linux-kernel - Re: [RFC PATCH 1/3] hugetlb: skip to end of PT page mapping when pte not present

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Date:   Mon, 30 May 2022 15:56:43 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Muchun Song <songmuchun@...edance.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>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC PATCH 1/3] hugetlb: skip to end of PT page mapping when pte
 not present

Hi, Mike,

On Fri, May 27, 2022 at 03:58:47PM -0700, Mike Kravetz wrote:
> +unsigned long hugetlb_mask_last_hp(struct hstate *h)
> +{
> +	unsigned long hp_size = huge_page_size(h);
> +
> +	if (hp_size == P4D_SIZE)
> +		return PGDIR_SIZE - P4D_SIZE;
> +	else if (hp_size == PUD_SIZE)
> +		return P4D_SIZE - PUD_SIZE;
> +	else if (hp_size == PMD_SIZE)
> +		return PUD_SIZE - PMD_SIZE;
> +
> +	return ~(0);
> +}

How about:

unsigned long hugetlb_mask_last_hp(struct hstate *h)
{
	unsigned long hp_size = huge_page_size(h);

	return hp_size * (PTRS_PER_PTE - 1);
}

?

This is definitely a good idea, though I'm wondering the possibility to go
one step further to make hugetlb pgtable walk just like the normal pages.

Say, would it be non-trivial to bring some of huge_pte_offset() into the
walker functions, so that we can jump over even larger than PTRS_PER_PTE
entries (e.g. when p4d==NULL for 2m huge pages)?  It's very possible I
overlooked something, though.

Thanks,

-- 
Peter Xu