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Tue, 1 Apr 2025 11:56:43 +0000 Message-ID: Date: Tue, 1 Apr 2025 13:56:36 +0200 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH net-next v5 2/2] page_pool: Track DMA-mapped pages and unmap them when destroying the pool To: Yunsheng Lin CC: Zhu Yanjun , =?UTF-8?Q?Toke_H=C3=B8iland-J=C3=B8rgensen?= , "David S. Miller" , Jakub Kicinski , "Jesper Dangaard Brouer" , Saeed Mahameed , "Leon Romanovsky" , Tariq Toukan , Andrew Lunn , Eric Dumazet , Paolo Abeni , Ilias Apalodimas , "Simon Horman" , Andrew Morton , "Mina Almasry" , Yonglong Liu , Pavel Begunkov , Matthew Wilcox , , , , , Qiuling Ren , Yuying Ma References: <20250328-page-pool-track-dma-v5-0-55002af683ad@redhat.com> <20250328-page-pool-track-dma-v5-2-55002af683ad@redhat.com> From: Alexander Lobakin Content-Language: en-US In-Reply-To: Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8bit X-ClientProxiedBy: WA2P291CA0028.POLP291.PROD.OUTLOOK.COM (2603:10a6:1d0:1f::23) To DS0PR11MB8718.namprd11.prod.outlook.com (2603:10b6:8:1b9::20) Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DS0PR11MB8718:EE_|MW4PR11MB6887:EE_ X-MS-Office365-Filtering-Correlation-Id: b11145d2-14e5-4018-f04d-08dd7114457a X-LD-Processed: 46c98d88-e344-4ed4-8496-4ed7712e255d,ExtAddr X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|376014|1800799024|7416014|7053199007; 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This causes resource leaks and/or >>>> crashes when there are pages still outstanding while the device is torn >>>> down, because page_pool will attempt an unmap through a non-existent DMA >>>> device on the subsequent page return. >>> >>> [...] >>> >>>> @@ -173,10 +212,10 @@ struct page_pool { >>>>       int cpuid; >>>>       u32 pages_state_hold_cnt; >>>>   -    bool has_init_callback:1;    /* slow::init_callback is set */ >>>> +    bool dma_sync;            /* Perform DMA sync for device */ >>> >>> Have you seen my comment under v3 (sorry but I missed that there was v4 >>> already)? Can't we just test the bit atomically? >> >> Perhaps test_bit series functions can test the bit atomically. Maybe there are more good options about this testing the bit atomically. But test_bit should implement the task that tests the bit atomically. > > There are two reading of dma_sync in this patch, the first reading is not > under rcu read lock and doing the reading without READ_ONCE(), the second > reading is under rcu read lock and do the reading with READ_ONCE(). > > The first one seems an optimization to avoid taking the rcu read lock, > why might need READ_ONCE() to make KCSAN happy if we do care about making > KCSAN happy. > > The second one does not seems to need the atomicity by using the READ_ONCE() > as it is always under RCU read lock(implicit or explicit one), and there is > a rcu sync after the clearing of that bit. IOW, are you saying this change is not needed at all? Thanks, Olek