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    SubjectRe: [PATCH v8 04/10] mm: thp: Support allocation of anonymous multi-size THP
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    >>
    >> Right, but you know from the first loop which order is applicable (and will be
    >> fed to the second loop) and could just pte_unmap(pte) + tryalloc. If that fails,
    >> remap and try with the next orders.
    >
    > You mean something like this?
    >
    > pte = pte_offset_map(vmf->pmd, vmf->address & PMD_MASK);
    > if (!pte)
    > return ERR_PTR(-EAGAIN);
    >
    > order = highest_order(orders);
    > while (orders) {
    > addr = ALIGN_DOWN(vmf->address, PAGE_SIZE << order);
    > if (!pte_range_none(pte + pte_index(addr), 1 << order)) {
    > order = next_order(&orders, order);
    > continue;
    > }
    >
    > pte_unmap(pte);
    >
    > folio = vma_alloc_folio(gfp, order, vma, addr, true);
    > if (folio) {
    > clear_huge_page(&folio->page, vmf->address, 1 << order);
    > return folio;
    > }
    >
    > pte = pte_offset_map(vmf->pmd, vmf->address & PMD_MASK);
    > if (!pte)
    > return ERR_PTR(-EAGAIN);
    >
    > order = next_order(&orders, order);
    > }
    >
    > pte_unmap(pte);
    >
    > I don't really like that because if high order folio allocations fail, then you
    > are calling pte_range_none() again for the next lower order; once that check has
    > succeeded for an order it shouldn't be required for any lower orders. In this
    > case you also have lots of pte map/unmap.

    I see what you mean.

    >
    > The original version feels more efficient to me.
    Yes it is. Adding in some comments might help, like

    /*
    * Find the largest order where the aligned range is completely prot_none(). Note
    * that all remaining orders will be completely prot_none().
    */
    ...

    /* Try allocating the largest of the remaining orders. */

    >
    >>
    >> That would make the code certainly easier to understand. That "orders" magic of
    >> constructing, filtering, walking is confusing :)
    >>
    >>
    >> I might find some time today to see if there is an easy way to cleanup all what
    >> I spelled out above. It really is a mess. But likely that cleanup could be
    >> deferred (but you're touching it, so ... :) ).
    >
    > I'm going to ignore the last 5 words. I heard the "that cleanup could be
    > deferred" part loud and clear though :)

    :)

    If we could stop passing orders into thp_vma_allowable_orders(), that would probably
    be the biggest win. It's just all a confusing mess.

    --
    Cheers,

    David / dhildenb

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