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=1792486600; b=uty4RXD0Xi3xHaQ/1lNPcoN3OXE8IXOfAFJw4rMU7s5egbHR44KVAVThhAlhSFmZd4iJ 6rI4c8peZw138nM8MIB4mpPaZIpvdWeuSc/6cXcKoZDL2jAqiGLvzvK69DqjlW/rGWmVn e51PSulWUUj2S9f8B8eFQWNpwqj8QENnYJBZBzcDGNsQuGgFfeD7IMkS0h7AJuVUfqyEV AXzeHxe2OLLC/KuPd0+lZCY5RAfq3fF9dKV4DjoCnyQ4T8+/KNh5uep8vrWGA4k4UIqdD 387q18xc7N/UAO7w6ZUwkx4h687NTyV+k2Mv+rGen+PwwueZi6oZ5cMSdXNcu3byMaA== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1792486600; 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=McrTiUbg6EZriE8j8VkFoRkisw7c+uzkDaypGtect48=; b=FzRlJDRIoLn7131FOTe8F6qLzydOrpWYtzfFpLHNWTgs0wgnokEXgTTIek+7YX30e8h7 yVJYMnqQEYbAWLWS3AYKf/PZmJvSSYPKFbnA55pCsdHeF6rLhxTKB4jnxXFQMVzIOqlho CaJWOMt1+gfyAihL2Y30QSCoX0IyaqJxAxIDJeCYciWCnvqabRgZdaH56xINx3TywrNTG lFAfOnYjwYcwz99tYcElCJVeakATF0orN9yhifqPLBa+tFIqECx/UYRlSzQRAGmPB25Ik O6pAewYvMlnuGT77ReYKGRVjPUk5zPfNWEkct8FTF3khmhcOEoDimJq95/NTQJIieCA== 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=h7GyUXZP; 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 1FDEA606A319 for ; Tue, 20 Oct 2026 08:56:39 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=h7GyUXZP Received: by smtp.kernel.org (Postfix) with ESMTPSA id 429E01F000C4; Tue, 20 Oct 2026 08:56:37 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1792486599; bh=McrTiUbg6EZriE8j8VkFoRkisw7c+uzkDaypGtect48=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=h7GyUXZPkJOIJO7nSf76sVYqy2RVTUXCQDJ6I8vqZ8YasE+voazj8H17eSgLcq6IE TC96O2bsiATCYbJgc9gXKqJO24FPu+bJ8JjAdyhdhoGFpkaYD394k3yDcmyUl7RIqW NE+Ct72HgraxCgWuCMKxtQBMTIv2gOz+vqRy6iTehEIl4Ed4j1Vmmzw2r6Bukf1bcj ky32EBS6BbjL9N9FCFmfDKDKQV06K7bJ8zTsn416qtNKoijZ0Lne1n+9gh3kOQqopJ uiEElzL+zD76qMPYaUwY5+/WkT5JqeTibBg07KDGsYSVUWp8ndX/GkrVj6Ys1v7Qk2 qr2VVt0RvjN5g== Message-ID: <91eb0dca-773a-4906-b8fb-9ed6854966ec@kernel.org> Date: Tue, 20 Oct 2026 10:56:34 +0200 X-Mailing-List: linux-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] x86/mm: Split a 1G direct mapping that hot-remove only partly covers To: Cong Wang Cc: Cong Wang , linux-kernel@vger.kernel.org, Dave Hansen , Andy Lutomirski , Peter Zijlstra , Thomas Gleixner , Ingo Molnar , Borislav Petkov , "H. Peter Anvin" , x86@kernel.org, Oscar Salvador , Andrew Morton References: <20261019041606.641716-1-xiyou.wangcong@gmail.com> 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: 8bit On 10/19/26 22:32, Cong Wang wrote: > On Mon, Oct 19, 2026 at 3:26 AM David Hildenbrand (Arm) > wrote: >> >> On 10/19/26 06:16, Cong Wang wrote: >>> From: Cong Wang >>> >>> Hot-remove works on memory blocks, 128M and up, while the kernel maps >>> boot memory with 1G pages where it can. Removing a block that sits in >>> such a 1G page covers only part of it, and remove_pud_table() then >>> walks the memory that 1G page maps as if it were a PMD table. The >>> removal succeeds, but it corrupts the first page of the 1G region, may >>> free that page while still in use, and leaves the removed memory >>> mapped. >>> >>> Map the 1G leaf through 2M leaves first, under pgd_lock as >>> set_memory_*() may change it for the rest of the 1G meanwhile. >>> >>> Reproduced in a 3G QEMU guest booted with movablecore=1280M. We used a >>> test module fills the first page of a 1G-mapped region with a known >>> pattern, then calls offline_and_remove_memory() on one 128M block in >>> that region. Before this patch, part of the pattern is zeroed and the >>> 1G page stays. After this patch, the pattern is intact, the block is >>> unmapped, the rest stays mapped through 2M pages, and adding the block >>> back works. >> >> Which in-tree mechanism can trigger this? >> >> I would imagine only DIMM hotremove on bare metal. But let's be real, that is >> never going to be < 1 GiB. >> >> All other mechanisms (including DIMMs in virtual environments) get removed in >> the same granularity they were added. > > You're right. > > The only in-tree path I can see is ACPI hot-remove of boot memory, > where the direct map was built over contiguous E820 RAM and a 1G page > can straddle DIMM boundaries that are not 1G aligned. I agree that is > unlikely on real firmware. > > Where I actually hit it is removing 128M blocks of boot memory > (offline_and_remove_memory()) for the multikernel work, which is not > upstream. Happy to carry this patch downstream. It should definitely not be tagged as a fix. Given that we don't expect in-tree users (and also don't endorse ones that remove arbitrary boot memory they don't actually own), I'm not sure if we want this at this point. I'll leave it up to x86 folks. (IMHO we should, however, definitely warn if we would run into this situation) -- Cheers, David