Re: [PATCH][RFC] x86, hotplug: Use zero page for monitor when resuming from hibernation

[Peter Zijlstra <peterz@infradead.org>]

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?