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

From: Peter Xu <peterx@redhat.com>
To: Mike Kravetz <mike.kravetz@oracle.com>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	Muchun Song <songmuchun@bytedance.com>,
	Michal Hocko <mhocko@suse.com>,
	Naoya Horiguchi <naoya.horiguchi@linux.dev>,
	James Houghton <jthoughton@google.com>,
	Mina Almasry <almasrymina@google.com>,
	"Aneesh Kumar K . V" <aneesh.kumar@linux.vnet.ibm.com>,
	Anshuman Khandual <anshuman.khandual@arm.com>,
	Paul Walmsley <paul.walmsley@sifive.com>,
	Christian Borntraeger <borntraeger@linux.ibm.com>,
	Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [RFC PATCH 1/3] hugetlb: skip to end of PT page mapping when pte not present
Date: Mon, 30 May 2022 15:56:43 -0400	[thread overview]
Message-ID: <YpUhe6BSfflOVz7b@xz-m1.local> (raw)
In-Reply-To: <20220527225849.284839-2-mike.kravetz@oracle.com>

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