On Mon, Oct 14, 2024 at 09:23:14AM +0200, Hannes Reinecke wrote: >> 3) some storage utilities >> - dm thin provisioning utility of thin_check >> - `dt`(https://github.com/RobinTMiller/dt) >> >> I looks like same user buffer is used in more than 1 dio. >> >> 4) some self cooked test code which does same thing with 1) >> >> In storage stack, the buffer provider is far away from the actual DMA >> controller operating code, which doesn't have the knowledge if >> DMA_ATTR_SKIP_CPU_SYNC should be set. >> >> And suggestions for avoiding this noise? >> > Can you check if this is the NULL page? Operations like 'discard' will > create bios with several bvecs all pointing to the same NULL page. > That would be the most obvious culprit. The only case I fully understand without looking into the details is raid1, and that will obviously map the same data multiple times because it writes it out multiple time. Now mapping a buffer multiple times for a DMA_TO_DEVICE is relatively harmless in practice as the data is transferred to the device, but it it still breaks the dma buffer ownership model in the dma which is really helpful to find bugs where people don't think about this at all. Not sure if there is any good solution here.
