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 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: List-Subscribe: List-Unsubscribe: On 24 Sep 2025, at 13:05, David Hildenbrand wrote: >>> >>>> >>>> What I can think of is: >>>> 0. split code always does a split to allowed minimal order, >>>> namely max(fs_min_order, order_from_caller); >>> >>> Wouldn't max mean "allowed maximum order" ? >>> >>> I guess what you mean is "split to this order or smaller" -- min? >> >> But LBS imposes a fs_min_order that is not 0. When a caller asks >> to split to 0, folio split code needs to use fs_min_order instead of 0. >> Thus the max. > > I'd say, the point is that if someone wants to split to 0 but that is imp= ossible, then we should fail :) I agree. > >> >>> >>>> 1. if split order cannot reach to order_from_caller, it just return fa= ils, >>>> so most of the caller will know about it; >>> >>> Yes, I think this would be the case here: if we cannot split to order-0= , we can just fail right away. >>> >>>> 2. for LBS code, when it sees a split failure, it should check the res= ulting >>>> folio order against fs min_order. If the orders match, it regards= it as >>>> a success. >>>> >>>> At least, most of the code does not need to be LBS aware. WDYT? >>> >>> Is my understand correct that it's either that the caller wants to >>> >>> (a) Split to order-0 -- no larger folio afterwards. >>> >>> (b) Split to smallest order possible, which might be the mapping min or= der. >> >> Right. IIRC, most of callers are (a), since folio split was originally >> called by code that cannot handle THPs (now large folios). For (b), >> I actually wonder if there exists such a caller. >> >>> If so, we could keep the interface simpler than allowing to specify arb= itrary orders as request. >> >> We might just need (a), since there is no caller of (b) in kernel, excep= t >> split_folio_to_order() is used for testing. There might be future uses >> when kernel wants to convert from THP to mTHP, but it seems that we are >> not there yet. >> > > Even better, then maybe selected interfaces could just fail if the min-or= der contradicts with the request to split to a non-larger (order-0) folio. Yep. Let=E2=80=99s hear what Luis and Pankaj will say about this. > >> >> >> +Luis and Pankaj for their opinions on how LBS is going to use split fol= io >> to any order. >> >> Hi Luis and Pankaj, >> >> It seems that bumping split folio order from 0 to mapping_min_folio_orde= r() >> instead of simply failing the split folio call gives surprises to some >> callers and causes issues like the one reported by this email. I cannot = think >> of any situation where failing a folio split does not work. If LBS code >> wants to split, it should supply mapping_min_folio_order(), right? Does >> such caller exist? >> >> Thanks. >> >> >> Best Regards, >> Yan, Zi >> > > > --=20 > Cheers > > David / dhildenb Best Regards, Yan, Zi