linux-kernel - Re: [PATCH 1/2] mm: hugetlb: proc: check for hugetlb shared PMD in /proc/PID/smaps

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Date:   Fri, 27 Jan 2023 17:12:05 -0800
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     David Hildenbrand <david@...hat.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Naoya Horiguchi <naoya.horiguchi@...ux.dev>,
        James Houghton <jthoughton@...gle.com>,
        Peter Xu <peterx@...hat.com>, Michal Hocko <mhocko@...e.com>,
        Yang Shi <shy828301@...il.com>,
        Vishal Moola <vishal.moola@...il.com>,
        Matthew Wilcox <willy@...radead.org>,
        Muchun Song <songmuchun@...edance.com>, stable@...r.kernel.org
Subject: Re: [PATCH 1/2] mm: hugetlb: proc: check for hugetlb shared PMD in
 /proc/PID/smaps

On 01/27/23 15:04, Andrew Morton wrote:
> On Fri, 27 Jan 2023 17:23:39 +0100 David Hildenbrand <david@...hat.com> wrote:
> 
> > On 26.01.23 23:27, Mike Kravetz wrote:
> > > A hugetlb page will have a mapcount of 1 if mapped by multiple processes
> > > via a shared PMD.  This is because only the first process increases the
> > > map count, and subsequent processes just add the shared PMD page to
> > > their page table.
> > > 
> > > page_mapcount is being used to decide if a hugetlb page is shared or
> > > private in /proc/PID/smaps.  Pages referenced via a shared PMD were
> > > incorrectly being counted as private.
> > > 
> > > To fix, check for a shared PMD if mapcount is 1.  If a shared PMD is
> > > found count the hugetlb page as shared.  A new helper to check for a
> > > shared PMD is added.
> > > 
> > ...
> >
> > > --- a/fs/proc/task_mmu.c
> > > +++ b/fs/proc/task_mmu.c
> > > @@ -749,8 +749,14 @@ static int smaps_hugetlb_range(pte_t *pte, unsigned long hmask,
> > >   
> > >   		if (mapcount >= 2)
> > >   			mss->shared_hugetlb += huge_page_size(hstate_vma(vma));
> > > -		else
> > > -			mss->private_hugetlb += huge_page_size(hstate_vma(vma));
> > > +		else {
> > 
> > Better:
> > 
> > if (mapcount >= 2 || hugetlb_pmd_shared(pte))
> > 	mss->shared_hugetlb += huge_page_size(hstate_vma(vma));
> > else
> > 	mss->private_hugetlb += huge_page_size(hstate_vma(vma));
> 
> Yup.  And that local doesn't add any value?
> 
> --- a/fs/proc/task_mmu.c~mm-hugetlb-proc-check-for-hugetlb-shared-pmd-in-proc-pid-smaps-fix
> +++ a/fs/proc/task_mmu.c
> @@ -745,18 +745,10 @@ static int smaps_hugetlb_range(pte_t *pt
>  			page = pfn_swap_entry_to_page(swpent);
>  	}
>  	if (page) {
> -		int mapcount = page_mapcount(page);
> -
> -		if (mapcount >= 2)
> +		if (page_mapcount(page) >= 2 || hugetlb_pmd_shared(pte))
>  			mss->shared_hugetlb += huge_page_size(hstate_vma(vma));
> -		else {
> -			if (hugetlb_pmd_shared(pte))
> -				mss->shared_hugetlb +=
> -						huge_page_size(hstate_vma(vma));
> -			else
> -				mss->private_hugetlb +=
> -						huge_page_size(hstate_vma(vma));
> -		}
> +		else
> +			mss->private_hugetlb += huge_page_size(hstate_vma(vma));
>  	}
>  	return 0;
>  }

Thank you both!  That looks much better.

-- 
Mike Kravetz