linux-kernel - Re: [PATCH 1/4] hugetlb: skip to end of PT page mapping when pte not present

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Date:   Fri, 17 Jun 2022 10:15:34 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-arm-kernel@...ts.infradead.org, linux-s390@...r.kernel.org,
        linux-sh@...r.kernel.org, sparclinux@...r.kernel.org,
        linux-ia64@...r.kernel.org, linux-mips@...r.kernel.org,
        linux-parisc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        Muchun Song <songmuchun@...edance.com>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        Michal Hocko <mhocko@...e.com>,
        Naoya Horiguchi <naoya.horiguchi@...ux.dev>,
        James Houghton <jthoughton@...gle.com>,
        Mina Almasry <almasrymina@...gle.com>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        catalin.marinas@....com, will@...nel.org,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 1/4] hugetlb: skip to end of PT page mapping when pte not
 present

Hi, Mike,

On Thu, Jun 16, 2022 at 02:05:15PM -0700, Mike Kravetz wrote:
> @@ -6877,6 +6896,39 @@ pte_t *huge_pte_offset(struct mm_struct *mm,
>  	return (pte_t *)pmd;
>  }
>  
> +/*
> + * Return a mask that can be used to update an address to the last huge
> + * page in a page table page mapping size.  Used to skip non-present
> + * page table entries when linearly scanning address ranges.  Architectures
> + * with unique huge page to page table relationships can define their own
> + * version of this routine.
> + */
> +unsigned long hugetlb_mask_last_page(struct hstate *h)
> +{
> +	unsigned long hp_size = huge_page_size(h);
> +
> +	switch (hp_size) {
> +	case P4D_SIZE:
> +		return PGDIR_SIZE - P4D_SIZE;
> +	case PUD_SIZE:
> +		return P4D_SIZE - PUD_SIZE;
> +	case PMD_SIZE:
> +		return PUD_SIZE - PMD_SIZE;
> +	default:

Should we add a WARN_ON_ONCE() if it should never trigger?

> +		break; /* Should never happen */
> +	}
> +
> +	return ~(0UL);
> +}
> +
> +#else
> +
> +/* See description above.  Architectures can provide their own version. */
> +__weak unsigned long hugetlb_mask_last_page(struct hstate *h)
> +{
> +	return ~(0UL);

I'm wondering whether it's better to return 0 rather than ~0 by default.
Could an arch with !CONFIG_ARCH_WANT_GENERAL_HUGETLB wrongly skip some
valid address ranges with ~0, or perhaps I misread?

Thanks,

-- 
Peter Xu