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b=ZONf+4+lZGaelMcILIEmfAnw3woDfcDmGEp0gD0f/5Fh/YeEpzOaMzpIN0HuM+sf2 B1fbntNHdm8ZxST2a1XJUZGECinTv07+BuFeAXo1bD/hn8iy7KABuQXE9MQEke60JR 5KcJaYCJduwYSUBIwArYuJMnt3T4tRYuxtBHM8wgY+PnI+yOwpoDeR+WQBCacWG5a8 zFpyjqJXugUVyiCr2/tr2DUsTuFDxyKUNHPbL1/0KDWg5Ze7Ta+l+qYyYXpUjBCVBm +cyegaaOmT+oQL+thPXZ3VtlNZGzYMEu+mvz7YzEdQSJWZdJ+H9N/YRDge+0hb9XLH oOiu0egcXuyyg== Message-ID: <8f60eae1-31ba-4ff4-adf9-951240561f13@kernel.org> Date: Fri, 25 Sep 2026 09:36:26 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 1/2] userfaultfd: clear the inherited uffd bit in move_swap_pte() To: Andrew Morton , Donggeun Yoo 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 References: <20260925042907.2330519-1-donggeunyoo.kernel@gmail.com> <20260925042907.2330519-2-donggeunyoo.kernel@gmail.com> <20260924214811.19ef8af417d37071ed8338c4@linux-foundation.org> 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-Stat-Signature: dy9dry1w7x4f6qrjtxk4hnpzryepeo8m X-Rspam-User: X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 52E2C40009 X-HE-Tag: 1790321794-331130 X-HE-Meta: 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 T0s/aBHt +wmjMZjnd0IRiaVTtVHlQmrKsYS1NDRj3beCvmFPbfpLbuliwvglSNXluVe0xSCqWNmp2DuJsahiTcvLKq6LYs8z06H+TUvgkkndHylvoAy8S0WUHhm4Hst9IxeErZPLcTxiRdHzyReq+J0a3FGJ3/JAoKTk4uHg3lCCWtEmx9bm15IKT9Ftj6xXfHJTrQL96UsACLL8la6XUTm8hholUZyeGRCOs6OAtu+PT9xB0unYvS9L4x5gpfw+swn1VOE9mNoVX8xJTF7+Pgq39pJiFdz7p6IYI9VH1mhBqyPlYM+8VShLOFCCKsdX8ZcspJRkL3S0CPuYhCy58PUyJNkOe7/VfuBWpAEC0nz4z+nJPa73R1BN7xUJy96Q9faWTUM4+JBpHAoTmSVP0LDr9gZxCwGS89w== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 9/25/26 06:48, Andrew Morton wrote: > On Fri, 25 Sep 2026 13:29:06 +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 >> 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