linux-kernel - RE: [PATCH][RFC] x86, hotplug: Use zero page for monitor when resuming from hibernation

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Date:	Mon, 6 Jun 2016 15:59:06 +0000
From:	"Chen, Yu C" <yu.c.chen@...el.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	"x86@...nel.org" <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Ingo Molnar" <mingo@...hat.com>,
	"H . Peter Anvin" <hpa@...or.com>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"Rafael J . Wysocki" <rjw@...ysocki.net>,
	Pavel Machek <pavel@....cz>,
	"Brown, Len" <len.brown@...el.com>, Borislav Petkov <bp@...e.de>,
	Zhu Guihua <zhugh.fnst@...fujitsu.com>,
	Juergen Gross <jgross@...e.com>
Subject: RE: [PATCH][RFC] x86, hotplug: Use zero page for monitor when
 resuming from hibernation

Hi Peter,

> -----Original Message-----
> From: Peter Zijlstra [mailto:peterz@...radead.org]
> Sent: Monday, June 06, 2016 10:25 PM
> To: Chen, Yu C
> Cc: x86@...nel.org; linux-kernel@...r.kernel.org; Thomas Gleixner; Ingo
> Molnar; H . Peter Anvin; linux-pm@...r.kernel.org; Rafael J . Wysocki; Pavel
> Machek; Brown, Len; Borislav Petkov; Zhu Guihua; Juergen Gross
> Subject: Re: [PATCH][RFC] x86, hotplug: Use zero page for monitor when
> resuming from hibernation
> 
> On Mon, Jun 06, 2016 at 10:19:09PM +0800, Chen Yu wrote:
> > @@ -1595,8 +1596,21 @@ static inline void mwait_play_dead(void)
> >  	 * This should be a memory location in a cache line which is
> >  	 * unlikely to be touched by other processors.  The actual
> >  	 * content is immaterial as it is not actually modified in any way.
> > +	 *
> > +	 * However in hibernation resume process, this address could be
> > +	 * touched by BSP when restoring page frames, if the page table
> > +	 * for this address is not coherent across hibernation(due to
> > +	 * inconsistence of e820 memory map), access from APs might
> > +	 * cause exception. So change the mwait address to zero page,
> > +	 * which is located in .bss, in this way we can avoid illegal
> > +	 * access from APs because page table for kernel mapping
> > +	 * of text/data/bss should keeps unchanged according to
> > +	 * hibernation semantic.
> >  	 */
> > -	mwait_ptr = &current_thread_info()->flags;
> > +	if (hibernation_in_resume())
> > +		mwait_ptr = empty_zero_page;
> > +	else
> > +		mwait_ptr = &current_thread_info()->flags;
> 
> Why is this conditional? Is there any case in which the zero page is not also
> correct?
I'm thinking of avoid unnecessary wakeup for normal CPU offline,  for example, 
if one  driver uses  the zero page and access it.