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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 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 X-Rspam-User: X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 58F4F120019 X-Stat-Signature: srwsqwpa65rfcbmgjip6oygzmn57rrfk X-HE-Tag: 1790319691-697159 X-HE-Meta: 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 kmvQOCwZ f/ORFD+rtDlaDyXpi3lOYctrjTxBsrn4aSD/bzfiRdD++ZCD3LPVFA3g5QAhmfXCBXHXFCGEFzVWqffszW8nVdpNFYFoBMzOarUh22tCoM4gTDCWG+fao3JqAc48/te4IEQLCbkW5hpDbSEMrGToL89DB4h4wSOwNP/Kw+F9Al0N2LfekeP9dCj4WRIznpdk+1V8wVcig6QZB7fjBSorDLBdEUq6yBDT7GuwBONNlhhEKChkmssBTPX2ZVulI0sNN7/gJIqZ4I5WeHKjfjQbpYZWRZnKFl+y/3dFGsTAs3iXIvzEkZrJEmR+M/5r9bZ+FIEyib8b/vriHzd9wrhXW1RPiRx3L+tU5QBLt88tFJ+Qb+DIju7gWWf9iePdx0UEOqbpk7shouyWn05jiMRrBN4kawukEfaJLRiEj4swDAqJAHYQ= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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