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

[m.smarduch@samsung.com (Mario Smarduch)]

>> 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?

- Mario