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(p200300cbc7383c008b014fd9b833e1e9.dip0.t-ipconnect.de. [2003:cb:c738:3c00:8b01:4fd9:b833:e1e9]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-43ec364a0c7sm137141685e9.29.2025.04.07.11.33.53 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 07 Apr 2025 11:33:54 -0700 (PDT) Message-ID: <12196206-16b0-4913-b087-1f59ff808603@redhat.com> Date: Mon, 7 Apr 2025 20:33:53 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v1] x86/mm/pat: (un)track_pfn_copy() fix + improvements To: Ingo Molnar Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, x86@kernel.org, kernel test robot , Dan Carpenter , Andrew Morton , Lorenzo Stoakes , Dave Hansen , Andy Lutomirski , Peter Zijlstra , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Rik van Riel , "H. Peter Anvin" , Linus Torvalds References: <20250404124931.2255618-1-david@redhat.com> <630caa8e-2ee2-4895-9e4e-8bf2fa079100@redhat.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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 B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZgEEwEIAEICGwMGCwkIBwMCBhUIAgkKCwQW AgMBAh4BAheAAhkBFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAl8Ox4kFCRKpKXgACgkQTd4Q 9wD/g1oHcA//a6Tj7SBNjFNM1iNhWUo1lxAja0lpSodSnB2g4FCZ4R61SBR4l/psBL73xktp rDHrx4aSpwkRP6Epu6mLvhlfjmkRG4OynJ5HG1gfv7RJJfnUdUM1z5kdS8JBrOhMJS2c/gPf wv1TGRq2XdMPnfY2o0CxRqpcLkx4vBODvJGl2mQyJF/gPepdDfcT8/PY9BJ7FL6Hrq1gnAo4 3Iv9qV0JiT2wmZciNyYQhmA1V6dyTRiQ4YAc31zOo2IM+xisPzeSHgw3ONY/XhYvfZ9r7W1l pNQdc2G+o4Di9NPFHQQhDw3YTRR1opJaTlRDzxYxzU6ZnUUBghxt9cwUWTpfCktkMZiPSDGd KgQBjnweV2jw9UOTxjb4LXqDjmSNkjDdQUOU69jGMUXgihvo4zhYcMX8F5gWdRtMR7DzW/YE BgVcyxNkMIXoY1aYj6npHYiNQesQlqjU6azjbH70/SXKM5tNRplgW8TNprMDuntdvV9wNkFs 9TyM02V5aWxFfI42+aivc4KEw69SE9KXwC7FSf5wXzuTot97N9Phj/Z3+jx443jo2NR34XgF 89cct7wJMjOF7bBefo0fPPZQuIma0Zym71cP61OP/i11ahNye6HGKfxGCOcs5wW9kRQEk8P9 M/k2wt3mt/fCQnuP/mWutNPt95w9wSsUyATLmtNrwccz63XOwU0EVcufkQEQAOfX3n0g0fZz Bgm/S2zF/kxQKCEKP8ID+Vz8sy2GpDvveBq4H2Y34XWsT1zLJdvqPI4af4ZSMxuerWjXbVWb T6d4odQIG0fKx4F8NccDqbgHeZRNajXeeJ3R7gAzvWvQNLz4piHrO/B4tf8svmRBL0ZB5P5A 2uhdwLU3NZuK22zpNn4is87BPWF8HhY0L5fafgDMOqnf4guJVJPYNPhUFzXUbPqOKOkL8ojk CXxkOFHAbjstSK5Ca3fKquY3rdX3DNo+EL7FvAiw1mUtS+5GeYE+RMnDCsVFm/C7kY8c2d0G NWkB9pJM5+mnIoFNxy7YBcldYATVeOHoY4LyaUWNnAvFYWp08dHWfZo9WCiJMuTfgtH9tc75 7QanMVdPt6fDK8UUXIBLQ2TWr/sQKE9xtFuEmoQGlE1l6bGaDnnMLcYu+Asp3kDT0w4zYGsx 5r6XQVRH4+5N6eHZiaeYtFOujp5n+pjBaQK7wUUjDilPQ5QMzIuCL4YjVoylWiBNknvQWBXS lQCWmavOT9sttGQXdPCC5ynI+1ymZC1ORZKANLnRAb0NH/UCzcsstw2TAkFnMEbo9Zu9w7Kv AxBQXWeXhJI9XQssfrf4Gusdqx8nPEpfOqCtbbwJMATbHyqLt7/oz/5deGuwxgb65pWIzufa N7eop7uh+6bezi+rugUI+w6DABEBAAHCwXwEGAEIACYCGwwWIQQb2cqtc1xMOkYN/MpN3hD3 AP+DWgUCXw7HsgUJEqkpoQAKCRBN3hD3AP+DWrrpD/4qS3dyVRxDcDHIlmguXjC1Q5tZTwNB boaBTPHSy/Nksu0eY7x6HfQJ3xajVH32Ms6t1trDQmPx2iP5+7iDsb7OKAb5eOS8h+BEBDeq 3ecsQDv0fFJOA9ag5O3LLNk+3x3q7e0uo06XMaY7UHS341ozXUUI7wC7iKfoUTv03iO9El5f XpNMx/YrIMduZ2+nd9Di7o5+KIwlb2mAB9sTNHdMrXesX8eBL6T9b+MZJk+mZuPxKNVfEQMQ a5SxUEADIPQTPNvBewdeI80yeOCrN+Zzwy/Mrx9EPeu59Y5vSJOx/z6OUImD/GhX7Xvkt3kq Er5KTrJz3++B6SH9pum9PuoE/k+nntJkNMmQpR4MCBaV/J9gIOPGodDKnjdng+mXliF3Ptu6 3oxc2RCyGzTlxyMwuc2U5Q7KtUNTdDe8T0uE+9b8BLMVQDDfJjqY0VVqSUwImzTDLX9S4g/8 kC4HRcclk8hpyhY2jKGluZO0awwTIMgVEzmTyBphDg/Gx7dZU1Xf8HFuE+UZ5UDHDTnwgv7E th6RC9+WrhDNspZ9fJjKWRbveQgUFCpe1sa77LAw+XFrKmBHXp9ZVIe90RMe2tRL06BGiRZr jPrnvUsUUsjRoRNJjKKA/REq+sAnhkNPPZ/NNMjaZ5b8Tovi8C0tmxiCHaQYqj7G2rgnT0kt WNyWQQ== Organization: Red Hat In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 07.04.25 18:50, Ingo Molnar wrote: > > * David Hildenbrand wrote: > >> On 06.04.25 19:28, Ingo Molnar wrote: >>> >>> * David Hildenbrand wrote: >>> >>>> We got a late smatch warning and some additional review feedback. >>>> >>>> smatch warnings: >>>> mm/memory.c:1428 copy_page_range() error: uninitialized symbol 'pfn'. >>> >>>> - if (!(src_vma->vm_flags & VM_PAT)) >>>> + if (!(src_vma->vm_flags & VM_PAT)) { >>>> + *pfn = 0; >>>> return 0; >>>> + } >>> >>>> static inline int track_pfn_copy(struct vm_area_struct *dst_vma, >>>> struct vm_area_struct *src_vma, unsigned long *pfn) >>>> { >>>> + *pfn = 0; >>>> return 0; >>>> } >>> >>> That's way too ugly. There's nothing wrong with not touching 'pfn' >>> in the error path: in fact it's pretty standard API where output >>> pointers may not get set on errors. >> >> We're not concerned about the error path, though. > > Sorry, indeed, not an error path, but the !VM_PAT path above - but > still a similar argument applies IMHO. > >>> If Smatch has a problem with it, Smatch should be fixed, or the false >>> positive warning should be worked around by initializing 'pfn' in the >>> callers. >> >> We could adjust the documentation of track_pfn_copy, to end up with the >> following: >> >> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h >> index e2b705c149454..b50447ef1c921 100644 >> --- a/include/linux/pgtable.h >> +++ b/include/linux/pgtable.h >> @@ -1511,8 +1511,9 @@ static inline void track_pfn_insert(struct vm_area_struct *vma, pgprot_t *prot, >> /* >> * track_pfn_copy is called when a VM_PFNMAP VMA is about to get the page >> - * tables copied during copy_page_range(). On success, stores the pfn to be >> - * passed to untrack_pfn_copy(). >> + * tables copied during copy_page_range(). Will store the pfn to be >> + * passed to untrack_pfn_copy() only if there is something to be untracked. >> + * Callers should initialize the pfn to 0. >> */ >> static inline int track_pfn_copy(struct vm_area_struct *dst_vma, >> struct vm_area_struct *src_vma, unsigned long *pfn) >> @@ -1522,7 +1523,9 @@ static inline int track_pfn_copy(struct vm_area_struct *dst_vma, >> /* >> * untrack_pfn_copy is called when a VM_PFNMAP VMA failed to copy during >> - * copy_page_range(), but after track_pfn_copy() was already called. >> + * copy_page_range(), but after track_pfn_copy() was already called. Can >> + * be called even if track_pfn_copy() did not actually track anything: >> + * handled internally. >> */ >> static inline void untrack_pfn_copy(struct vm_area_struct *dst_vma, >> unsigned long pfn) >> diff --git a/mm/memory.c b/mm/memory.c >> index 2d8c265fc7d60..1a35165622e1c 100644 >> --- a/mm/memory.c >> +++ b/mm/memory.c >> @@ -1361,7 +1361,7 @@ copy_page_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma) >> struct mm_struct *dst_mm = dst_vma->vm_mm; >> struct mm_struct *src_mm = src_vma->vm_mm; >> struct mmu_notifier_range range; >> - unsigned long next, pfn; >> + unsigned long next, pfn = 0; > > Ack. > > I hate it how uninitialized variables are even a thing in C, and why > there's no compiler switch to turn it off for the kernel. (At least for > non-struct variables. Even for structs I would zero-initialize and > *maybe* allow a non-initialized opt-in for cases where it matters. It > matters in very few cases in praxis. And don't get me started about the > stupidity that is to not initialize holes in struct members ...) > > Over the decades we've lived through numerous nasty bugs for very > little tangible code generation benefits. Ok, let me resend with that. (I'll still tag it as a fix due do the weird UB scenario when passing uninitialized values to a non-inline function ...) -- Cheers, David / dhildenb