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

[Pavel Machek <pavel@ucw.cz>]

On Mon 2016-06-06 22:19:09, Chen Yu wrote:
> Stress test from Varun Koyyalagunta reports that, the
> nonboot CPU would hang occasionally, when resuming from
> hibernation. Further investigation shows that, the precise
> phase when nonboot CPU hangs, is the time when the nonboot
> CPU been woken up incorrectly, and tries to monitor the
> mwait_ptr for the second time, then an exception is
> triggered due to illegal vaddr access, say, something like,
> 'Unable to handler kernel address of 0xffff8800ba800010...'
> 
> One of the possible scenarios for this issue is illustrated below,
> when the boot CPU tries to resume from hibernation:
> 1. puts the nonboot CPUs offline, so the nonboot CPUs are monitoring
>    at the address of the task_struct.flags.
> 2. boot CPU copies pages to their original address, which includes
>    task_struct.flags, thus wakes up one of the nonboot CPUs.
> 3. nonboot CPU tries to monitor the task_struct.flags again, but since
>    the page table for task_struct.flags has been overwritten by
>    boot CPU, and there is probably a changed across hibernation
>    (because of inconsistence of e820 memory map), an exception is
>    triggered.

If memory map changes between suspend and resume, there'll be fun. If
that's suspected, should we attach md5 sum of e820 to the hibernation
image?

I doubt mwait fix is enough to catch everything in that case...

Best regards,
									Pavel
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