linux-kernel - Re: [PATCH 2/6] mm/hugetlb: Fix uffd-wp bit lost when unsharing happens

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Message-ID: <a8bc8c64-e436-7bd8-a160-11253cd269ad@redhat.com>
Date:   Fri, 14 Apr 2023 11:23:56 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Peter Xu <peterx@...hat.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Cc:     Axel Rasmussen <axelrasmussen@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        linux-stable <stable@...r.kernel.org>
Subject: Re: [PATCH 2/6] mm/hugetlb: Fix uffd-wp bit lost when unsharing
 happens

On 14.04.23 01:11, Peter Xu wrote:
> When we try to unshare a pinned page for a private hugetlb, uffd-wp bit can
> get lost during unsharing.  Fix it by carrying it over.
> 
> This should be very rare, only if an unsharing happened on a private
> hugetlb page with uffd-wp protected (e.g. in a child which shares the same
> page with parent with UFFD_FEATURE_EVENT_FORK enabled).
> 
> Cc: linux-stable <stable@...r.kernel.org>
> Fixes: 166f3ecc0daf ("mm/hugetlb: hook page faults for uffd write protection")
> Reported-by: Mike Kravetz <mike.kravetz@...cle.com>
> Signed-off-by: Peter Xu <peterx@...hat.com>
> ---
>   mm/hugetlb.c | 7 +++++--
>   1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 7320e64aacc6..083aae35bff8 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -5637,13 +5637,16 @@ static vm_fault_t hugetlb_wp(struct mm_struct *mm, struct vm_area_struct *vma,
>   	spin_lock(ptl);
>   	ptep = hugetlb_walk(vma, haddr, huge_page_size(h));
>   	if (likely(ptep && pte_same(huge_ptep_get(ptep), pte))) {
> +		pte_t newpte = make_huge_pte(vma, &new_folio->page, !unshare);
> +
>   		/* Break COW or unshare */
>   		huge_ptep_clear_flush(vma, haddr, ptep);
>   		mmu_notifier_invalidate_range(mm, range.start, range.end);
>   		page_remove_rmap(old_page, vma, true);
>   		hugepage_add_new_anon_rmap(new_folio, vma, haddr);
> -		set_huge_pte_at(mm, haddr, ptep,
> -				make_huge_pte(vma, &new_folio->page, !unshare));
> +		if (huge_pte_uffd_wp(pte))
> +			newpte = huge_pte_mkuffd_wp(newpte);
> +		set_huge_pte_at(mm, haddr, ptep, newpte);
>   		folio_set_hugetlb_migratable(new_folio);
>   		/* Make the old page be freed below */
>   		new_folio = page_folio(old_page);

LGTM, thanks

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

-- 
Thanks,

David / dhildenb