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charset=UTF-8 Date: Tue, 18 Aug 2026 19:45:31 -0400 Message-Id: Subject: Re: [syzbot] [mm?] WARNING in ep_write_iter Cc: "Andrew Morton" , "syzbot" , , , , , , , , , , , , "Greg Kroah-Hartman" , , "Vlastimil Babka" , "Suren Baghdasaryan" , "Michal Hocko" , "Brendan Jackman" , "Johannes Weiner" To: "Alan Stern" From: "Zi Yan" X-Mailer: aerc 0.21.0 References: <9787b33b-b30e-4c5e-a0ee-7f14515c7166@rowland.harvard.edu> <20260816194213.0e813ed338144ebc81ed4050@linux-foundation.org> <472add4b-f16a-4b87-bbc3-98c8aa385cf5@rowland.harvard.edu> <7deabd9c-04d5-4a45-8b8a-39929c046152@rowland.harvard.edu> <2bf01548-723c-465d-8368-247a814f61eb@rowland.harvard.edu> <6FC11E12-9428-4975-B135-E24805C2D9BF@nvidia.com> <15b086cc-7f10-4766-9b0c-88c3302e7398@rowland.harvard.edu> In-Reply-To: <15b086cc-7f10-4766-9b0c-88c3302e7398@rowland.harvard.edu> X-ClientProxiedBy: YQBPR0101CA0101.CANPRD01.PROD.OUTLOOK.COM (2603:10b6:c01:4::34) To IA0PR12MB8374.namprd12.prod.outlook.com (2603:10b6:208:40e::7) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: IA0PR12MB8374:EE_|MN0PR12MB5835:EE_ X-MS-Office365-Filtering-Correlation-Id: b06c68be-5075-41af-d249-08defd82cb67 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|376014|7416014|366016|23010399003|10067099003|56012099006|4143699003|22082099003|11063799006|18002099003; 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In other words, prevent it from >> >>> crashing production systems. >> > >> > You didn't respond to this. >>=20 >> I do not get your argument here. Why does this WARN need to be condition= al >> on CONFIG_EXPERT? What makes it special? Or you mean you want all WARNs >> to be conditional CONFIG_EXPERT? > > This may be a moot point now, but I'll answer the questions anyway. > > This WARN should be conditional on something indicating that the kernel=20 > is meant for testing rather than production (not necessarily=20 > CONFIG_EXPERT but something like it) because it does not indicate that=20 > an operation has failed badly enough to leave the kernel in a=20 > dangerously unstable state. Rather, it indicates that a programmer has=20 > tried to do something that can't possibly succeed. The result should=20 > not be dangerous because the caller should always check kmalloc()'s=20 > return value. If the caller is unable to recover from a memory=20 > allocation failure then it is the caller's responsibility to WARN, not=20 > kmalloc()'s. I agree with you that this situation, kmalloc cannot satisfy the too-large-to-allocate request, is not fatal and is recoverable since the caller handles NULL result. But without the warning, some callers have no way to know if they are sending an unexpected/undesirable request that might come from a bug in their code. With the patch, kmalloc returning NULL means two ambiguous things: 1. the request cannot be satisfied due to current memory system condition, e.g., insufficient free memory; 2. the request itself is unreasonable/unexpected. It could make debugging harder. > > The same is true for all other WARN statements. If they merely indicate= =20 > a programming infelicity and not a dangerous failure then they should=20 > not fire in production kernels. It is hard to tell whether a condition is dangerous or not. A caller sending a too-large-to-allocate request can come from a miscalculation or a wrong user input. It can also come from some data corruption/overwrite, clobbering the request size itself. --=20 Best Regards, Yan, Zi