Re: Prerequisites for Large Anon Folios

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

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.


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?
> 
> 
>