(page);
> > +
> > +	if (balloon_stats.hotplug_pages)
> > +		--balloon_stats.hotplug_pages;
> > +	else
> > +		--balloon_stats.balloon_hotplug;
> > +
> > +	mutex_unlock(&balloon_mutex);
> > +
> > +	return NOTIFY_STOP;
> > +}
>
> I'm not a _huge_ fan of these notifier chains, but I guess it works.

Could you tell me why ??? I think that in that case new
(faster, simpler, etc.) mechanism is an overkill. I prefer
to use something which is writen, tested and ready for usage.

> However, if you're going to use these notifier chains, then we probably
> should use them to full effect.  Have a notifier list like this:
>
> 	1. generic online_page()
> 	2. xen_online_page_notifier() (returns NOTIFY_STOP)
> 	3. free_online_page()
>
> Where finish_online_page() does something like this:
>
> finish_online_page(...)
> {
>         ClearPageReserved(page);
>         init_page_count(page);
>         __free_page(page);
> }

OK.

Daniel
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