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(p200300d82f4efd008e13e3b590c81159.dip0.t-ipconnect.de. [2003:d8:2f4e:fd00:8e13:e3b5:90c8:1159]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-453647071f4sm114418395e9.34.2025.06.23.08.18.38 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 23 Jun 2025 08:18:39 -0700 (PDT) Message-ID: Date: Mon, 23 Jun 2025 17:18:38 +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 v2 2/4] userfaultfd: remove (VM_)BUG_ON()s To: Tal Zussman Cc: Andrew Morton , Peter Xu , "Jason A. Donenfeld" , Alexander Viro , Christian Brauner , Jan Kara , Andrea Arcangeli , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org References: <20250607-uffd-fixes-v2-0-339dafe9a2fe@columbia.edu> <20250607-uffd-fixes-v2-2-339dafe9a2fe@columbia.edu> 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: 8bit On 18.06.25 01:10, Tal Zussman wrote: > On Tue, Jun 10, 2025 at 3:26 AM David Hildenbrand wrote: >> >> On 07.06.25 08:40, Tal Zussman wrote: >>> >>> if (ctx->features & UFFD_FEATURE_SIGBUS) >>> goto out; >>> @@ -411,12 +411,11 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason) >>> * to be sure not to return SIGBUS erroneously on >>> * nowait invocations. >>> */ >>> - BUG_ON(vmf->flags & FAULT_FLAG_RETRY_NOWAIT); >>> + VM_WARN_ON_ONCE(vmf->flags & FAULT_FLAG_RETRY_NOWAIT); >>> #ifdef CONFIG_DEBUG_VM >>> if (printk_ratelimit()) { >>> - printk(KERN_WARNING >>> - "FAULT_FLAG_ALLOW_RETRY missing %x\n", >>> - vmf->flags); >>> + pr_warn("FAULT_FLAG_ALLOW_RETRY missing %x\n", >>> + vmf->flags); >> >> You didn't cover that in the patch description. >> >> I do wonder if we really still want the dump_stack() here and could >> simplify to >> >> pr_warn_ratelimited(). >> >> But I recall that the stack was helpful at least once for me (well, I >> was able to reproduce and could have figured it out differently.). > > I'll include this in the description as well. Given that you found the stack > helpful before, I'll leave it as-is for now. > >> [...] >> >>> err = uffd_move_lock(mm, dst_start, src_start, &dst_vma, &src_vma); >>> if (err) >>> @@ -1867,9 +1865,9 @@ ssize_t move_pages(struct userfaultfd_ctx *ctx, unsigned long dst_start, >>> up_read(&ctx->map_changing_lock); >>> uffd_move_unlock(dst_vma, src_vma); >>> out: >>> - VM_WARN_ON(moved < 0); >>> - VM_WARN_ON(err > 0); >>> - VM_WARN_ON(!moved && !err); >>> + VM_WARN_ON_ONCE(moved < 0); >>> + VM_WARN_ON_ONCE(err > 0); >>> + VM_WARN_ON_ONCE(!moved && !err); >>> return moved ? moved : err; >> >> >> Here you convert VM_WARN_ON to VM_WARN_ON_ONCE without stating it in the >> description (including the why). > > Will update in the description. These checks represent impossible conditions > and are analogous to the BUG_ON()s in move_pages(), but were added later. > So instead of BUG_ON(), they use VM_WARN_ON() as a replacement when > VM_WARN_ON_ONCE() is likely a better fit, as per other conversions. > >>> @@ -1956,9 +1954,9 @@ int userfaultfd_register_range(struct userfaultfd_ctx *ctx, >>> for_each_vma_range(vmi, vma, end) { >>> cond_resched(); >>> >>> - BUG_ON(!vma_can_userfault(vma, vm_flags, wp_async)); >>> - BUG_ON(vma->vm_userfaultfd_ctx.ctx && >>> - vma->vm_userfaultfd_ctx.ctx != ctx); >>> + VM_WARN_ON_ONCE(!vma_can_userfault(vma, vm_flags, wp_async)); >>> + VM_WARN_ON_ONCE(vma->vm_userfaultfd_ctx.ctx && >>> + vma->vm_userfaultfd_ctx.ctx != ctx); >>> WARN_ON(!(vma->vm_flags & VM_MAYWRITE)); >> >> Which raises the question, why this here should still be a WARN > > After checking it, it looks like the relevant condition is enforced in the > registration path, so this can be converted to a debug check. I'll update > the patch accordingly. Sorry for the late reply. Yeah, a lot of that probably needs a cleanup. > > However, I think the checks in userfaultfd_register() may be redundant. > First, it checks !(cur->vm_flags & VM_MAYWRITE). Then, after a hugetlb > check, it checks > ((vm_flags & VM_UFFD_WP) && !(cur->vm_flags & VM_MAYWRITE)), which should > never be hit. Yes, likely. > > Am I missing something? > > Additionally, the comment above the first !(cur->vm_flags & VM_MAYWRITE) > check is a bit confusing. It seems to be based on the fact that file-backed > MAP_SHARED mappings without write permissions will not have VM_MAYWRITE set, > while MAP_PRIVATE mappings will always(?) have it set, but doesn't say it as > explicitly. Am I understanding this check correctly? private anon mappings should always have VM_MAYWRITE. (there is no driver to really change that). For other mappings (MAP_SHARED), VM_MAYWRITE is cleared for R/O files I think. VM_MAYWRITE really just tells you that you can do mprotect(PROT_WRITE) later, to effectively set VM_WRITE. So if VM_MAYWRITE is clear, we cannot possibly write to such mappings later (not even after mprotect()). -- Cheers, David / dhildenb