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=1793112170; b=tRCDpLMseVXFR0OCsA9Lw0W0W2VySLY5Twj6lt57c7xjKlEcXYL4IQ24M9EYIfJZ3rAB hZxt7DrrvwiqsKhDadWDRU38yQS8u4OWO3FJJoyJ8u94XelaOW5emKPSpaAXDbtH5+kG4 6SqzKNPiTQdn9kUW9ZSRmbb2at0k5EuA4qj5UDlSOZkawLWIUfXljFmOv6aQ2AlcKa/yW uXO5Qq05bvxRoFnofYmUIvptqk8y7iaJfGlO0180zHW0C16SaeC7rzEuenQLWakb4cdCD FawKOjGlKfdm3vyFXQ/Nkev06P3TmJPD1UdhBqCHPKGh5lLYY9wLZno5wV0zwSIMXQw== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793112170; 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=xoLSuofN7cQ2tWXwUvc/DSgwxulevn9ENLUaotbNxMY=; b=XT1L13zuPU6KZ13I60Ngx2T0rasexpgRKfGFgN/3Y+AGc/BPXb03tr7+DEne449VTPxw Dnet6LudhdyBLoJEUu6ADVEV85DVgRJ2sYSi4MLDxtvB3jIhiYbI3HbwgRhlB8Etrt0Qm I6wE83S6+gs+RGkNdLUmHEGyvrMDpqlNr49DeOer2PJUa/+qnQ24WV9FHWtqogqVopcoi BuWi3q6ahJJpsLUUKsQt29h7TnelrfILT9y8iBG3wVtNJ01gxVaA9y6AxTTf/kX5pwKIo 1aMEVsOYPtyi9YLT/bQ7OsCFBFzfXb4tNUcKeBMykOOgW4K3rshY3VGrcfQ61I0+vCQ== 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=nCibtCOv; 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 C3B6C606A31B; Tue, 27 Oct 2026 14:42:49 +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=nCibtCOv Received: by smtp.kernel.org (Postfix) with ESMTPSA id 0F6B11F031E2; Tue, 27 Oct 2026 14:42:42 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793112169; bh=xoLSuofN7cQ2tWXwUvc/DSgwxulevn9ENLUaotbNxMY=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=nCibtCOvcqjhtwxq9rIemKZCtHNwaNf0BlFYC+FouftiBZosSYpxUL0mYyiauWLpd T2Y6UhXpbH0x1mHHjwNPbem/AT56EcTY0wEs4ibeYgTQGRswuJDpCv2LXbJ+CxbD6X a3MNJHAVtaQVmSxaDIlgWzSKkR19zx+tcBS53yPUPCaggBn1R319Y+LeTJTjoPQdt4 U9EtvqFBnxhwsG3bg1rjQB+ZU2EMtiGPYNKf8ulZoxR+zqhKLcr7A3W4el3jE2UZme ESkSApBkTcsYRn2/FolhS5eGkQDuIExsSFIm+G8gM1QCufx8s3kvrSwfMUkqaeUqZg 7DbTbx3UW5nSA== Message-ID: <829f5a0f-4d9a-4c65-9703-fbf6e49d2b02@kernel.org> Date: Tue, 27 Oct 2026 15:42:40 +0100 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 v4 3/4] mm/huge_memory: read 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?= , Lorenzo Stoakes , Shuah Khan , linux-kselftest@vger.kernel.org, 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 , Sashiko References: <20261021-b4-prep-vstor-149847-copy_huge_pmd-recheck-special-pmd-v4-0-66f00b7a564c@virtuozzo.com> <20261021-b4-prep-vstor-149847-copy_huge_pmd-recheck-special-pmd-v4-3-66f00b7a564c@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: <20261021-b4-prep-vstor-149847-copy_huge_pmd-recheck-special-pmd-v4-3-66f00b7a564c@virtuozzo.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 10/21/26 22:36, 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. > > 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, for example: > > - 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 > the driver has released them. > > The same branch deposits no page table under the copy. ppc64 in hash > mode keeps the hash slot array of every huge PMD in that table, so the > child dereferences a NULL table on the first hash fault, in > __hash_page_thp(), and on the zap, in hash__pmdp_huge_get_and_clear(). > > Take the locks first and let vm_normal_page_pmd() tell a PFN mapping > from a folio under them, as copy_present_ptes() does with > vm_normal_page(). The zap clears the parent PMD under the same lock, so > either fork finds it none and skips it, or the copy is in place before > the zap reaches the child. > > The copy of a PFN mapping then goes through the same tail as a folio: > > - it gets a page table where the architecture needs one; > - in a shared mapping it is marked clean in the child, as a PTE is; > - in a CoW mapping, which only VM_MIXEDMAP allows, both sides are write > protected instead of a warning being printed. > > Fixes: bc02afbd4d73 ("mm/fork: accept huge pfnmap entries") > Fixes: d1503aa9ab80 ("powerpc/64s: Add support for huge pfnmaps") > Reported-by: Sashiko > Closes: https://sashiko.dev/#/patchset/20261014-b4-prep-vstor-149847-copy_huge_pmd-recheck-special-pmd-v1-1-d225c60bc316@virtuozzo.com?part=1 > Suggested-by: David Hildenbrand > Signed-off-by: Konstantin Khorenko > --- > mm/huge_memory.c | 24 +++--------------------- > 1 file changed, 3 insertions(+), 21 deletions(-) > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > index 13e3674fb6115..da5918a0ade87 100644 > --- a/mm/huge_memory.c > +++ b/mm/huge_memory.c > @@ -1961,25 +1961,6 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm, > int ret = -ENOMEM; > bool writable; > > - pmd = pmdp_get_lockless(src_pmd); > - if (unlikely(pmd_present(pmd) && pmd_special(pmd) && > - !is_huge_zero_pmd(pmd))) { > - dst_ptl = pmd_lock(dst_mm, dst_pmd); > - 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. > - */ > - VM_WARN_ON_ONCE(vma_is_cow_mapping(src_vma) && pmd_write(pmd)); > - goto set_pmd; > - } > - > if (vma_is_anonymous(dst_vma) || arch_needs_pgtable_deposit()) { > pgtable = pte_alloc_one(dst_mm); > if (unlikely(!pgtable)) > @@ -2021,7 +2002,9 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm, > goto copy_pmd; > } > > - src_page = pmd_page(pmd); > + src_page = vm_normal_page_pmd(src_vma, addr, pmd); > + if (!src_page) > + goto copy_pmd; > VM_BUG_ON_PAGE(!PageHead(src_page), src_page); > src_folio = page_folio(src_page); > > @@ -2061,7 +2044,6 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm, > } > if (src_vma->vm_flags & VM_SHARED) > pmd = pmd_mkclean(pmd); > -set_pmd: > pmd = pmd_mkold(pmd); > set_pmd_at(dst_mm, addr, dst_pmd, pmd); > > Looks correct. Reading the above, I wonder about Cc: stable. It would affect v6.12, so not too crazy to backport. We'd have to backport patch #2 as well in that case (or have a simpler fix, but I am not sure that's really worth it). Acked-by: David Hildenbrand (Arm) -- Cheers, David