Re: [PATCH v6 4/4] add 2nd stage page fault handling during live migration

Mario Smarduch
2014-05-29T19:10:44Z

On 05/29/2014 10:57 AM, Christoffer Dall wrote:
> On Thu, May 29, 2014 at 10:08:07AM -0700, Mario Smarduch wrote:
>>
>>>> So this needs to be cleared up given this is key to logging.
>>>> Cases this code handles during migration -
>>>> 1. huge page fault described above - write protect fault so you breakup
>>>> the huge page.
>>>> 2. All other faults - first time access, pte write protect you again wind up in
>>>> stage2_set_pte().
>>>>
>>>> Am I missing something here?
>>>>
>>>
>>> no, I forgot about the fact that we can take the permission fault now.
>>> Hmm, ok, so either we need to use the original approach of always
>>> splitting up huge pages or we need to just follow the regular huge page
>>> path here and just mark all 512 4K pages dirty in the log, or handle it
>>> in stage2_set_pte().
>>>
>>> I would say go with the most simple appraoch for now (which may be going
>>> back to splitting all pmd_huge() into regular pte's), and we can take a
>>> more careful look in the next patch iteration.
>>>
>>
>> Looking at the overall memslot update architecture and various
>> fail scenarios - user_mem_abort() appears to be the most
>> optimal and reliable place. First Write Protect huge pages after
>> memslots are committed and deal with rest in user_mem_abort().
>>
>> Still need some feedback on the pud_huge() before revising for
>> next iteration?
>>
> Just assume it's not used for now, and that you don't have to consider
> it, and make that assumption clear in the commit message, so it doesn't
> block this work. I have a feeling we need to go through a few
> iterations here, so let's get that rolling.
>
> Thanks.
>
Ok thanks I'm on it now.

- Mario