From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1794385077; b=vP3BI+YUDXwQ72pzj/hMF5Qq6v0Tz8nwEy1LZTgTx6vx0mJZDvxU4/wmx2RIyKeM9Wly 4v9CPtJBnoTFQXLWdo3NCtrf1GIbKe9nNoJhEyErYszMH0+R3/kQ+jOngXxmqo4ZeIT9w HhjqvDhIpOz3HQ6QNz3w0kekRw3x5uE2dEkuPX8KUTown1KNamHZdAXrj1NrKx3vlYDSI QL3Ii/RXw6vIFrdcJCIfkAbDaqwZtYRbyXvu7ysYFsyMo9ikROTfbGicPA0JVqqKveUcQ xUZRcGN9gOtCXclbXcTNyUq7ObDc6Nh7k1W3oU48rJ+3r497l8Vd4JbwmwCIJARnMWQ== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1794385077; h=Received:DKIM-Signature:Message-ID:Date:MIME-Version:User-Agent: Subject:To:Cc:References:From:Content-Language:Autocrypt:In-Reply-To: Content-Type:Content-Transfer-Encoding; bh=JjqXs3HoQpEHN6otr6mHB8M84/EWiaNnxYY5mSAu4Q0=; b=lyf9/JNNVISEEobd/FkkOYS6lYUhgC6KPJi7kB/55/E1xsM9k7pg7/YNLtPu9TR/+PrC 3SoE6uN+Eyuq6QeQIrsBY7Vd29atVLhz9tFftqcGfoSqeDYdGHBJBmYAHvpcVBgAceDlH /RUah1pExeY8zKCy/+3lveH+/KsjqXpBJuySws7Pg+W4LtmRxSH0h3x7++NOhb2WEIZX6 m6DxAP0I4vpX+RBAqYwLWofCXg0aj6z5LP9iRDoHkUD8JtZO9GbJ+JpTtgttkg3RAJAXZ adaBaLB2myLwhGwxuGYK/Zp8LG3QnmWwbpkxqLk5md7EjoMPH1q/ai2v+bt9C126BzA== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=Z0lg2dMz; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 39B0C606A320; Wed, 11 Nov 2026 08:17:56 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=Z0lg2dMz Received: by smtp.kernel.org (Postfix) with ESMTPSA id B1DBA1F00A05; Wed, 11 Nov 2026 08:17:47 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1794385076; bh=JjqXs3HoQpEHN6otr6mHB8M84/EWiaNnxYY5mSAu4Q0=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=Z0lg2dMzPTK2I/fb78yKkOyDefR41RgDE6wgeC5vIbTKMnC3M7veH0J5PL0PnYO2r UkrfGCByL7Wya/iziyNm/lHZOO9yKX6OgBvNZbZL5ASWu3hoM1UJ+7XTFSGXN3emB+ ivyOa/JbkeIgjkMKGAUZtsx2Cw/AUyKiRfoEoIKmrWUleGk3J+7Xk0bnjcWqrQ3Hc7 f4ZFLrznIqd2imPtCkvhJ6mPRraoc+xDyd7RyX2ECrEXXcEUu9xqzpuoeUgfnqhsL9 6dBC3QVeNEdRJ6p/ZznhT2vi5iihjT712vi0Do7Utqd8Pa5MdMnSqSxhh9D7eAE8YS MDVSudr6R6u7w== Message-ID: Date: Wed, 11 Nov 2026 09:17:42 +0100 X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 2/2] mm/khugepaged: check for empty PTE tables before retracting To: "Lorenzo Stoakes (ARM)" Cc: sashiko-reviews@lists.linux.dev, Jonathan Corbet , Shuah Khan , Randy Dunlap , Andrew Morton , "Liam R. Howlett" , Vlastimil Babka , Jann Horn , Pedro Falcato , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Peter Xu , Steven Rostedt , Masami Hiramatsu , Mathieu Desnoyers , Zi Yan , Baolin Wang , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Kiryl Shutsemau , Axel Rasmussen , linux-mm@kvack.org, linux-trace-kernel@vger.kernel.org References: <20261107-b4-uffd_marker_loss-v1-0-82926b22704c@kernel.org> <20261107-b4-uffd_marker_loss-v1-2-82926b22704c@kernel.org> <28b818cb-f038-43ce-a2e2-a9c97befba09@kernel.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzS5EYXZpZCBIaWxk ZW5icmFuZCAoQ3VycmVudCkgPGRhdmlkQGtlcm5lbC5vcmc+wsGQBBMBCAA6AhsDBQkmWAik AgsJBBUKCQgCFgICHgUCF4AWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaYJt/AIZAQAKCRBN 3hD3AP+DWriiD/9BLGEKG+N8L2AXhikJg6YmXom9ytRwPqDgpHpVg2xdhopoWdMRXjzOrIKD g4LSnFaKneQD0hZhoArEeamG5tyo32xoRsPwkbpIzL0OKSZ8G6mVbFGpjmyDLQCAxteXCLXz ZI0VbsuJKelYnKcXWOIndOrNRvE5eoOfTt2XfBnAapxMYY2IsV+qaUXlO63GgfIOg8RBaj7x 3NxkI3rV0SHhI4GU9K6jCvGghxeS1QX6L/XI9mfAYaIwGy5B68kF26piAVYv/QZDEVIpo3t7 /fjSpxKT8plJH6rhhR0epy8dWRHk3qT5tk2P85twasdloWtkMZ7FsCJRKWscm1BLpsDn6EQ4 jeMHECiY9kGKKi8dQpv3FRyo2QApZ49NNDbwcR0ZndK0XFo15iH708H5Qja/8TuXCwnPWAcJ DQoNIDFyaxe26Rx3ZwUkRALa3iPcVjE0//TrQ4KnFf+lMBSrS33xDDBfevW9+Dk6IISmDH1R HFq2jpkN+FX/PE8eVhV68B2DsAPZ5rUwyCKUXPTJ/irrCCmAAb5Jpv11S7hUSpqtM/6oVESC 3z/7CzrVtRODzLtNgV4r5EI+wAv/3PgJLlMwgJM90Fb3CB2IgbxhjvmB1WNdvXACVydx55V7 LPPKodSTF29rlnQAf9HLgCphuuSrrPn5VQDaYZl4N/7zc2wcWM7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 11/10/26 18:08, Lorenzo Stoakes (ARM) wrote: > On Mon, Nov 09, 2026 at 04:08:23PM +0100, David Hildenbrand (Arm) wrote: >> On 11/8/26 00:51, sashiko-bot@kernel.org wrote: >>> [Severity: Medium] >>> Can removing the VM_COPY_ON_FORK flag checks here cause retract_page_tables() >>> to race with copy_page_range() during fork()? >>> >>> In dup_mmap(), the child VMA is linked into mapping->i_mmap and >>> i_mmap_unlock_write(mapping) is dropped before copy_page_range() runs. >>> vma_needs_copy() copies page tables not only when vma_has_anon_rmap() is set, >>> but also when VM_COPY_ON_FORK flags (VM_MAYBE_GUARD, VM_MAYBE_UFFD_POISON, >>> VM_UFFD_WP, or VM_UFFD_RWP) are set on a file or shmem VMA whose anon_vma is >>> still NULL. >> >> Ah, try_collapse_pte_mapped_thp() runs withe the mmap lock, whereby >> retract_page_tables() only holds the rmap lock. > > [ ... ] > >>> Once the PMD is pmd_none, the next fault in finish_fault() installs a huge >>> PMD before checking userfaultfd_armed(vma): >>> >>> mm/memory.c:finish_fault() { >>> if (pmd_none(*vmf->pmd)) { >>> if (!needs_fallback && folio_test_pmd_mappable(folio)) { >>> ret = do_set_pmd(vmf, folio, page); >>> if (ret != VM_FAULT_FALLBACK) >>> return ret; >>> } >>> ... >>> /* Using per-page fault to maintain the uffd semantics */ >>> if (unlikely(userfaultfd_armed(vma)) || unlikely(needs_fallback)) { >>> nr_pages = 1; >>> } >>> >>> When userspace later issues UFFDIO_RWPROTECT on that file or shmem VMA, >>> change_pmd_range() sees pgtable_split_needed() is true and calls >>> __split_huge_pmd(), which zaps the file huge PMD back to pmd_none without >>> populating a PTE table (since pgtable_populate_needed() is false for >>> MM_CP_UFFD_RWP), so RWP protection is lost and subsequent accesses fault the >>> unprotected huge PMD back in without delivering RWP faults. Installing a >>> huge PMD also causes PAGEMAP_SCAN to report all 512 pages as accessed or >>> written after a single 4KB page fault. >> >> "Since pgtable_populate_needed() is false for MM_CP_UFFD_RWP" I think RWP also >> needs proper uffd marker support, independent of the change here. > > Hey, > > Wondered if you planned to do a respin on this? Want to know whether to delay > review for that or not, thanks! Let's move forward with patch #1 for now. Patch #2 is mostly a cleanup on top that can wai. -- Cheers, David