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[2003:cb:c719:1700:56dc:6a88:b509:d3f3]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-43656b41904sm624530315e9.37.2025.01.07.01.20.36 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 07 Jan 2025 01:20:37 -0800 (PST) Message-ID: <2339600b-ebd5-49f3-a0be-414bc400a858@redhat.com> Date: Tue, 7 Jan 2025 10:20:31 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v1] mm: fix div by zero in bdi_ratio_from_pages To: Stefan Roesch , willy@infradead.org, zzqq0103.hey@gmail.com, akpm@linux-foundation.org, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org References: <20250104012037.159386-1-shr@devkernel.io> From: David Hildenbrand 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: <20250104012037.159386-1-shr@devkernel.io> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: 2SlOBL3khZhfE4uLxI2A9QveHPR0lv_e7VQ3iFxVyws_1736241639 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Stat-Signature: z6wmoqowezwqnwyqp9buuee6ngdhgmt5 X-Rspam-User: X-Rspamd-Queue-Id: 3D59C8000A X-Rspamd-Server: rspam08 X-HE-Tag: 1736241644-324392 X-HE-Meta: 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 nGFj29s/ 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 04.01.25 02:20, Stefan Roesch wrote: > During testing it has been detected, that it is possible to get div by > zero error in bdi_set_min_bytes. The error is caused by the function > bdi_ratio_from_pages(). bdi_ratio_from_pages() calls > global_dirty_limits. If the dirty threshold is 0, the div by zero is > raised. This can happen if the root user is setting: > > echo 0 > /proc/sys/vm/dirty_ration. > > The following is a test case: > > echo 0 > /proc/sys/vm/dirty_ratio > cd /sys/class/bdi/ > echo 1 > strict_limit > echo 8192 > min_bytes > > ==> error is raised. > > The problem is addressed by returning -EINVAL if dirty_ratio or > dirty_bytes is set to 0. > > Reported-by: cheung wall > Closes: https://lore.kernel.org/linux-mm/87pll35yd0.fsf@devkernel.io/T/#t > Signed-off-by: Stefan Roesch > --- > mm/page-writeback.c | 2 ++ > 1 file changed, 2 insertions(+) > > diff --git a/mm/page-writeback.c b/mm/page-writeback.c > index d213ead95675..91aa7a5c0078 100644 > --- a/mm/page-writeback.c > +++ b/mm/page-writeback.c > @@ -692,6 +692,8 @@ static unsigned long bdi_ratio_from_pages(unsigned long pages) > unsigned long ratio; > > global_dirty_limits(&background_thresh, &dirty_thresh); > + if (!dirty_thresh) > + return -EINVAL; > ratio = div64_u64(pages * 100ULL * BDI_RATIO_SCALE, dirty_thresh); > > return ratio; bdi_set_min_bytes() calls bdi_ratio_from_pages() and passes the result to __bdi_set_min_ratio(). __bdi_set_min_ratio() expects an "unsigned int min_ratio". I assume this will work because "max_ratio > 100 * BDI_RATIO_SCALE", but it is rather confusing ... Maybe we want something like: /* Use 101% to indicate "invalid" */ #define BDI_RATIO_INVALID (101 * BDI_RATIO_SCALE) Or alternatively, just handle it in the callers of bdi_ratio_from_pages(), checking for -EINVAL manually. -- Cheers, David / dhildenb