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=1794595267; b=CxdTWDlWcEJP6A9rMF7hZh4Wel+8q0jHbkXEvQFIKSOxD3AGi90XAfOyy2Heg4CkNqIz 8LPapM4GWXk1qmACBxUHjOJiHb/8xUpeRsw6LSgbv424eeiqcLj6IyaIS7GtSnL60W3XB ZDsvX1OXkCI73xmAU6uA4F1iySYRjNakmq5QA8EwyNJ0BdUBQNkBLylu1AojP0cOvWkPP 8OkD3zg/cSPbIKVnaVzSOYf6r08wLyn6LUiYbEC478CZvz+Td3YK1fklGve+Fue0vxDgv xOazKQ219UT/eDO5QWUnci4JJ2MP8xxFqTylV8onls5pqBXhhsSNo8FiLSvqQxaRNpA== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1794595267; h=Received:DKIM-Signature:Message-ID:Date:MIME-Version:User-Agent: Subject:From:To:Cc:References:Content-Language:Autocrypt:In-Reply-To: Content-Type:Content-Transfer-Encoding; bh=XpptrgK2l80VgLjm5xpa2pL7+GMcRlYHRpTi5z7EAjU=; b=1jnBwPn/aMtY6THcM29mqClR8vPH5BQioBoFUncfsIRMJvFzR+4wR5FneJ3uqhxvFWaK agjDtTgoLWBdgWMvyhoVwkbNzTfyoBUjSEikF6nmsbv52+7lIc6xqqELnWj7kGxny9FMh fTuWzoYZWFpklX/Qb2sRZqk4gu0lGEO6nor6BrZ3mlYHtTLehj6Xu4OFumDbWnZ1fCsZ+ 7N6zZzU4qFbG4KFn9iiyRi9honipb44C37NNM5JBeETPVCljVdrI9QTmauIKXnxwbrdAH czSbl1SjoBE0rCHWs7z1g82gcz7cZ6D0BJJZrVJ1acJ+KscddbTjbm7H+kyRbJ0knWg== 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=UPltpBH5; 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 27CD0606A33F; Fri, 13 Nov 2026 18:41:06 +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=UPltpBH5 Received: by smtp.kernel.org (Postfix) with ESMTPSA id 050CF1F000C5; Fri, 13 Nov 2026 18:40:57 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1794595266; bh=XpptrgK2l80VgLjm5xpa2pL7+GMcRlYHRpTi5z7EAjU=; h=Date:Subject:From:To:Cc:References:In-Reply-To; b=UPltpBH5YLh1QQnrefheBr7BYwXIXoWGQCLJA1Zdq5XfCsvRTcbVSjBjmckRpr6gj p1mSYPsTCxeL93RU9a6/RHPQeqqwzwr71lDQiLJigcyNIASreO27Us2sljcDA/txMt l9bN2KgqB0xysRCnnIH+h6RnzBFZZAb6oDHgThHQMshrJd32N32hC55ooTjVb5ir1K J+cJ2azt+UXtcfuPAbjFaH4x3AFFdzQWl02/ZP8G92warfRjI97NymrClN1xsJeVkn AWqDLNOI8WVqN8AUUxubqgV9rG1I2dgahcOH4WA8R5GfCoTQQfmm+rFiJ99iLkA16e J64/nx+9hlzyQ== Message-ID: <8b186ea8-7a3b-4bb6-9354-de4b8892799f@kernel.org> Date: Fri, 13 Nov 2026 19:40:55 +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 v2 5/9] mm/mprotect: keep writable entries writable on permission upgrades From: "David Hildenbrand (Arm)" To: =?UTF-8?Q?Carlos_L=C3=B3pez?= , kvm@vger.kernel.org, linux-mm@kvack.org Cc: linux-kernel@vger.kernel.org, linux-s390@vger.kernel.org, linux-riscv@lists.infradead.org, seanjc@google.com, pbonzini@redhat.com, akpm@linux-foundation.org, peterx@redhat.com, ljs@kernel.org, imbrenda@linux.ibm.com, borntraeger@linux.ibm.com, frankja@linux.ibm.com, Zi Yan , Baolin Wang , "Liam R. Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Kiryl Shutsemau , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Jann Horn , Pedro Falcato References: <20261105165004.2663799-4-clopez@suse.de> <20261105165004.2663799-9-clopez@suse.de> <60c4f0a0-e3c9-42dc-b2e0-0860ab6e30f0@kernel.org> 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: <60c4f0a0-e3c9-42dc-b2e0-0860ab6e30f0@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 11/5/26 18:06, David Hildenbrand (Arm) wrote: > On 11/5/26 17:50, Carlos López wrote: >> change_protection() applies vma->vm_page_prot, which lacks the write bit >> for CoW and write-notified mappings, so it write-protects every entry >> in them. When an entry would simply be made writable on fault without >> further checks (e.g. dirty PTEs in shared mappings, exclusive anonymous >> pages), MM_CP_TRY_CHANGE_WRITABLE makes the entry writable again to >> avoid the future fault. >> >> However, writable entries in shared mappings that are not dirty still >> become read-only. For example, a uffd-wp unprotect makes clean shmem >> PTEs read-only even if they were never write-protected, e.g. when they >> were mapped by a read fault before uffd-wp was registered. The next >> write to each of them causes a fault that only restores the write bit. >> >> Now that MM_CP_PERM_UPGRADE indicates whether mprotect() only adds >> permissions, change_protection() can tell, together with the >> userfaultfd unprotect flags, when a change revokes no access. Keep >> writable entries writable in these cases, at the PTE, PMD and PUD >> levels. This is consistent with the write fault handler, as an entry >> that is already writable needs no write fault. >> >> Signed-off-by: Carlos López >> --- >> mm/huge_memory.c | 7 +++++++ >> mm/internal.h | 7 +++++++ >> mm/mprotect.c | 4 ++++ >> 3 files changed, 18 insertions(+) >> >> diff --git a/mm/huge_memory.c b/mm/huge_memory.c >> index 506e1e61c0fd..6e23d0e4ae66 100644 >> --- a/mm/huge_memory.c >> +++ b/mm/huge_memory.c >> @@ -2809,6 +2809,10 @@ int change_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma, >> if (userfaultfd_rwp(vma) && pmd_uffd(entry)) >> entry = pmd_modify(entry, PAGE_NONE); >> >> + /* See change_present_ptes(). */ >> + if (mm_cp_is_perm_upgrade(cp_flags) && pmd_write(oldpmd)) >> + entry = pmd_mkwrite(entry, vma); >> + >> /* See change_pte_range(). */ >> if ((cp_flags & MM_CP_TRY_CHANGE_WRITABLE) && !pmd_write(entry) && >> can_change_pmd_writable(vma, addr, entry)) >> @@ -2885,6 +2889,9 @@ int change_huge_pud(struct mmu_gather *tlb, struct vm_area_struct *vma, >> */ >> oldpud = pudp_invalidate(vma, addr, pudp); >> entry = pud_modify(oldpud, newprot); >> + /* See change_present_ptes(). */ >> + if (mm_cp_is_perm_upgrade(cp_flags) && pud_write(oldpud)) >> + entry = pud_mkwrite(entry); > > That looks completely hacked on. We do have can_change_pud_writable() below to > make such decisions. > > Whaatever you propose here is either wrong or way too shaky, sorry. For some reason, your reply neither reached my inbox through CC nor the mailing list folder through the mailing list. I see that you send a v3 where you keep it simple for now, good. >> >> That looks completely hacked on. We do have can_change_pud_writable() below to >> make such decisions. > > Okay, for v3 I can pass the old entry to can_change_{pmd,pud}_writable() > and return true if it was writable, after the existing write-tracking > checks (uffd-wp/soft-dirty). For PTEs, can_change_pte_writable() is not > used on the change_protection() path, so the same check would go in > set_write_prot_commit_flush_ptes(). > > Does that sound more reasonable? Let's keep it simple for now. Whenever we mess with protection changes there is the chance that we mess up and cause more trouble, either now or in the future when we change calling code. > > Whaatever you propose here is either wrong or way too shaky, sorry. > > The idea is that MM_CP_TRY_CHANGE_WRITABLE relies on the dirty bit to > make entries in shared mappings writable, which misses clean entries > that were already writable (e.g. shmem read-faulted before uffd-wp > registration). Such entries can stay writable, as anything that relies > on write faults write-protects the entry first. Right, but that should be optimized separately with a clear motivation why we even care about optimizing this. Not hidden in a patch set that tries to achieve something completely different. -- Cheers, David