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O2R8b0WKj/i0ksDUoXiGWcLWXKGdRvDX4Xp74c4TInTTGrxDgO01yvCJPoDSe+xPlC138UeqAXxtc1Nu+rPjzEGIqMTW2lD91CD+6bT+PAp0CNXWYSEVxXBAtVPnxVgZBZtMnnMUbipVxK9u8/Z4mMY4Atn74cWOYUpjiFtT68RU1lQ3wEN19DvJk32j53cSKy4d6R28UxdJcCoNspM4i8DjwdRIJYHcqRHr/UxCFQpvPzdUqBknff/v/90jobATXFC6X31k8ofG/jFw9ZTsWT4Jn0w== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 7/24/2023 5:46 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. > > 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? All these questions are good questions and I don't have answer. :). I'd like to confirm whether this is an issue for anon large folio first. Regards Yin, Fengwei > >> >> >> 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? >>> >>> >>> >