linux-kernel - Re: [PATCH 2/2] migrate: hugetlb: Check for hugetlb shared PMD in node migration

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Date:   Fri, 27 Jan 2023 17:23:59 +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 2/2] migrate: hugetlb: Check for hugetlb shared PMD in
 node migration

On 26.01.23 23:27, Mike Kravetz wrote:
> migrate_pages/mempolicy semantics state that CAP_SYS_NICE is required
> to move pages shared with another process to a different node.
> page_mapcount > 1 is being used to determine if a hugetlb page is shared.
> However, a hugetlb page will have a mapcount of 1 if mapped by multiple
> processes via a shared PMD.  As a result, hugetlb pages shared by multiple
> processes and mapped with a shared PMD can be moved by a process without
> CAP_SYS_NICE.
> 
> To fix, check for a shared PMD if mapcount is 1.  If a shared PMD is
> found consider the page shared.
> 
> Fixes: e2d8cf405525 ("migrate: add hugepage migration code to migrate_pages()")
> Cc: stable@...r.kernel.org
> Signed-off-by: Mike Kravetz <mike.kravetz@...cle.com>
> ---
>   mm/mempolicy.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 85a34f1f3ab8..72142fbe7652 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -600,7 +600,8 @@ static int queue_pages_hugetlb(pte_t *pte, unsigned long hmask,
>   
>   	/* With MPOL_MF_MOVE, we migrate only unshared hugepage. */
>   	if (flags & (MPOL_MF_MOVE_ALL) ||
> -	    (flags & MPOL_MF_MOVE && page_mapcount(page) == 1)) {
> +	    (flags & MPOL_MF_MOVE && page_mapcount(page) == 1 &&
> +	     !hugetlb_pmd_shared(pte))) {
>   		if (isolate_hugetlb(page, qp->pagelist) &&
>   			(flags & MPOL_MF_STRICT))
>   			/*

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Thanks,

David / dhildenb