From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 15DC7126C02; Fri, 25 Sep 2026 07:01:29 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790319691; cv=none; b=Q/QlNGODmKiMf4vlcftpw+YA1cQJe9Cfvq4IWD4OCPuokryMrUGFO41yqGnsFbyAPe/MuWEmyOp0lb3WpTJ3tYDrnEPSsOF8vSRS+nOHAPldHZt4J8QztJgSrhfme7ec98mWuYnzPOxF33nLaLzLaqNiw0QcP0kD3OlXg2SPZXM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790319691; c=relaxed/simple; bh=ybrqyG5jd+0ZNmw6bxWvXnpAFn0qEQAc0SpEc+wlL/w=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=Md7KJNkrgpt6BVtwS+UQqzzaMYChUwCHpiynWc4hHFIcPtNcwrSWBq/C7b8Xhtq2PPEUyRMqkrqlq+AOQffB7TMR2207L+av3VxRrWBDKHeVjn5uN7miJqUvUR2fSn7sOuSJeeQP/bpAQ41qWOYk0tswahC78sOJyF/DaVH3FzQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=ZsFUjKBg; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="ZsFUjKBg" Received: by smtp.kernel.org (Postfix) with ESMTPSA id CCD041F000FF; Fri, 25 Sep 2026 07:01:25 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1790319689; bh=12GQTQF+CbA5YWBSY494mtmthnVLIa76obNaRDLDI/c=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=ZsFUjKBgrpHW8BPAYX+7NWH4Sgkrz1IewWel2Oqzog+gTBQMFg6F22pt+peCYKq3s 0erWR043KfeVfU7c9hbkT5gFacZ2S2SVvoQRngbPCHTQliIyjgMrzNi4YpWpgKEHAW yc6BU1+ypEBxx3O9T0wnjWani0UonVjC9NA8sauVwF1q0j9wdcKY4Q5log9VP3jvPf F+DUFg4l1MOFJwRIOvefvcRyHtzzRgAXJe+34GgThhIulZ4Mz5sFTjAiSxqKyS2Twf vVZewlqy6kOIGqFtCPEeQ36qkpjc9DDEIjbR9E3JdGeHjCxr1BOsxbGFnkAgU9dfah 6cfKfYb3HfnnQ== Message-ID: <90fc724a-08c4-4324-9420-89fab05d53d1@kernel.org> Date: Fri, 25 Sep 2026 09:01:22 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v1 1/2] userfaultfd: clear the inherited uffd bit in move_swap_pte() To: Kiryl Shutsemau , Donggeun Yoo Cc: akpm@linux-foundation.org, 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, linux-mm@kvack.org, linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org References: <20260919004630.1159895-1-donggeunyoo.kernel@gmail.com> <20260919004630.1159895-2-donggeunyoo.kernel@gmail.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 9/23/26 13:18, Kiryl Shutsemau wrote: > On Sat, Sep 19, 2026 at 09:46:29AM +0900, Donggeun Yoo 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 is >> then permanent: the bit is dropped only by change_protection() under >> MM_CP_UFFD_{WP,RWP}_RESOLVE, which uffd_wp_range(), mrwprotect_range() >> and userfaultfd_clear_vma() issue only for a VMA registered in that >> mode. pagemap reports the page as uffd-tracked, and MADV_COLLAPSE >> refuses the range while the bit is set, because collapse_scan_pmd() is >> strict about uffd on swap entries. >> >> move_present_ptes() and move_zeropage_pte() build the destination PTE >> from dst_vma->vm_page_prot and arm RWP only when dst_vma asks for it, so >> the destination's own registration decides the result. move_swap_pte() >> copies the source PTE instead and only ever sets the bit, never clears >> it, so one UFFDIO_MOVE behaves differently depending on whether the page >> happened to be resident. >> >> Clear the uffd bit on the moved swap entry unless the destination is >> RWP-registered, as copy_nonpresent_pte() does where it installs a PTE >> into a destination that may not be armed. A WP-registered destination >> stops inheriting the bit as well, which is already what it gets when the >> moved page is resident. >> >> Fixes: adef440691ba ("userfaultfd: UFFDIO_MOVE uABI") >> Cc: >> Signed-off-by: Donggeun Yoo >> --- >> mm/userfaultfd.c | 2 ++ >> 1 file changed, 2 insertions(+) >> >> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c >> index 74f04c323c50..6495666c596b 100644 >> --- a/mm/userfaultfd.c >> +++ b/mm/userfaultfd.c >> @@ -1452,6 +1452,8 @@ static int move_swap_pte(struct mm_struct *mm, struct vm_area_struct *dst_vma, >> /* Re-arm RWP on the moved swap entry if dst_vma is RWP-registered. */ >> if (userfaultfd_rwp(dst_vma)) >> orig_src_pte = pte_swp_mkuffd(orig_src_pte); >> + else >> + orig_src_pte = pte_swp_clear_uffd(orig_src_pte); > > Putting it in the 'else' is wrong. Clear uffd on the source > unconditionally and re-arm based on the target VMA: > > orig_src_pte = pte_swp_clear_uffd(orig_src_pte); > /* Re-arm RWP on the moved swap entry if dst_vma is RWP-registered. */ > if (userfaultfd_rwp(dst_vma)) > orig_src_pte = pte_swp_mkuffd(orig_src_pte); I'm confused why the original code proposed would be "wrong"? I prefer your way of writing it as well, but I don't understand why it would be "wrong"? Maybe I need more coffee :) -- Cheers, David