Re: [PATCH v3 09/10] PM / Hibernate: Publish pages restored in-place to arch code

[James Morse <james.morse@arm.com>]

Hi Pavel,

On 05/12/15 09:35, Pavel Machek wrote:
> On Thu 2015-11-26 17:32:47, James Morse wrote:
>> Some architectures require code written to memory as if it were data to be
>> 'cleaned' from any data caches before the processor can fetch them as new
>> instructions.
>>
>> During resume from hibernate, the snapshot code copies some pages directly,
>> meaning these architectures do not get a chance to perform their cache
>> maintenance. Create a new list of pages that were restored in place, so
>> that the arch code can perform this maintenance when necessary.
> 
> Umm. Could the copy function be modified to do the neccessary
> flushing, instead?

The copying is done by load_image_lzo() using memcpy() if you have
compression enabled, and by load_image() using swap_read_page() if you
don't.

I didn't do it here as it would clean every page copied, which was the
worrying part of the previous approach. If there is an architecture
where this cache-clean operation is expensive, it would slow down
restore. I was trying to benchmark the impact of this on 32bit arm when
I spotted it was broken.

This allocated-same-page code path doesn't happen very often, so we
don't want this to have an impact on the 'normal' code path. On 32bit
arm I saw ~20 of these allocations out of ~60,000 pages.

This new way allocates a few extra pages during restore, and doesn't
assume that flush_cache_range() needs calling. It should have no impact
on architectures that aren't using the new list.


> Alternatively, can you just clean the whole cache before jumping to
> the new kernel?

On arm64, cleaning the whole cache means cleaning all of memory by
virtual address, which would be a high price to pay when we only need to
clean the pages we copied. The current implementation does clean all the
page it copies, the problem is the ~0.03% that are copied behind its
back. This patch publishes where those pages are.



Thanks!

James