linux-kernel - Re: [PATCH 2/2 v2, RFC] Driver core: Introduce offline/online callbacks for memory blocks

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Message-ID: <1824290.fKsAJTo9gA@vostro.rjw.lan>
Date:	Tue, 21 May 2013 13:15:54 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Tang Chen <tangchen@...fujitsu.com>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Toshi Kani <toshi.kani@...com>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	isimatu.yasuaki@...fujitsu.com,
	vasilis.liaskovitis@...fitbricks.com, Len Brown <lenb@...nel.org>,
	linux-mm@...ck.org
Subject: Re: [PATCH 2/2 v2, RFC] Driver core: Introduce offline/online callbacks for memory blocks

On Tuesday, May 21, 2013 02:37:53 PM Tang Chen wrote:
> Hi Rafael,
> 
> Please see below.
> 
> On 05/04/2013 07:21 PM, Rafael J. Wysocki wrote:
> ......
> >   static BLOCKING_NOTIFIER_HEAD(memory_chain);
> > @@ -278,33 +283,64 @@ static int __memory_block_change_state(s
> >   {
> >   	int ret = 0;
> >
> > -	if (mem->state != from_state_req) {
> > -		ret = -EINVAL;
> > -		goto out;
> > -	}
> > +	if (mem->state != from_state_req)
> > +		return -EINVAL;
> >
> >   	if (to_state == MEM_OFFLINE)
> >   		mem->state = MEM_GOING_OFFLINE;
> >
> >   	ret = memory_block_action(mem->start_section_nr, to_state, online_type);
> > -
> >   	if (ret) {
> >   		mem->state = from_state_req;
> > -		goto out;
> > +	} else {
> > +		mem->state = to_state;
> > +		if (to_state == MEM_ONLINE)
> > +			mem->last_online = online_type;
> 
> Why do we need to remember last online type ?
> 
> And as far as I know, we can obtain which zone a page was in last time it
> was onlined by check page->flags, just like online_pages() does. If we
> use online_kernel or online_movable, the zone boundary will be 
> recalculated.
> So we don't need to remember the last online type.
> 
> Seeing from your patch, I guess memory_subsys_online() can only handle
> online and offline. So mem->last_online is used to remember what user has
> done through the original way to trigger memory hot-remove, right ? And 
> when
> user does it in this new way, it just does the same thing as user does last
> time.
> 
> But I still think we don't need to remember it because if finally you call
> online_pages(), it just does the same thing as last time by default.
> 
> online_pages()
> {
> 	......
> 	if (online_type == ONLINE_KERNEL ......
> 
> 	if (online_type == ONLINE_MOVABLE......
> 
> 	zone = page_zone(pfn_to_page(pfn));
> 
> 	/* Here, the page will be put into the zone which it belong to last 
> time. */

To be honest, it wasn't entirely clear to me that online_pages() would do the
same thing as last time by default.  Suppose, for example, that the previous
online_type was ONLINE_MOVABLE.  How online_pages() is supposed to know that
it should do the move_pfn_zone_right() if we don't tell it to do that?  Or
is that unnecessary, because it's already been done previously?

> 	......
> }
> 
> I just thought of it. Maybe I missed something in your design. Please tell
> me if I'm wrong.

Well, so what should be passed to __memory_block_change_state() in
memory_subsys_online()?  -1?

> Reviewed-by: Tang Chen <tangchen@...fujitsu.com>
> 
> Thanks. :)

Thanks for your comments,
Rafael


-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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