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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! To be clear, those others were when we were debating about your and my method, that is why I said it is a waste of time revisiting them :) > >> >> 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---] >>> >> >