On Sun, Oct 01, 2017 at 02:22:08PM -0700, Dan Williams wrote: > On Sun, Oct 1, 2017 at 2:11 PM, Dave Chinner <david@xxxxxxxxxxxxx> wrote: > > On Sun, Oct 01, 2017 at 10:58:06AM -0700, Dan Williams wrote: > >> On Sun, Oct 1, 2017 at 12:57 AM, Christoph Hellwig <hch@xxxxxx> wrote: > >> > While this looks like a really nice cleanup of the code and removes > >> > nasty race conditions I'd like to understand the tradeoffs. > >> > > >> > This now requires every dax device that is used with a file system > >> > to have a struct page backing, which means not only means we'd > >> > break existing setups, but also a sharp turn from previous policy. > >> > > >> > Unless I misremember it was you Intel guys that heavily pushed for > >> > the page-less version, so I'd like to understand why you've changed > >> > your mind. > >> > >> Sure, here's a quick recap of the story so far of how we got here: > >> > >> * In support of page-less I/O operations envisioned by Matthew I > >> introduced pfn_t as a proposal for converting the block layer and > >> other sub-systems to use pfns instead of pages [1]. You helped out on > >> that patch set with some work on the DMA api. [2] > >> > >> * The DMA api conversion effort came to a halt when it came time to > >> touch sparc paths and DaveM said [3]: "Generally speaking, I think > >> that all actual physical memory the kernel operates on should have a > >> struct page backing it." > >> > >> * ZONE_DEVICE was created to solve the DMA problem and in developing / > >> testing that discovered plenty of proof for Dave's assertion (no fork, > >> no ptrace, etc). We should have made the switch to require struct page > >> at that point, but I was persuaded by the argument that changing the > >> dax policy may break existing assumptions, and that there were larger > >> issues to go solve at the time. > >> > >> What changed recently was the discussions around what the dax mount > >> option means and the assertion that we can, in general, make some > >> policy changes on our way to removing the "experimental" designation > >> from filesystem-dax. It is clear that the page-less dax path remains > >> experimental with all the way it fails in several kernel paths, and > >> there has been no patches for several months to revive the effort. > >> Meanwhile the page-less path continues to generate maintenance > >> overhead. The recent gymnastics (new ->post_mmap file_operation) to > >> make sure ->vm_flags are safely manipulated when dynamically changing > >> the dax mode of a file was the final straw for me to pull the trigger > >> on this series. > >> > >> In terms of what breaks by changing this policy it should be noted > >> that we automatically create pages for "legacy" pmem devices, and the > >> default for "ndctl create-namespace" is to allocate pages. I have yet > >> to see a bug report where someone was surprised by fork failing or > >> direct-I/O causing a SIGBUS. So, I think the defaults are working, it > >> is unlikely that there are environments dependent on page-less > >> behavior. > > > > Does this imply that the hardware vendors won't have > > tens of terabytes of pmem in systems in the near to medium term? > > That's what we were originally told to expect by 2018-19 timeframe > > (i.e. 5 years in), and that's kinda what we've been working towards. > > Indeed, supporting systems with a couple of orders of magnitude more > > pmem than ram was the big driver for page-less DAX mappings in the > > first place. i.e. it was needed to avoid the static RAM overhead of > > all the static struct pages for such large amounts of physical > > memory. > > > > If we decide that we must have struct pages for pmem, then we're > > essentially throwing away the ability to support the very systems > > the hardware vendors were telling us we needed to design the pmem > > infrastructure for. If that reality has changed, then I'd suggest > > that we need to determine what the long term replacement for > > pageless IO on large pmem systems will be before we throw what we > > have away. > > No, we can support large pmem with struct page capacity reserved from > pmem itself rather than ram. A 1.5% capacity tax does not appear to be > prohibitive. The "capacity tax" had nothing to do with it - the major problem with self hosting struct pages was that page locks can be hot and contention on them will rapidly burn through write cycles on the pmem. That's still a problem, yes? I don't want to have to ask about all the issues one by one, so I'll ask you to explain in one go: what has changed (both hardware and software!) since we last discussed these problems with self hosting and make it a viable solution? Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx
