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

[Sean Christopherson <seanjc@google.com>]

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.

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