From: "David Hildenbrand (Arm)" <david@kernel.org>
To: Andrew Morton <akpm@linux-foundation.org>,
Donggeun Yoo <donggeunyoo.kernel@gmail.com>
Cc: rppt@kernel.org, peterx@redhat.com, surenb@google.com,
aarcange@redhat.com, ljs@kernel.org, liam@infradead.org,
vbabka@kernel.org, mhocko@suse.com, shuah@kernel.org,
kirill@shutemov.name, linux-mm@kvack.org,
linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org,
stable@vger.kernel.org
Subject: Re: [PATCH v2 1/2] userfaultfd: clear the inherited uffd bit in move_swap_pte()
Date: Fri, 25 Sep 2026 09:36:26 +0200 [thread overview]
Message-ID: <8f60eae1-31ba-4ff4-adf9-951240561f13@kernel.org> (raw)
In-Reply-To: <20260924214811.19ef8af417d37071ed8338c4@linux-foundation.org>
On 9/25/26 06:48, Andrew Morton wrote:
> On Fri, 25 Sep 2026 13:29:06 +0900 Donggeun Yoo <donggeunyoo.kernel@gmail.com> wrote:
>
>> UFFDIO_MOVE on a swapped-out page installs the source PTE at the
>> destination unchanged, so a uffd bit set on the source lands in a
>> destination VMA that was never registered for write protection. It
>> can't be unset there, and THP collapse can't happen either, because a
>> swap entry with the uffd bit set makes the scan bail.
>>
>> I don't think it's intentional because for an unswapped page the bit
>> doesn't come along, and I don't see why being swapped out should change
>> that.
>>
>> Clear the uffd bit on the moved swap entry unless the destination is
>> RWP-registered.
>
> Thanks, but...
>
> When fixing a bug, please always fully describe the userspace-visible
> effects of that bug.
It would also be helpful to describe if this was actually hit in practice so
far. IOW, how relevant is this in practice.
Will this trigger a kernel splat later?
Someone would have to use UFFD_WP/UFFD_RWP with UFFDIO_MOVE, and have
UFFD_WP/UFFD_RWP disabled on the destination vma ... and then work on a swap entry.
Code itself LGTM
--
Cheers,
David
next prev parent reply other threads:[~2026-09-25 7:36 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-09-25 4:29 [PATCH v2 0/2] " Donggeun Yoo
2026-09-25 4:29 ` [PATCH v2 1/2] " Donggeun Yoo
2026-09-25 4:48 ` Andrew Morton
2026-09-25 7:36 ` David Hildenbrand (Arm) [this message]
2026-09-25 4:29 ` [PATCH v2 2/2] selftests/mm: add a test for UFFDIO_MOVE of a write-protected swap entry Donggeun Yoo
2026-09-25 7:41 ` David Hildenbrand (Arm)
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