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    SubjectRe: [PATCH 2/2] migrate: hugetlb: Check for hugetlb shared PMD in node migration
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    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@vger.kernel.org
    > Signed-off-by: Mike Kravetz <mike.kravetz@oracle.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@redhat.com>

    --
    Thanks,

    David / dhildenb

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