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It is possible that >>>> the folio is unmapped fully, but it is unnecessary to add the folio >>>> to deferred split list at all. Fix it by checking folio->_nr_pages_m= apped >>>> before adding a folio to deferred split list. If the folio is alread= y >>>> on the deferred split list, it will be skipped. This issue applies t= o >>>> both PTE-mapped THP and mTHP. >>>> >>>> Commit 98046944a159 ("mm: huge_memory: add the missing >>>> folio_test_pmd_mappable() for THP split statistics") tried to exclud= e >>>> mTHP deferred split stats from THP_DEFERRED_SPLIT_PAGE, but it does = not >>>> fix the above issue. A fully unmapped PTE-mapped order-9 THP was sti= ll >>> >>> Once again: your patch won't fix it either. >>> >>>> added to deferred split list and counted as THP_DEFERRED_SPLIT_PAGE,= >>>> since nr is 512 (non zero), level is RMAP_LEVEL_PTE, and inside >>>> deferred_split_folio() the order-9 folio is folio_test_pmd_mappable(= ). >>>> However, this miscount was present even earlier due to implementatio= n, >>>> since PTEs are unmapped individually and first PTE unmapping adds th= e THP >>>> into the deferred split list. >>> >>> It will still be present. Just less frequently. >> >> OK. Let me reread the email exchanges between you and Yang and clarify= >> the details in the commit log. > > Likely something like: > > -- > In __folio_remove_rmap(), a large folio is added to deferred split list= > if any page in a folio loses its final mapping. But, it is possible tha= t > the folio is now fully unmapped and adding it to the deferred split lis= t is unnecessary. > > For PMD-mapped THPs, that was not really an issue, because removing the= last PMD mapping in the absence of PTE mappings would not have added the= folio to the deferred split queue. > > However, for PTE-mapped THPs, which are now more prominent due to mTHP,= we will always end up adding them to the deferred split queue. > > One side effect of this is that we will frequently increase the THP_DEF= ERRED_SPLIT_PAGE stat for PTE-mapped THP, making it look like we frequent= ly get many partially mapped folios -- although we are simply > unmapping the whole thing stepwise. > > Core-mm will now try batch-unmapping consecutive PTEs of PTE-mapped THP= s where possible. If we're lucky, we unmap the whole thing in one go and = can avoid adding the folio to the deferred split queue, reducing the THP_= DEFERRED_SPLIT_PAGE noise. > > But there will still be noise when we cannot batch-unmap a complete PTE= -mapped folio in one go -- or where this type of batching is not implemen= ted yet. > -- > > Feel free to reuse what you consider reasonable. Sure. Thank you a lot for drafting it! -- Best Regards, Yan, Zi --=_MailMate_8878B8EE-4BC3-4587-A43A-51F5B64E621C_= Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename=signature.asc Content-Type: application/pgp-signature; name=signature.asc -----BEGIN PGP SIGNATURE----- iQJDBAEBCgAtFiEE6rR4j8RuQ2XmaZol4n+egRQHKFQFAmYqc9IPHHppeUBudmlk aWEuY29tAAoJEOJ/noEUByhUme8P/jtVbLpaTivbJaO3O7dHNq0Tu/1N28oJzW4J SJFubeu7wRo2ifTiWBxr1psaqRNBEwJkSRpLN9u7uzVuBKPRTdZ1z2sMNdK85Nx/ ESNHLkMZx2ANF6NXxcT23iYAhvnskG+KBT5ZnrE4u0E3fXsXj3pjoWCyavUUxXtT /Ys/1pJK955crI/ICd+VP4I6GuGq1lLzWL1pBCFPXwsWtUGy8POLbfPjDLA/5Nk+ rWPKQ3j/24NOsWcxpUCM/avBZuegysSU7ob6HQH3g2+DNZiHiipZ19RZafEM6/sg 0BMvgcYY4JGAb9rbSzGXLVwYiFl3sA1+V48q/Kx3R8ItsbmHKqgTbdUpL8TArqHN IbuH0Tv6F0iWFQ+6vGtBxw/XImKXQzzvKM3W5ZHAxtaUgONeXQnYURkM9K/+vjHK 8qwJ4c/T57wavugMgEJXVjtftwlNu8GaZ4pEXMPdrkaLJR1dJgVKBrtGdiLv/mGq vJHQlQentH34/12prNRCWqxA2vR0MNQwWf5U1gzPVsWkiGvmVaeYHPwVjKHKifBv LkVefIlpcZnICxe7RmWKXhRPZjyUogmDvHMRPviETg7kANNXdF12xCzk4E4ELpTT JRlbT+gkP/MHDa6qr8L2oVhG3XPDz0s97Ag9ko5CaG15t3niq/PJRASKJvNsUP2c Qst+FF1R =1uvL -----END PGP SIGNATURE----- --=_MailMate_8878B8EE-4BC3-4587-A43A-51F5B64E621C_=--