linux-kernel - Re: [PATCH 1/6] mm/hugetlb: Fix uffd-wp during fork()

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Date:   Fri, 14 Apr 2023 15:17:39 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        linux-stable <stable@...r.kernel.org>
Subject: Re: [PATCH 1/6] mm/hugetlb: Fix uffd-wp during fork()

On 04/13/23 19:11, Peter Xu wrote:
> There're a bunch of things that were wrong:
> 
>   - Reading uffd-wp bit from a swap entry should use pte_swp_uffd_wp()
>     rather than huge_pte_uffd_wp().

That was/is quite confusing to me at least.

> 
>   - When copying over a pte, we should drop uffd-wp bit when
>     !EVENT_FORK (aka, when !userfaultfd_wp(dst_vma)).
> 
>   - When doing early CoW for private hugetlb (e.g. when the parent page was
>     pinned), uffd-wp bit should be properly carried over if necessary.
> 
> No bug reported probably because most people do not even care about these
> corner cases, but they are still bugs and can be exposed by the recent unit
> tests introduced, so fix all of them in one shot.
> 
> Cc: linux-stable <stable@...r.kernel.org>
> Fixes: bc70fbf269fd ("mm/hugetlb: handle uffd-wp during fork()")
> Signed-off-by: Peter Xu <peterx@...hat.com>
> ---
>  mm/hugetlb.c | 26 ++++++++++++++++----------
>  1 file changed, 16 insertions(+), 10 deletions(-)

No issues except losing information in pte entry as pointed out by Mika.

-- 
Mike Kravetz