From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 6761238A2B4 for ; Wed, 14 Jan 2026 11:14:48 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1768389288; cv=none; b=EGFz47AmedVlHC8it3vsGPOhH25eYZ0NaFkVqukbtLY1dWGC7c3Pxg3sYBw9999es2rkfCFjxY6oWsSlcYZo1TKyJY80dHm17NYAD/wg+O7e3RMe9JRvCPMIo40MPaHaMbfdhIc2Zhn0xHrUvPgaXkllel9ugeFGz33pC1WtkKs= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1768389288; c=relaxed/simple; bh=l+5wlrp0jYxNQOqBaU6nlefHrlSNjYjWSsYitXtpAv4=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=KILqTCvCTTNtahg8RSdbpD9O3XuXTsOPA4bYkZJcr1PtDYAD8yRbSv9zU1KPOHEyPAIg8bNybClZSmf67CTk+LIxF8way5EjdfY15ZwDQyKMiXYnno7tKiqYIgaJXHh2La01J+eOMiabWFWRyjiPOR3+6432TFNN8JmGd8WU9F4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=dcnZFRw+; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="dcnZFRw+" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 5C92FC4CEF7; Wed, 14 Jan 2026 11:14:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1768389288; bh=l+5wlrp0jYxNQOqBaU6nlefHrlSNjYjWSsYitXtpAv4=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=dcnZFRw+cbJrDxYJFMr5mgj8kXqvSSa1VIr5LTnBjZQEy0DVHrN/4VWgX4+NfgEjG +cL+7K4tRYZu2W0QsbNiDJAZ44zaSQv/FQJsc8f2yaTduI5xb1VcjLb9MXmE9AlUAf dkUBkj5QOqoK2suJkmymaGeJcazZQUTh31uVZCfU0jOgp0m/Ls+q3qAyBBdqd0tCCi Ggk/cFZ3/FBcTBig+blaqW/vgD27pe6mE0mukRh6NlF7NKlQMY1BzxpkcnXcXRpV3L JsJlUPOLmyzGD8Lvf6koD9M6fvYaOVurf9vyLlQ6B9zRFTvNq1CLD1BSd8Wcl0RLk1 z3KmgLiQqI0YQ== Message-ID: <9b43a8ca-e6fb-49f2-9a95-b5088c6d3352@kernel.org> Date: Wed, 14 Jan 2026 12:14:43 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH mm-hotfixes] mm: do not copy page tables unnecessarily for VM_UFFD_WP To: Lorenzo Stoakes , Andrew Morton Cc: "Liam R . Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Pedro Falcato , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Chris Mason References: <20260114110006.1047071-1-lorenzo.stoakes@oracle.com> From: "David Hildenbrand (Red Hat)" 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 B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAa2VybmVsLm9yZz7CwY0EEwEIADcWIQQb2cqtc1xMOkYN/MpN3hD3 AP+DWgUCaKYhwAIbAwUJJlgIpAILCQQVCgkIAhYCAh4FAheAAAoJEE3eEPcA/4Naa5EP/3a1 9sgS9m7oiR0uenlj+C6kkIKlpWKRfGH/WvtFaHr/y06TKnWn6cMOZzJQ+8S39GOteyCCGADh 6ceBx1KPf6/AvMktnGETDTqZ0N9roR4/aEPSMt8kHu/GKR3gtPwzfosX2NgqXNmA7ErU4puf zica1DAmTvx44LOYjvBV24JQG99bZ5Bm2gTDjGXV15/X159CpS6Tc2e3KvYfnfRvezD+alhF XIym8OvvGMeo97BCHpX88pHVIfBg2g2JogR6f0PAJtHGYz6M/9YMxyUShJfo0Df1SOMAbU1Q Op0Ij4PlFCC64rovjH38ly0xfRZH37DZs6kP0jOj4QdExdaXcTILKJFIB3wWXWsqLbtJVgjR YhOrPokd6mDA3gAque7481KkpKM4JraOEELg8pF6eRb3KcAwPRekvf/nYVIbOVyT9lXD5mJn IZUY0LwZsFN0YhGhQJ8xronZy0A59faGBMuVnVb3oy2S0fO1y/r53IeUDTF1wCYF+fM5zo14 5L8mE1GsDJ7FNLj5eSDu/qdZIKqzfY0/l0SAUAAt5yYYejKuii4kfTyLDF/j4LyYZD1QzxLC MjQl36IEcmDTMznLf0/JvCHlxTYZsF0OjWWj1ATRMk41/Q+PX07XQlRCRcE13a8neEz3F6we 08oWh2DnC4AXKbP+kuD9ZP6+5+x1H1zEzsFNBFXLn5EBEADn1959INH2cwYJv0tsxf5MUCgh Cj/CA/lc/LMthqQ773gauB9mN+F1rE9cyyXb6jyOGn+GUjMbnq1o121Vm0+neKHUCBtHyseB fDXHA6m4B3mUTWo13nid0e4AM71r0DS8+KYh6zvweLX/LL5kQS9GQeT+QNroXcC1NzWbitts 6TZ+IrPOwT1hfB4WNC+X2n4AzDqp3+ILiVST2DT4VBc11Gz6jijpC/KI5Al8ZDhRwG47LUiu Qmt3yqrmN63V9wzaPhC+xbwIsNZlLUvuRnmBPkTJwwrFRZvwu5GPHNndBjVpAfaSTOfppyKB Tccu2AXJXWAE1Xjh6GOC8mlFjZwLxWFqdPHR1n2aPVgoiTLk34LR/bXO+e0GpzFXT7enwyvF FFyAS0Nk1q/7EChPcbRbhJqEBpRNZemxmg55zC3GLvgLKd5A09MOM2BrMea+l0FUR+PuTenh 2YmnmLRTro6eZ/qYwWkCu8FFIw4pT0OUDMyLgi+GI1aMpVogTZJ70FgV0pUAlpmrzk/bLbRk F3TwgucpyPtcpmQtTkWSgDS50QG9DR/1As3LLLcNkwJBZzBG6PWbvcOyrwMQUF1nl4SSPV0L LH63+BrrHasfJzxKXzqgrW28CTAE2x8qi7e/6M/+XXhrsMYG+uaViM7n2je3qKe7ofum3s4v q7oFCPsOgwARAQABwsF8BBgBCAAmAhsMFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAmic2qsF CSZYCKEACgkQTd4Q9wD/g1oq0xAAsAnw/OmsERdtdwRfAMpC74/++2wh9RvVQ0x8xXvoGJwZ rk0Jmck1ABIM//5sWDo7eDHk1uEcc95pbP9XGU6ZgeiQeh06+0vRYILwDk8Q/y06TrTb1n4n 7FRwyskKU1UWnNW86lvWUJuGPABXjrkfL41RJttSJHF3M1C0u2BnM5VnDuPFQKzhRRktBMK4 GkWBvXlsHFhn8Ev0xvPE/G99RAg9ufNAxyq2lSzbUIwrY918KHlziBKwNyLoPn9kgHD3hRBa Yakz87WKUZd17ZnPMZiXriCWZxwPx7zs6cSAqcfcVucmdPiIlyG1K/HIk2LX63T6oO2Libzz 7/0i4+oIpvpK2X6zZ2cu0k2uNcEYm2xAb+xGmqwnPnHX/ac8lJEyzH3lh+pt2slI4VcPNnz+ vzYeBAS1S+VJc1pcJr3l7PRSQ4bv5sObZvezRdqEFB4tUIfSbDdEBCCvvEMBgoisDB8ceYxO cFAM8nBWrEmNU2vvIGJzjJ/NVYYIY0TgOc5bS9wh6jKHL2+chrfDW5neLJjY2x3snF8q7U9G EIbBfNHDlOV8SyhEjtX0DyKxQKioTYPOHcW9gdV5fhSz5tEv+ipqt4kIgWqBgzK8ePtDTqRM qZq457g1/SXSoSQi4jN+gsneqvlTJdzaEu1bJP0iv6ViVf15+qHuY5iojCz8fa0= In-Reply-To: <20260114110006.1047071-1-lorenzo.stoakes@oracle.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 1/14/26 12:00, Lorenzo Stoakes wrote: > Commit ab04b530e7e8 ("mm: introduce copy-on-fork VMAs and make > VM_MAYBE_GUARD one") aggregates flags checks in vma_needs_copy(), including > VM_UFFD_WP. > > However in doing so, it incorrectly performed this check against > src_vma. This check was done on the assumption that all relevant flags are > copied upon fork. > > However the userfaultfd logic is very innovative in that it implements > custom logic on fork in dup_userfaultfd(), including a rather well hidden > case where lacking UFFD_FEATURE_EVENT_FORK causes VM_UFFD_WP to not be > propagated to the destination VMA. > > And indeed, vma_needs_copy(), prior to this patch, did check this property > on dst_vma, not src_vma. > > Since all the other relevant flags are copied on fork, we can simply fix > this by checking against dst_vma. > > While we're here, we fix a comment against VM_COPY_ON_FORK (noting that it > did indeed already reference dst_vma) to make it abundantly clear that we > must check against the destination VMA. > > Reported-by: Chris Mason > Closes: https://lore.kernel.org/all/20260113231257.3002271-1-clm@meta.com/ > Fixes: ab04b530e7e8 ("mm: introduce copy-on-fork VMAs and make VM_MAYBE_GUARD one") > Signed-off-by: Lorenzo Stoakes Acked-by: David Hildenbrand (Red Hat) -- Cheers David