On Sat, Sep 29, 2018 at 09:21:17AM -0700, Matthew Wilcox wrote: > > being slow to pick it up. It looks like there are several patterns, and > > we have to support both set_page_dirty() and set_page_dirty_lock(). So > > the best combination looks to be adding a few variations of > > release_user_pages*(), but leaving put_user_page() alone, because it's > > the "do it yourself" basic one. Scatter-gather will be stuck with that. > > I think our current interfaces are wrong. We should really have a > get_user_sg() / put_user_sg() function that will set up / destroy an > SG list appropriate for that range of user memory. This is almost > orthogonal to the original intent here, so please don't see this as a > "must do first" kind of argument that might derail the whole thing.
The SG list really is the wrong interface, as it mixes up information about the pages/phys addr range and a potential dma mapping. I think the right interface is an array of bio_vecs. In fact I've recently been looking into a get_user_pages variant that does fill bio_vecs, as it fundamentally is the right thing for doing I/O on large pages, and will really help with direct I/O performance in that case.
