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 16:25:06 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Chen Yu <yu.c.chen@...el.com>
Cc:	x86@...nel.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,
	"Rafael J . Wysocki" <rjw@...ysocki.net>,
	Pavel Machek <pavel@....cz>, Len Brown <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

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?