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=1794670795; b=cXEvFr6kHZBkhVwXmqhs1FdkM3u2T4tlqGTDc8pg5ocANL9B0IYc1AszVDyd/kt2kUhG kf51aP55ngOw/PTJ8WRofho1fKy5jcwrpigCW2jQZ/2A0ZqWesNrUJVNBmErE/k9hpqhu zeQWt5LDSI20IoYIndukdAEuy9P7MJcfiE7oueyldHRRvPTe5roJlQzPQ/hdwebEQT3aR +XoBtnFGixvlixDmtRQUhHVylRsi+DAds+sPA6f5MHHzT/6EHIzU7PJNjN6qTrlQB2brB pfkNLAy+/LfsGWq3/Z9o/hmM8tRk1v/WxVw3E35yBcjs3zcQTbhbE2eCan4BEn4eaeg== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1794670795; 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=X5eNB7t15oqibkYLEbf3vFkeTE0MI+KiGdQxvQFH3vg=; b=rYNCqleqNxSliQY8tjYHsgoSJN+zw6cowe2/KkradpiXiVQeaEeearVr6pFHW1w6x2Vi J4VQAoFVuJKt1DAm68dKLg7sdpTGQhHLSwn9QqTKJRJEc/v0Emv1sjhBm2GfYkMZluJef sdkTA2MfYVxDDAXAZo7N5SVBGaCapMXakjTBzRbL95Sm7m9aYiFdRsWPVFan2Byu5Nior GhXGRTXTzkcf675iWUPwmyM0Gio9YowQleHVnWj1oQrlu/AbPzuSVtm9ooAIOm1+76CA5 wfahHmMmYhKVOqdLeqGQ4Tbu0pTi7twi4/38pLf7e9qgvo+BNLKiScTZGfjq6GkY4MA== 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=R9IZPu56; 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 CC790606A327; Sat, 14 Nov 2026 15:39:55 +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=R9IZPu56 Received: by smtp.kernel.org (Postfix) with ESMTPSA id 1E2E61F000C4; Sat, 14 Nov 2026 15:39:48 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1794670795; bh=X5eNB7t15oqibkYLEbf3vFkeTE0MI+KiGdQxvQFH3vg=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=R9IZPu56rcOtuEEtMLDxI46lPqueOi50zstru67q8h5p5BHM4vSyaZe8p+7nnG0YW emOitgZWSXZvi8MHAANezIPSoH3rzGObtF177H/gG6ZNm7/YVnUiz1F+sZlxrCBYz1 g+9auJWpWmSQl9aqTQ89LdUUAn1z891utpLgCvPVv3qA49q5tAeKchWnfA7bLERAOi IttAJOudp9wQXqLoJAl9jTG7Iii+j1yjJj29ptRwWcvMDu8v7UbP6qrpNm3fAPNqyK /wJt08rgmxK1t2GfhYxADkh6MrDnRhOINHy4qyNxenT9MncKmBRn6Qif3nRxJtV71K mF2G5t7uBO2mQ== Message-ID: <822c1c7e-bfdf-4570-bf30-08f187e7253d@kernel.org> Date: Sat, 14 Nov 2026 16:39:44 +0100 X-Mailing-List: linux-doc@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 1/2] mm/userfaultfd: prevent poison marker loss To: Jonathan Corbet , Shuah Khan , Randy Dunlap , Andrew Morton , "Liam R. Howlett" , Lorenzo Stoakes , 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 Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-doc@vger.kernel.org, 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-v1-1-82926b22704c@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: <20261107-b4-uffd_marker_loss-v1-1-82926b22704c@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 11/7/26 00:51, David Hildenbrand (Arm) wrote: > Unregistering userfaultfd leaves poison markers in the page tables. > However, once unregistered, hugetlb PMD tables can be shared again. > > Further, shmem page-table retraction can discard poison markers even > while registered in missing mode, and fork() can skip copying page > tables containing poison markers. > > To fix it, add VM_MAYBE_UFFD_POISON and set it before installing poison > markers. Preserve it across unregistration, splits, merges and fork(), > just like we do with guard markers. > > Use the new atomic VMA flag to disable PMD sharing and page-table > retraction, and to force page-table copying during fork(). > > Move VM_UFFD_WP to bit 44 and make HAVE_ARCH_USERFAULTFD_WP depend on > 64BIT, freeing bit 12 for the new flag on both 32-bit and 64-bit systems. > Expose the flag as "up" in smaps. > > Losing poison markers, while unpleasant, is not really fatal, and the > use case is rather a niche thing. In addition, we rely on the atomic > VMA flags infrastructure ... so let's not CC stable. > > Fixes: fc71884a5f59 ("mm: userfaultfd: add new UFFDIO_POISON ioctl") > Fixes: 8a13897fb0da ("mm: userfaultfd: support UFFDIO_POISON for hugetlbfs") > Signed-off-by: David Hildenbrand (Arm) > --- Ping on this one. -- Cheers, David