Kernel scatterlists contiguous in virtual address?

Question

I'd like to ask/verify whether an array of struct scatterlist that were mapped with blk_rq_map_sg are all contiguous at least in virtual address domain. Well we all know that scatterlists are primarily for DMA, where each scatter-gather (sg) array element describes a chunk of memory and those chunks are discontiguous in their physical addresses.

I'm writing a block device driver. Suppose I receive struct requests from my request_queue, and I want to do something with the I/O data that my request is holding, like memcpy ing them to somewhere. That operation will need a virtual address and length parameter. Now all I know is that I can access the request's data via the scatterlist mapped through the helper function blk_map_rq_sg() . Is my assumption correct, that those sg elements are contiguous in virtual, and that I can get the data address via sg_virt() , and use that address and total length for memcpy ? Or they are discontiguous in virtual, and I have to walk through each sg array element and memcpy to/from each sg memory chunk (in virtual) which is harder?

Answer 1

Functions like writev allow to submit discontiguous requests, and pages in high memory do not have a virtual address to begin with.

To access SG list contents with the CPU, use something like sg_copy_from_buffer , which automatically maps the pages temporarily.