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 17:23:39 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Cc:     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>,
        Andrew Morton <akpm@...ux-foundation.org>,
        stable@...r.kernel.org
Subject: Re: [PATCH 1/2] mm: hugetlb: proc: check for hugetlb shared PMD in
 /proc/PID/smaps

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.
> 
> Fixes: 25ee01a2fca0 ("mm: hugetlb: proc: add hugetlb-related fields to /proc/PID/smaps")
> Cc: stable@...r.kernel.org
> Signed-off-by: Mike Kravetz <mike.kravetz@...cle.com>
> ---
>   fs/proc/task_mmu.c      | 10 ++++++++--
>   include/linux/hugetlb.h | 12 ++++++++++++
>   2 files changed, 20 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index e35a0398db63..cb9539879402 100644
> --- 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));



-- 
Thanks,

David / dhildenb