Re: [RFC PATCH 1/3] mm: hugetlb: Simplify logic in dequeue_hugetlb_folio_vma()

From: Sean Christopherson
Date: Wed Oct 30 2024 - 10:34:49 EST


On Fri, Oct 11, 2024, Ackerley Tng wrote:
> Replace arguments avoid_reserve and chg in dequeue_hugetlb_folio_vma()
> so dequeue_hugetlb_folio_vma() is more understandable.
>
> The new argument, use_hstate_resv, indicates whether the folio to be
> dequeued should be taken from reservations in hstate.
>
> If use_hstate_resv is true, the folio to be dequeued should be taken
> from reservations in hstate and hence h->resv_huge_pages is
> decremented, and the folio is marked so that the reservation is
> restored.
>
> If use_hstate_resv is false, then a folio needs to be taken from the
> pool and hence there must exist available_huge_pages(h), failing
> which, goto err.
>
> The bool use_hstate_resv can be reused within
> dequeue_hugetlb_folio_vma()'s caller, alloc_hugetlb_folio().
>
> No functional changes are intended.
>
> As proof, the original two if conditions
>
> !vma_has_reserves(vma, chg) && !available_huge_pages(h)
>
> and
>
> avoid_reserve && !available_huge_pages(h)
>
> can be combined into
>
> (avoid_reserve || !vma_has_reserves(vma, chg))
> && !available_huge_pages(h).

The period here, and the comma later, are weird.

> Applying de Morgan's theorem on
>
> avoid_reserve || !vma_has_reserves(vma, chg)
>
> yields
>
> !avoid_reserve && vma_has_reserves(vma, chg),
>
> hence the simplification is correct.

Whitespace exists for a reason :-)

---

As proof, the original two if conditions

!vma_has_reserves(vma, chg) && !available_huge_pages(h)

and

avoid_reserve && !available_huge_pages(h)

can be combined into

(avoid_reserve || !vma_has_reserves(vma, chg)) && !available_huge_pages(h)

Applying de Morgan's theorem on

avoid_reserve || !vma_has_reserves(vma, chg)

yields

!avoid_reserve && vma_has_reserves(vma, chg)

hence the simplification is correct.

---