On Mon, Oct 01, 2018 at 08:29:29AM -0700, Matthew Wilcox wrote: > I don't understand the dislike of the sg list. Other than for special > cases which we should't be optimising for (ramfs, brd, loopback > filesystems), when we get a page to do I/O, we're going to want a dma > mapping for them. It makes sense to already allocate space to store > the mapping at the outset. We don't actually need the space - the scatterlist forces it on us, otherwise we could translate directly in the on-disk format and save that duplicate space. I have prototypes for NVMe and RDMA that do away with the scatterlist entirely. And even if we are still using the scatterlist as we do right now we'd need a second scatterlist at least for block / file system based I/O as we can't plug the scatterlist into the I/O stack (nevermind that due to splitting merging the lower one might not map 1:1 to the upper one). > [1] Can we ever admit that the bio_vec and the skb_frag_t are actually > the same thing? When I brought this up years ago the networking folks insisted that their use of u16 offset/size fields was important for performance, while for bio_vecs we needed the larger ones for some cases. Since then networking switched to 32-bit fields for what is now the fast path, so it might be worth to give it another spin. Than should also help with using my new bio_vec based dma-mapping helpers to batch iommu mappings in networking, which Jesper had on his todo list as all the indirect calls are causing performance issues.
