Re: Prerequisites for Large Anon Folios

[Ryan Roberts <ryan.roberts@arm.com>]

On 31/08/2023 01:01, Yin, Fengwei wrote:
> 
> 
> On 8/30/2023 6:08 PM, Ryan Roberts wrote:
>> On 24/07/2023 10:33, Yin, Fengwei wrote:
>>>
>>>
>>> On 7/24/2023 5:04 PM, Ryan Roberts wrote:
>>>> On 23/07/2023 13:33, Yin, Fengwei wrote:
>>>>>
>>>>>
>>>>> On 7/20/2023 5:41 PM, Ryan Roberts wrote:
>>>>>> Hi All,
>>>>>>
>>>>>> As discussed at Matthew's call yesterday evening, I've put together a list of
>>>>>> items that need to be done as prerequisites for merging large anonymous folios
>>>>>> support.
>>>>>>
>>>>>> It would be great to get some review and confirmation as to whether anything is
>>>>>> missing or incorrect. Most items have an assignee - in that case it would be
>>>>>> good to check that my understanding that you are working on the item is correct.
>>>>>>
>>>>>> I think most things are independent, with the exception of "shared vs exclusive
>>>>>> mappings", which I think becomes a dependency for a couple of things (marked in
>>>>>> depender description); again would be good to confirm.
>>>>>>
>>>>>> Finally, although I'm concentrating on the prerequisites to clear the path for
>>>>>> merging an MVP Large Anon Folios implementation, I've included one "enhancement"
>>>>>> item ("large folios in swap cache"), solely because we explicitly discussed it
>>>>>> last night. My view is that enhancements can come after the initial large anon
>>>>>> folios merge. Over time, I plan to add other enhancements (e.g. retain large
>>>>>> folios over COW, etc).
>>>>>>
>>>>>> I'm posting the table as yaml as that seemed easiest for email. You can convert
>>>>>> to csv with something like this in Python:
>>>>>>
>>>>>>   import yaml
>>>>>>   import pandas as pd
>>>>>>   pd.DataFrame(yaml.safe_load(open('work-items.yml'))).to_csv('work-items.csv')
>>>>>>
>>>>>> Thanks,
>>>>>> Ryan
>>>>> Should we add the mremap case to the list? Like how to handle the case that mremap
>>>>> happens in the middle of large anonymous folio and fails to split it.
>>>>
>>>> What's the issue that you see here? My opinion is that if we do nothing special
>>>> for mremap(), it neither breaks correctness nor performance when we enable large
>>>> anon folios. So on that basis, its not a prerequisite and I'd rather leave it
>>>> off the list. We might want to do something later as an enhancement though?
>>> The issue is related with anonymous folio->index.
>>>
>>> If mremap happens in the middle of the large folio, current code doesn't split it.
>>> So the large folio will be split to two parts: one is in original place and another
>>> is in the new place. These two parts which are in different VMA have same folio->index.
>>> Can rmap_walk_anon() work with this situation? vma_address() combined with head page.
>>> Can it work for the pages not in same vma as head page?
>>>
>>> I could miss something here. Will try to build test against it.
>>
>> Hi Fengwei,
>>
>> Did you ever reach a conclusion on this? Based on David's comment, I'm assuming
>> this is not a problem and already handled correctly for pte-mapped THP?
> Yes. It's not a real problem.

Great - thanks!

> 
>>
>> I guess vma->vm_pgoff is fixed up in the new vma representing the remapped
>> portion to take account of the offset? (just a guess).
> Yes. vma->vm_pgoff keep unchanged for mremap target vma. So the rmap walk can
> walk the source vma and target vma.
> 
> 
> Regards
> Yin, Fengwei
> 
>>
>> Thanks,
>> Ryan
>>
>>
>>>
>>>
>>> Regards
>>> Yin, Fengwei
>>>
>>>>
>>>> If we could always guarrantee that large anon folios were always naturally
>>>> aligned in VA space, then that would make many things simpler to implement. And
>>>> in that case, I can see the argument for doing something special in mremap().
>>>> But since splitting a folio may fail, I guess we have to live with non-naturally
>>>> aligned folios for the general case, and therefore the simplification argument
>>>> goes out of the window?
>>>>
>>>>
>>>>
>>