On Thu, Oct 4, 2018 at 11:35 PM Johannes Thumshirn <jthumshirn@xxxxxxx> wrote: > > On Thu, Oct 04, 2018 at 11:25:24PM -0700, Christoph Hellwig wrote: > > Since when is an article on some website a promise (of what exactly) > > by linux kernel developers? > > Let's stop it here, this doesn't make any sort of forward progress. > I do think there is some progress we can make if we separate DAX as an access mechanism vs DAX as a resource utilization contract. My attempt at representing Christoph's position is that the kernel should not be advertising / making access mechanism guarantees. That makes sense. Even with MAP_SYNC+DAX the kernel reserves the right to write-protect mappings at will and trap access into a kernel handler. Additionally, whether read(2) / write(2) does anything different behind the scenes in DAX mode, or not should be irrelevant to the application. That said what is certainly not irrelevant is a kernel giving userspace visibility and control into resource utilization. Jan's MADV_DIRECT_ACCESS let's the application make assumptions about page cache utilization, we just need to another mechanism to read if a mapping is effectively already in that state.
