Re: Prerequisites for Large Anon Folios

["Yin, Fengwei" <fengwei.yin@intel.com>]

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.

> 
> 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?
>>>
>>>
>>>
>