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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 11.07.24 06:48, Oscar Salvador wrote: > On Thu, Jul 11, 2024 at 02:15:38AM +0200, David Hildenbrand wrote: >>>> (as a side note, cont-pte/cont-pmd should primarily be a hint from arch code >>>> on how many entries we can batch, like we do in folio_pte_batch(); point is >>>> that we want to batch also on architectures where we don't have such bits, >>>> and prepare for architectures that implement various sizes of batching; >>>> IMHO, having cont-pte/cont-pmd checks in common code is likely the wrong >>>> approach. Again, folio_pte_batch() is where we tackled the problem >>>> differently from the THP perspective) >>> >>> I must say I did not check folio_pte_batch() and I am totally ignorant >>> of what/how it does things. >>> I will have a look. >>> >>>> I have an idea for a better page table walker API that would try batching >>>> most entries (under one PTL), and walkers can just register for the types >>>> they want. Hoping I will find some time to at least scetch the user >>>> interface soon. >>>> >>>> That doesn't mean that this should block your work, but the >>>> cont-pte/cont/pmd hugetlb stuff is really nasty to handle here, and I don't >>>> particularly like where this is going. >>> >>> Ok, let me take a step back then. >>> Previous versions of that RFC did not handle cont-{pte-pmd} wide in the >>> open, so let me go back to the drawing board and come up with something >>> that does not fiddle with cont- stuff in that way. >>> >>> I might post here a small diff just to see if we are on the same page. >>> >>> As usual, thanks a lot for your comments David! >> >> Feel free to reach out to discuss ways forward. I think we should >> >> (a) move to the automatic cont-pte setting as done for THPs via >> set_ptes(). >> (b) Batching PTE updates at all relevant places, so we get no change in >> behavior: cont-pte bit will remain set. >> (c) Likely remove the use of cont-pte bits in hugetlb code for anything >> that is not a present folio (i.e., where automatic cont-pte bit >> setting would never set it). Migration entries might require >> thought (we can easily batch to achieve the same thing, but the >> behavior of hugetlb likely differs to the generic way of handling >> migration entries on multiple ptes: reference the folio vs. >> the respective subpages of the folio). > > Uhm, I see, but I am bit confused. > Although related, this seems orthogonal to this series and more like for > a next-thing to do, right? Well, yes and no. The thing is, that the cont-pte/cont-pmd stuff is not as easy to handle like the PMD/PUD stuff, and sorting that out sounds like some "pain". That's the ugly part of hugetlb, where it's simply ... quite different :( > > It is true that this series tries to handle cont-{pmd,pte} in the > pagewalk api for hugetlb vmas, but in order to raise less eye brows I > can come up with a way not to do that for now, so we do not fiddle with > cont-stuff in this series. > > > Or am I misunderstanding you? I can answer once I know more details about the approach you have in mind :) -- Cheers, David / dhildenb