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.
Ahh, I see. So the rmap is broken for large anon folios that have pages mapped
non-contiguously in VA? In that case, I agree that this is a big issue for
correctness and therefore a prerequisite!
Do you have any thoughts for how we could reliably fix this? What are the
reasons that split_folio could fail? Is it an option to copy the contents to new
pages in this case? - I'm guessing not if the folio has the exclusive bit set.
I'm guessing its not really an option to fail the mremap either. What about
waiting for split to succeed - will it succeed eventually, or could it fail
indefinitely? Is there anything we can do to me rmap aware of the discontiguous
large folio and still find the other VAs?
>
>
> 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?
>>
>>
>>
