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Subject: Re: [PATCH v3 4/4] hugetlb/userfaultfd: Unshare all pmds for
 hugetlbfs when register wp
To:     Peter Xu <peterx@redhat.com>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Axel Rasmussen <axelrasmussen@google.com>,
        "Kirill A . Shutemov" <kirill@shutemov.name>,
        Matthew Wilcox <willy@infradead.org>
References: <20210218215434.10203-1-peterx@redhat.com>
 <20210218215555.10710-1-peterx@redhat.com>
From:   Mike Kravetz <mike.kravetz@oracle.com>
Message-ID: <ff2fa34e-2a9b-a386-fd56-426eeac7d429@oracle.com>
Date:   Thu, 18 Feb 2021 14:18:44 -0800
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On 2/18/21 1:55 PM, Peter Xu wrote:
> Huge pmd sharing for hugetlbfs is racy with userfaultfd-wp because
> userfaultfd-wp is always based on pgtable entries, so they cannot be shared.
> 
> Walk the hugetlb range and unshare all such mappings if there is, right before
> UFFDIO_REGISTER will succeed and return to userspace.
> 
> This will pair with want_pmd_share() in hugetlb code so that huge pmd sharing
> is completely disabled for userfaultfd-wp registered range.
> 
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>  fs/userfaultfd.c        |  4 ++++
>  include/linux/hugetlb.h |  3 +++
>  mm/hugetlb.c            | 51 +++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 58 insertions(+)

Thanks,
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
-- 
Mike Kravetz