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Each bit >>> represents a utilized region of order KHUGEPAGED_MIN_MTHP_ORDER ptes. If >>> mTHPs are enabled we remove the restriction of max_ptes_none during the >>> scan phase so we dont bailout early and miss potential mTHP candidates. >>> >>> After the scan is complete we will perform binary recursion on the >>> bitmap to determine which mTHP size would be most efficient to collapse >>> to. max_ptes_none will be scaled by the attempted collapse order to >>> determine how full a THP must be to be eligible. >>> >>> If a mTHP collapse is attempted, but contains swapped out, or shared >>> pages, we dont perform the collapse. >>> >>> For non PMD collapse we much leave the anon VMA write locked until after >>> we collapse the mTHP >> Why? I know that Hugh pointed out locking errors; I am yet to catch up >> on that thread, but you need to explain in the description why you do >> what you do. >> >> [--snip---] >> >>> - >>> - spin_lock(pmd_ptl); >>> - BUG_ON(!pmd_none(*pmd)); >>> - folio_add_new_anon_rmap(folio, vma, address, RMAP_EXCLUSIVE); >>> - folio_add_lru_vma(folio, vma); >>> - pgtable_trans_huge_deposit(mm, pmd, pgtable); >>> - set_pmd_at(mm, address, pmd, _pmd); >>> - update_mmu_cache_pmd(vma, address, pmd); >>> - deferred_split_folio(folio, false); >>> - spin_unlock(pmd_ptl); >>> + if (order == HPAGE_PMD_ORDER) { >>> + pgtable = pmd_pgtable(_pmd); >>> + _pmd = folio_mk_pmd(folio, vma->vm_page_prot); >>> + _pmd = maybe_pmd_mkwrite(pmd_mkdirty(_pmd), vma); >>> + >>> + spin_lock(pmd_ptl); >>> + BUG_ON(!pmd_none(*pmd)); >>> + folio_add_new_anon_rmap(folio, vma, _address, RMAP_EXCLUSIVE); >>> + folio_add_lru_vma(folio, vma); >>> + pgtable_trans_huge_deposit(mm, pmd, pgtable); >>> + set_pmd_at(mm, address, pmd, _pmd); >>> + update_mmu_cache_pmd(vma, address, pmd); >>> + deferred_split_folio(folio, false); >>> + spin_unlock(pmd_ptl); >>> + } else { /* mTHP collapse */ >>> + mthp_pte = mk_pte(&folio->page, vma->vm_page_prot); >>> + mthp_pte = maybe_mkwrite(pte_mkdirty(mthp_pte), vma); >>> + >>> + spin_lock(pmd_ptl); >> Nico, >> >> I've noticed a few occasions where my review comments have not been acknowledged - >> for example, [1]. It makes it difficult to follow up and contributes to some >> frustration on my end. I'd appreciate if you could make sure to respond to >> feedback, even if you are disagreeing with my comments. Thanks! > I'm sorry you feel that way, are there any others? I feel like I've > been pretty good at responding to all comments. I've also been out of > the office for the last month, so keeping up with upstream has been > more difficult, but i'm back now. No issues, there were others but I don't want to waste our time digging them up, when we are on the same page! > > Sorry I never got back to you on that one! I will add the BUG_ON, but > I believe it's unnecessary. Your changeset was focused on different > functionality and it seems that you had a bug in it if you were > hitting that often. In my original reply, when I said "I hit the BUG_ON a lot of times", I meant, during testing. It was quite difficult to extend for non-PMD sized VMAs, and the BUG_ON was getting hit due to rmap reaching the non-isolated folios and somehow installing the PMD again. That is why I say that the BUG_ON is important since it will help us catch bugs early. And we have that for the PMD case anyways so why not for mTHP... > > Cheers, > -- Nico >> >> [1] https://lore.kernel.org/all/08d13445-5ed1-42ea-8aee-c1dbde24407e@arm.com/ >> >> >> [---snip---] >> >