From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 E208A5304BC; Thu, 15 Oct 2026 14:14:16 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1792073658; cv=none; b=hean5uL+z+nQrMTHZoC8qM++EOvjAw2wIZy1L6UbiZYI+aY8Q9rJLdelfMdttZHyvn4ckLYfiCBU3o++KcKOb7HdM0B7Od56PfQuV4AZl3vr59DcH1y1PJVEyMfr79zSQzdfwxeYxtRftcP79T7SpbAKwtv4iiCuJ+3C0ugfs0k= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1792073658; c=relaxed/simple; bh=ZCejfvmafwnBOk5vT8e9YPV7RwSvPgoJ0T7baK1aNEc=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=Ft+GU2C9v2U4+31ABZLthKUOxQ5rVu1Qarq17TeHvbkBBLcHkmrwQnMjisEET4sEmU0G0+kizwmY5jNpjnYvGNU+SOn9YtioH4MAcwFetldffr9zQqDZZ7FjHCZeWhUuqnTTFQebJsxH1ZCeRUHzpxDaCy+C0cd0xDatcm6d1Fk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=MtXhzWod; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="MtXhzWod" Received: by smtp.kernel.org (Postfix) with ESMTPSA id EFBEA1F000C4; Thu, 15 Oct 2026 14:14:10 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1792073656; bh=UgdqyF6FEN8olbz6l9+spc+Bapy75+BcU13cALLW+eI=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=MtXhzWod9jpMv2PHNRke7nFXeXMTCxfQmq1SByHN/ACefsv1CJ40kxnEmYNPzEqHm knVkDXnJrIKBc5IkEo/4zRzB59Vm2fD+RdnEwKresx9fecoGg6SEvizJu6nOwq2v0t XChs7uSw9Rn6UwqnLvI4FQUHke9LV6VhBDdmQODyh75Wj82lKG5hl4Cd5xd37+B7YH 7/tuNjR7X1cHa0nBeuEek/QA863n2R+XLbnnnyckmFO6o2UiJktkSKVS7Yu5LtHphA aM3Yb4Md1bBwpIeDA+NkUo/xOzJ1wGHTv27be6RQIhBnWeIDa2aSmWu71s/kh2slh8 5g+/y/3hlS2yA== Message-ID: <5800bc6e-7671-41eb-bb09-7fbc8f2d085c@kernel.org> Date: Thu, 15 Oct 2026 16:14:07 +0200 Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 1/2] mm/huge_memory: recheck a special PMD under the lock in copy_huge_pmd() To: Konstantin Khorenko , Andrew Morton , Zi Yan , Baolin Wang , "Liam R. Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Kiryl Shutsemau , Peter Xu Cc: Alex Williamson , kvm@vger.kernel.org, =?UTF-8?Q?Lo=C3=AFc_Molinari?= , dri-devel@lists.freedesktop.org, Madhavan Srinivasan , "Ritesh Harjani (IBM)" , linuxppc-dev@lists.ozlabs.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Jason Gunthorpe , stable@vger.kernel.org References: <20261014-b4-prep-vstor-149847-copy_huge_pmd-recheck-special-pmd-v3-0-85eaec8417f6@virtuozzo.com> <20261014-b4-prep-vstor-149847-copy_huge_pmd-recheck-special-pmd-v3-1-85eaec8417f6@virtuozzo.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: <20261014-b4-prep-vstor-149847-copy_huge_pmd-recheck-special-pmd-v3-1-85eaec8417f6@virtuozzo.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 10/14/26 17:46, Konstantin Khorenko wrote: > copy_huge_pmd() reads a special PMD without the page table lock and > copies that value to the child. The mmap write lock held by fork does > not stop unmap_mapping_range(): it zaps under i_mmap_rwsem and the PMD > lock. For a special PMD (that is not the huge zero folio), we need to go through vmf_insert_pfn_pmd()->insert_pmd(). I would have thought that unmap_mapping_pages() is only called in corner cases when we would have concurrent fork(). Like vfio_pci_zap_bars(). But yeah, likely there is a theoretical race. > > The zap visits the parent VMA before the child's, as dup_mmap() links > the child right after the parent in i_mmap. If it clears the parent PMD > after fork has read it, and passes the child before fork copies it, the > child keeps a mapping the driver has revoked: > > - vfio-pci: the child reaches the BAR after the device memory was > disabled or the device was reset, instead of getting SIGBUS; > - panthor: the child maps the pages of an evicted buffer object after > they are freed. > > Read the PMD again under the source PMD lock, which the zap takes to > clear it. Either the zap got there first, and the PMD is none, or the > copy is in place before the zap reaches the child. Faults are locked > out, so only a zap can change the PMD, and pmd_none() is the test. > > Fixes: bc02afbd4d73 ("mm/fork: accept huge pfnmap entries") > Cc: stable@vger.kernel.org > Assisted-by: LLM > Signed-off-by: Konstantin Khorenko > --- > mm/huge_memory.c | 23 ++++++++++++++++------- > 1 file changed, 16 insertions(+), 7 deletions(-) > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > index 1e5d68acf62a5..bf788bb231691 100644 > --- a/mm/huge_memory.c > +++ b/mm/huge_memory.c > @@ -1965,13 +1965,22 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm, > src_ptl = pmd_lockptr(src_mm, src_pmd); > spin_lock_nested(src_ptl, SINGLE_DEPTH_NESTING); > /* > - * No need to recheck the pmd, it can't change with write > - * mmap lock held here. > - * > - * Meanwhile, making sure it's not a CoW VMA with writable > - * mapping, otherwise it means either the anon page wrongly > - * applied special bit, or we made the PRIVATE mapping be > - * able to wrongly write to the backend MMIO. > + * The mmap and VMA write locks of the source keep faults > + * out, but unmap_mapping_range() does not take the mmap > + * lock and may have cleared the pmd since the lockless > + * read. Let copy_pmd_range() skip it then. > + */ TBH, I think we can drop this comment entirely. That's just simply the right thing to do? :) > + pmd = pmdp_get(src_pmd); > + if (unlikely(pmd_none(pmd))) { > + spin_unlock(src_ptl); > + spin_unlock(dst_ptl); > + return -EAGAIN; > + } > + /* > + * Make sure it's not a CoW VMA with writable mapping, > + * otherwise it means either the anon page wrongly applied > + * special bit, or we made the PRIVATE mapping be able to > + * wrongly write to the backend MMIO. > */ > VM_WARN_ON_ONCE(vma_is_cow_mapping(src_vma) && pmd_write(pmd)); > goto set_pmd; > But why do we do this early copy dance *at all*? I think I complained about this vma_is_anonymous handling in the past. vma_needs_copy() is the one deciding whether something should get copied or not. So, can't we just get rid of the vma_is_anonymous() handling and perform the weird special-pte check (that I hate entirely) when already holding the PTL? -- Cheers, David