linux-kernel - Re: [PATCH 21/46] hugetlb: use struct hugetlb_pte for walk_hugetlb

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Message-ID: <Y78+/wleTEC3gyqu@x1n>
Date:   Wed, 11 Jan 2023 17:58:07 -0500
From:   Peter Xu <peterx@...hat.com>
To:     James Houghton <jthoughton@...gle.com>
Cc:     Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <songmuchun@...edance.com>,
        David Hildenbrand <david@...hat.com>,
        David Rientjes <rientjes@...gle.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Mina Almasry <almasrymina@...gle.com>,
        Zach O'Keefe <zokeefe@...gle.com>,
        Manish Mishra <manish.mishra@...anix.com>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Yang Shi <shy828301@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 21/46] hugetlb: use struct hugetlb_pte for
 walk_hugetlb_range

James,

On Thu, Jan 05, 2023 at 10:18:19AM +0000, James Houghton wrote:
> @@ -751,9 +761,9 @@ static int smaps_hugetlb_range(pte_t *pte, unsigned long hmask,
>  		int mapcount = page_mapcount(page);
>  
>  		if (mapcount >= 2)
> -			mss->shared_hugetlb += huge_page_size(hstate_vma(vma));
> +			mss->shared_hugetlb += hugetlb_pte_size(hpte);
>  		else
> -			mss->private_hugetlb += huge_page_size(hstate_vma(vma));
> +			mss->private_hugetlb += hugetlb_pte_size(hpte);
>  	}
>  	return 0;

One thing interesting I found with hgm right now is mostly everything will
be counted as "shared" here, I think it's because mapcount is accounted
always to the huge page even if mapped in smaller sizes, so page_mapcount()
to a small page should be huge too because the head page mapcount should be
huge.  I'm curious the reasons behind the mapcount decision.

For example, would that risk overflow with head_compound_mapcount?  One 1G
page mapping all 4K takes 0.25M counts, while the limit should be 2G for
atomic_t.  Looks like it's possible.

Btw, are the small page* pointers still needed in the latest HGM design?
Is there code taking care of disabling of hugetlb vmemmap optimization for
HGM?  Or maybe it's not needed anymore for the current design?

Thanks,

-- 
Peter Xu