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Fri, 25 Sep 2026 04:47:35 -0400 (EDT) Date: Fri, 25 Sep 2026 09:47:34 +0100 From: Kiryl Shutsemau To: "David Hildenbrand (Arm)" Cc: Donggeun Yoo , 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 Subject: Re: [PATCH v1 1/2] userfaultfd: clear the inherited uffd bit in move_swap_pte() Message-ID: References: <20260919004630.1159895-1-donggeunyoo.kernel@gmail.com> <20260919004630.1159895-2-donggeunyoo.kernel@gmail.com> <90fc724a-08c4-4324-9420-89fab05d53d1@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <90fc724a-08c4-4324-9420-89fab05d53d1@kernel.org> X-Rspam-User: X-Stat-Signature: xxbwukk3wjqz66ods6hkumo6itpfbyco X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: 938DBA0007 X-HE-Tag: 1790326058-609917 X-HE-Meta: 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 8z4hPPRA 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Fri, Sep 25, 2026 at 09:01:22AM +0200, David Hildenbrand (Arm) wrote: > 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 :) It gives the right result, but communicates wrong reasoning to the reader. -- Kiryl Shutsemau / Kirill A. Shutemov