linux-kernel - Re: [PATCH 3/3] PM / hibernate: Remove the restriction when checking memory size before/after hibernation

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Date:	Fri, 28 Aug 2015 07:37:52 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Chen Yu <yu.c.chen@...el.com>
Cc:	tglx@...utronix.de, mingo@...hat.com, rjw@...ysocki.net,
	pavel@....cz, hpa@...or.com, len.brown@...el.com,
	yinghai@...nel.org, joeyli.kernel@...il.com, rui.zhang@...el.com,
	linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] PM / hibernate: Remove the restriction when checking
 memory size before/after hibernation


* Chen Yu <yu.c.chen@...el.com> wrote:

> Sometimes the resuming of hibernation might fail, because the
> system before/after hibernation have different number of page
> frames, and in current implementation, this situation will be
> regarded as invalud resuming process. However, consider the following
> scenario: The resuming system has a larger memory capacity than
> the one before hibernation, and the former memory region is a
> superset of the latter, it should be allowed to resume. For example,
> someone plugs more DRAMs before resuming from hibernation.
> Here's a case for this situation:
> 
> e820 memory map before hibernation:
> BIOS-e820: [mem 0x0000000020200000-0x0000000077517fff] usable
> BIOS-e820: [mem 0x0000000077518000-0x0000000077567fff] reserved
> 
> e820 memory map during resuming:
> BIOS-e820: [mem 0x0000000020200000-0x000000007753ffff] usable
> BIOS-e820: [mem 0x0000000077540000-0x0000000077567fff] reserved
> 
> In current code, the resuming process will be terminated, because
> they have different memory size(usable region), but actually we should
> let it continue to resume because [0x0000000020200000-0x000000007753ffff]
> is a superset of [0x0000000020200000-0x0000000077517fff].
> 
> This patch removes the constraint that number of page frames should
> be strictly the same before/after hibernation.
> 
> Note: This patch can only work after:
> Commit ec93ef809f34 ("PM / hibernate: avoid unsafe pages in e820
>  reserved regions") applied.
> 
> Signed-off-by: Chen Yu <yu.c.chen@...el.com>
> ---
>  kernel/power/snapshot.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/power/snapshot.c b/kernel/power/snapshot.c
> index c24d5a2..5b1a071 100644
> --- a/kernel/power/snapshot.c
> +++ b/kernel/power/snapshot.c
> @@ -2072,8 +2072,12 @@ static int check_header(struct swsusp_info *info)
>  	char *reason;
>  
>  	reason = check_image_kernel(info);
> -	if (!reason && info->num_physpages != get_num_physpages())
> -		reason = "memory size";
> +	/*
> +	 * No need to check num_physpages with get_num_physpages
> +	 * as we did before(please refer to git log), because
> +	 * is_nosave_page will ensure that each page is safe
> +	 * to be restored.
> +	 */
>  	if (reason) {
>  		printk(KERN_ERR "PM: Image mismatch: %s\n", reason);
>  		return -EPERM;

No, this removes a useful sanity check that protects against data corruption.

If you want to relax it then you should add code that checks whether the 
before/after memory image is truly a superset of each other, and warn and deny the 
hibernation in any other case. (For example when RAM got removed or moved.)

Thanks,

	Ingo
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