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=1794234172; b=nhlo5IiGkjm6M51DtH5yDfNLW8SgQtyBbUVLJkxtWGU8ofHyoxjPNQcZgfECtsn+wg89 1zvA7L6xi0Kaq8LC9dh2sc17NkCEN70R6X/S9hxrnqa5ST2X2JNAJj2u1z0tpwpzcPRXa ahYpr97wroFd4Zp6NSwdykTpnkqipIjO7riZF87pdwZhOoazeQRNpEO0JJxpD2vRg3E4J 0nfNbuR0WUlJiSUVjNUGAD/fg890jx3RPfFa51YNy9KaQNSZ39RHPq5hEgVPWGMkO+0Oj vIaFBC+PGcn8ZbdctHWldrfMBdkPMyfPrsfPCn6E91xCOSfKT1K8yvjjaITcoSm53lw== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1794234172; 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=7isCUsuGWC/Pk0kPq18bunZ15LbusZD3620YiOmbCWs=; b=zfFNJlBl51NyMgsTh7zCfUmhOWwCFdoxeg/pq8+bQPwhLNBHPe5E4lWnL9/sjX/jvuZk RMJDatRt05xskEGPMeLdmmH5SEAuax/7LwMaBTT2nF2aSG+NcyrJuNT/GRacZY6MSFWM0 6YLbTDbpLkq+Dt0v4e0Fc90QtwI3I8klevNpX38q5DbHNHvai1i4k61R/lvPvgL6BX4QD ARX/8SlPycgIXesjln842N59TNO4j/67XuHoEIMyQRhjfumFTtSaR6Or+L6wNWCPV7dsP sU5f/ZJ93iXIuw/4y5/aOUGV25LWQz2LDolKi+0psfIKQ8rpk2uyp5jEyxYUZkmny7g== 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=JKkUdmMG; 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 C66FE606A323; Mon, 9 Nov 2026 14:22:51 +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=JKkUdmMG Received: by smtp.kernel.org (Postfix) with ESMTPSA id A840B1F0089F; Mon, 9 Nov 2026 14:22:47 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1794234171; bh=7isCUsuGWC/Pk0kPq18bunZ15LbusZD3620YiOmbCWs=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=JKkUdmMG3WEmwG9PZDtBPn8mPQCAfcvbK9QCxG9DKs7GZAjaO5hftP7/XX0ZpjpN2 dC8zSyBeIAYic0NYI3EnSQOx/TBuHEiPKh1aWXjnrfofmUxuvsYRD6JIhEi9K1rmVK 3EVea9LwkQGb4qOQApraD63OJ60bw1OMn/vSwz/k2hj+FJmOO1jktxLEwK7uIFRhmd SSRKF+Qec6l/Wl88ahehwFcbN+xbRLQSR2/F0f75ArHLipgdSQhlOLpKAJBGAm4bya f8BjU4jWjQtFdy80oic/KcFkoQLjcMdBj2FQmVSVMsKMIPinX/8gWiPYEr9r23nkTi fNmfqgFQInt6g== Message-ID: <3a2d4135-f531-4797-8dd2-a2732a846e45@kernel.org> Date: Mon, 9 Nov 2026 15:22:45 +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 v3 1/6] mm: khugepaged: ignore MADV_HUGEPAGE for shadow stacks To: Yeoreum Yun , linux-mm@kvack.org, linux-kernel@vger.kernel.org Cc: Jason Gunthorpe , John Hubbard , Peter Xu , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Andrew Morton , Rik van Riel , Yuan-Hao Hsu , stable@vger.kernel.org References: <20261109-refactor_gup_fast-v3-0-2aaef02df0e1@arm.com> <20261109-refactor_gup_fast-v3-1-2aaef02df0e1@arm.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: <20261109-refactor_gup_fast-v3-1-2aaef02df0e1@arm.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 11/9/26 13:32, Yeoreum Yun wrote: > commit 3efb9800557b ("mm: do not map the shadow stack as THP") makes > shadow stack mappings default to VM_NOHUGEPAGE. However, userspace can > still clear this flag and set VM_HUGEPAGE through madvise(). > > Allowing large folios in x86 shadow stacks exposes a limitation in > folio_pte_batch_flags(). For example: > > 1. create a shadow stack mapping. > 2. apply MADV_HUGEPAGE and populate the mapping with a PTE-mapped > large folio. > 3. apply MADV_DONTFORK to the first base page of that folio. > 4. call fork(). > > In the parent, the first page remains writable and anonymous-exclusive, > while the remaining pages shared with the child become write-protected > and lose PageAnonExclusive(). These pages can still belong to the same > large folio. > > On x86, writable shadow-stack PTEs have _PAGE_RW clear and _PAGE_DIRTY > set. Write-protected PTEs have both bits clear, with the dirty state > preserved in _PAGE_SAVED_DIRTY. > > When FPB_RESPECT_WRITE is set without FPB_RESPECT_DIRTY, > folio_pte_batch_flags() normalizes PTEs using pte_mkclean(), which clears > both dirty bits. This loses the distinction between writable > shadow-stack PTEs and write-protected PTEs, allowing them to be grouped > into the same batch. GUP-fast can encounter such a batch when walking > across the VMA boundary created by MADV_DONTFORK. > > Since there is no practical use-case shadow stack with large folio, > ignore MADV_HUGEPAGE for shadow stack mappings, preserving their > VM_NOHUGEPAGE setting and preventing large-folio allocation. > > Fixes: 3efb9800557b ("mm: do not map the shadow stack as THP") > Suggested-by: David Hildenbrand (Arm) > Signed-off-by: Yeoreum Yun > Cc: > --- > mm/khugepaged.c | 3 +++ > 1 file changed, 3 insertions(+) > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index 40fad28c52f6f..51ec043fa26c3 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -367,6 +367,9 @@ int hugepage_madvise(struct vm_area_struct *vma, > { > switch (advice) { > case MADV_HUGEPAGE: > + /* PTE batching doesn't work reliably. */ > + if (vma_test_single_mask(vma, VMA_SHADOW_STACK)) > + break; > *vm_flags &= ~VM_NOHUGEPAGE; > *vm_flags |= VM_HUGEPAGE; > break; > Ack, I prefer this solution for now until we know how to make pte_mkclean() on x86 shadow stack PTEs not result in an implicit pte_wrprotect(). Acked-by: David Hildenbrand (Arm) -- Cheers, David