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Date:   Thu, 5 Jan 2023 14:23:32 -0800
From:   Mike Kravetz <mike.kravetz@oracle.com>
To:     James Houghton <jthoughton@google.com>
Cc:     Muchun Song <songmuchun@bytedance.com>,
        Peter Xu <peterx@redhat.com>,
        Axel Rasmussen <axelrasmussen@google.com>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] hugetlb: unshare some PMDs when splitting VMAs
Message-ID: <Y7dN5GWQVdG5BvPw@monkey>
References: <20230104231910.1464197-1-jthoughton@google.com>
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On 01/04/23 23:19, James Houghton wrote:
> PMD sharing can only be done in PUD_SIZE-aligned pieces of VMAs;
> however, it is possible that HugeTLB VMAs are split without unsharing
> the PMDs first.
> 
> Without this fix, it is possible to hit the uffd-wp-related WARN_ON_ONCE
> in hugetlb_change_protection [1]. The key there is that
> hugetlb_unshare_all_pmds will not attempt to unshare PMDs in
> non-PUD_SIZE-aligned sections of the VMA.
> 
> It might seem ideal to unshare in hugetlb_vm_op_open, but we need to
> unshare in both the new and old VMAs, so unsharing in
> hugetlb_vm_op_split seems natural.
> 
> [1]: https://lore.kernel.org/linux-mm/CADrL8HVeOkj0QH5VZZbRzybNE8CG-tEGFshnA+bG9nMgcWtBSg@mail.gmail.com/
> 
> Fixes: 6dfeaff93be1 ("hugetlb/userfaultfd: unshare all pmds for hugetlbfs when register wp")
> Signed-off-by: James Houghton <jthoughton@google.com>
> ---
>  mm/hugetlb.c | 44 +++++++++++++++++++++++++++++++++++---------
>  1 file changed, 35 insertions(+), 9 deletions(-)

Thanks for the updates and additional investigation,

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