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wrote: > On Tue, Sep 16, 2025 at 11:06:30AM -0700, Zach O'Keefe wrote: > > So, since we are trying to aim for consistency here, I think we ought > > to also support the anonymous case. > > > > I don't have a patch, but can spot at least two things we'd need to adjust: > > > > First, we are defeated by the check in __thp_vma_allowable_orders(); > > > > /* > > * THPeligible bit of smaps should show 1 for proper VMAs even > > * though anon_vma is not initialized yet. > > * > > * Allow page fault since anon_vma may be not initialized until > > * the first page fault. > > */ > > if (!vma->anon_vma) > > return (smaps || in_pf) ? orders : 0; > > > > I think we can probably just delete that check, but would need to confirm. > > Do you want MADV_COLLAPSE to work on VMAs that never got a page fault? > > I think it should be fine as long as we agree that MADV_COLLAPSE implies > memory population. I think it should, but I want to be sure we are on > the same page. I'm definitely in favour of MADV_COLLAPSE not requiring pre-faulting like that. As long as nothing is assuming that an anon_vma already exists, which surely is not possible as we ignore in smaps/pf case. (Oh how I hate how anon_vma works) Though would this not also impact khugepaged? thp_vma_allowable_order() called with TVA_KHUGEPAGED are in: - khugepaged_enter_vma() - does't matter really, one VMA having anon_vma or not should be immaterial to adding the mm to khugepaged, not hugely likely _none_ would have anon_vma... - khugepaged_scan_mm_slot() - well, we will require at least 1 PTE for this to succeed anyway right? We might waste some time though. But probably not much. hpage_collapse_scan_pmd() -> collapse_huge_page() tries an anon VMA lock, but would have to have found PTEs to succeed... So actually it is fine to just remove the check. > > I also brings a question on holes in the files on MADV_COLLAPSE. We > might want to populate them too. But it means the logic between > MADV_COLLAPSE and khugepaged will diverge. It requires larger > refactoring. I think all of THP requires a larger refactoring :) but interesting point. > > > And second, madvise_collapse() doesn't route SCAN_PMD_NULL to > > collapse_pte_mapped_thp(). I think we just need to audit places where > > we return this code, to make sure it's faithfully describing a > > situation where we can go ahead and install a new pmd. As a hasty > > check, the return codes in check_pmd_state() don't look to follow > > that, with !present and pmd_bad() returning SCAN_PMD_NULL. Likewise, > > there are many underlying failure reasons for > > pte_offset_map_ro_nolock()=>___pte_offset_map() that aren't "no PMD > > entry". > > Sounds like a plan :) There being differences is kind of horrible. So yes indeed, one to look at :) > > -- > Kiryl Shutsemau / Kirill A. Shutemov Cheers, Lorenzo