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[87.161.174.152]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-45b7e8ab14esm466857795e9.21.2025.09.08.00.50.17 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 08 Sep 2025 00:50:18 -0700 (PDT) Message-ID: Date: Mon, 8 Sep 2025 09:50:17 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/mempolicy: prevent the risk of division by 0 To: "Huang, Ying" , Chelsy Ratnawat , Joshua Hahn Cc: akpm@linux-foundation.org, ziy@nvidia.com, rakie.kim@sk.com, byungchul@sk.com, apopple@nvidia.com, linux-mm@kvack.org References: <20250907235332.931050-1-joshua.hahnjy@gmail.com> <87ldmpwqmz.fsf@DESKTOP-5N7EMDA> 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 ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZoEEwEIAEQCGwMCF4ACGQEFCwkIBwICIgIG FQoJCAsCBBYCAwECHgcWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaJzangUJJlgIpAAKCRBN 3hD3AP+DWhAxD/9wcL0A+2rtaAmutaKTfxhTP0b4AAp1r/eLxjrbfbCCmh4pqzBhmSX/4z11 opn2KqcOsueRF1t2ENLOWzQu3Roiny2HOU7DajqB4dm1BVMaXQya5ae2ghzlJN9SIoopTWlR 0Af3hPj5E2PYvQhlcqeoehKlBo9rROJv/rjmr2x0yOM8qeTroH/ZzNlCtJ56AsE6Tvl+r7cW 3x7/Jq5WvWeudKrhFh7/yQ7eRvHCjd9bBrZTlgAfiHmX9AnCCPRPpNGNedV9Yty2Jnxhfmbv Pw37LA/jef8zlCDyUh2KCU1xVEOWqg15o1RtTyGV1nXV2O/mfuQJud5vIgzBvHhypc3p6VZJ lEf8YmT+Ol5P7SfCs5/uGdWUYQEMqOlg6w9R4Pe8d+mk8KGvfE9/zTwGg0nRgKqlQXrWRERv cuEwQbridlPAoQHrFWtwpgYMXx2TaZ3sihcIPo9uU5eBs0rf4mOERY75SK+Ekayv2ucTfjxr Kf014py2aoRJHuvy85ee/zIyLmve5hngZTTe3Wg3TInT9UTFzTPhItam6dZ1xqdTGHZYGU0O otRHcwLGt470grdiob6PfVTXoHlBvkWRadMhSuG4RORCDpq89vu5QralFNIf3EysNohoFy2A LYg2/D53xbU/aa4DDzBb5b1Rkg/udO1gZocVQWrDh6I2K3+cCs7BTQRVy5+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: <87ldmpwqmz.fsf@DESKTOP-5N7EMDA> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: S6iYU42p9Pcjo399er540yS14jEI6kFO2vuGuAE7Z2U_1757317819 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 4C8B9A0005 X-Stat-Signature: ismejbecp4s11tekbi68eg3r5ixdsah3 X-Rspam-User: X-HE-Tag: 1757317824-278263 X-HE-Meta: U2FsdGVkX1+x53u1W8tgUQOTSTy1qkoZn2FmCjJhAALXfm6jjDfvbskh9WnX6W8RfglIGdrvmtJ0oAiPivFvhKX13Wn5gbUGAKz1XkBxUkVYiqUA8M+3TFVa5cLHUFtXCmEPgFT2I8SmMYIemt6uWuuY/0TnpQQrpwmRx+H+RAN5g1QHtL1m2PLiIk2ptH9cUHxin/oE9Zm2Y9l8SkbCUcA1FshS5DuHqRwx7dQivdeQyEqqxSjQgeD5Uk4FGa2l1dgVEj+cDiaOMTmRRT+6l7fJ4UY92l1eokQ3ULy6y9LOgqzOTlGiSUkOvz+bmSQRLoqSyo6qJrCRLfnMeJ/QdYxUY+IGVueR40P5+gXemouMM5JpWvHz02RyMXxkLfOgeVeqNNTaqVSuxhVHWEzrN7/+zqoA+P+PBqcyFogiLjkWDbffSUhTS8KvNzylgAmidTpX9kX3JmsIZ7zorhK/ESDBdZ5iX1seaRxu8HvGRanweMSBbNb/MeeQms+68ntr37Yh7SydSwWk9c6grFXjWCYo/iIa4j1v4gdaFpGPeMPRS49PKx3fXTXVKdW6oCTSLgGHHUuKzNus7RTXaV8Cx0PQWSgjrdPNsNfcopatOID3rP/zt0DUjSC3PYX8UaKK5wY65bHYiz5z9LVwnidjpOtiUbwhCSFyE3YFm382gIlP34rAze7lzePN2GOthYx9OYdMwH6615VcGhv0pR3z25FVH9zwo4emELnEcPn9iOSBT6y1OgMYAWEu5zqe8gqfThoBxj5krC2R0cz9uaUC99q7BOjWnbyj04la4U0qr2QOeRltOuslIGeQhw1QdQid2FzQpJY8vutDlkLDjgsVRiVg3XSA9KxSFaFpQENtCJmhGHY2FCXyMIRDOjuAFx/gUUnIFGDo19yLAIwZEFhvNVVLFU8icVaIlAmlSezqYkxzl/wuv1gQKxAy1kEV0IJpuIecFr6hZkYFssL93lE dR+bASLV LyulBD83/3O29R01MjBsU2unIi3XYFrVfwpfiOryW/VOd1lCc1oPxTXMW6ETD/21Hr4K2tyByB2ENifHVDqujJRBC2c/+9Q+pvj+RNy4tupKK9w8tOltQfzkhXmPza8g+8DLcjzln7vsm1tyG/PErJ7VIpBkgoFCDf3dUIDQfhUyevIFW0SR7stUMBFvAX2FTq3Jzi6w4pl/3wzEgmFnmcQLL2iTW8TSE29KSeYDy3XtkYRFuY6huxvHOzVgQ0VTKSZ6HGj1uTRG95OauUiPFpJFYgooC4gWpIWFjXJbEjlF0kF7K1UcIwBh9jWEtBMnb/ii5UC88J9xyeoifS1uxnA0rLIhv2qkv3OPhJZTtIcByZst8/GE2tmpOMsVqY71GUhTBZqrtehIsA1SDWzDaxtLd+twIBesnR1XeEJtnxW56LflLMynFQWsrIwd8t1VvQtTseu7qVVGdRyFbKx9kVhC8hKpJChXeJGrN 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 08.09.25 05:48, Huang, Ying wrote: > Joshua Hahn writes: > >> On Sun, 7 Sep 2025 09:08:29 -0700 Chelsy Ratnawat wrote: >> >>> If no bits are set in the policy's node mask, then nodes will be 0. >>> This patch adds a check if nodes == 0 before dividing. >> >> Hello Chelsy, >> >> Thank you for the patch! I will start off by saying that it probably does not >> hurt to add a check like this. With that said, I think it's best if we clarify: >> >> (1) Have we seen this happen before? >> (2) Should we add a Fixes tag? >> (3) Do we expect to hit the nodes == 0 case, or is this just to be safe? >> (4) If so, should we do more than just return 0 (i.e. WARN_ON?) >> >> From what I can tell, I think it should not be possible for the policy to be >> interleave, and the nodes to be null. If we look at mpol_new, there is an >> explicit call to nodes_empty that checks if the nodes are empty, and returns >> an error if that is the case (of course, excluding MPOL_DEFAULT, which actually >> throws an error if the nodes are not empty). >> >> After all, it doesn't really make semantic sense to allow interleaving across >> 0 nodes ; -) >> >> But as always, it is very possible that I am missing something that you are >> seeing : -) Please feel free to correct me if you believe that is the case. >> >> With all of this said, I do agree that it's a bit scary to do a div0, even if >> I am confident that the value shouldn't be zero. I don't have any strong >> feelings about adding redundant checks, so I will let other mempolicy reviewers >> chime in with thier opinions. One thing I do feel strongly though, is that if >> we are to move forward in the patch, some explicit clarification that we >> have/have not seen it happen before, and a WARN_ON would make sense. > > IIUC, the patch is to catch the bug during kernel development. And, if > an exception can be generated for dividing by 0, it can be used to catch > the bug already. If so, IMHO, the patch appears unnecessary. We don't want to add runtime checks for things that are supposed to be caught early during testing. Usually we use VM_WARN_* for sanity checking such things, but the div0 would also already trigger early during testing, no? -- Cheers David / dhildenb