From: Nikita Yushchenko <nikita.yo...@cogentembedded.com> Date: Mon, 10 Oct 2016 12:51:28 +0300
> Hmm... I'm not about device writing to memory. This absolutely is about whether the device wrote into the area or not. > Sequence in igb driver is: > > dma_map(full_page) > <device writes here> > sync_to_cpu(half_page); > skb_add_rx_frag(skb, half_page); > napi_gro_receive(skb); > ... > dma_unmap(full_page) > > What I'm concerned about is - same area is first passed up to network > stack, and _later_ dma_unmap()ed. Is this indeed safe? dma_unmap() should never write anything unless the device has meanwhile written to that chunk of memory. If the device made no intervening writes into the area, dma_unmap() should not cause any data to be written to that area, period. In your example above, consider the case where the device never writes into the memory area after sync_to_cpu(). In that case there is nothing that dma_unmap() can possibly write. All the data has been synced, and no device writes into the memory are have occurred.
