From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Mon, 2 Oct 2017 08:59:59 +1100 From: Dave Chinner To: Dan Williams Cc: Christoph Hellwig , Andrew Morton , Michal Hocko , Jan Kara , "linux-nvdimm@lists.01.org" , Linux MM , Jeff Moyer , linux-fsdevel , Ross Zwisler , "Kirill A. Shutemov" Subject: Re: [PATCH v2 0/4] dax: require 'struct page' and other fixups Message-ID: <20171001215959.GF15067@dastard> References: <150664806143.36094.11882924009668860273.stgit@dwillia2-desk3.amr.corp.intel.com> <20171001075701.GB11554@lst.de> <20171001211147.GE15067@dastard> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org List-ID: On Sun, Oct 01, 2017 at 02:22:08PM -0700, Dan Williams wrote: > On Sun, Oct 1, 2017 at 2:11 PM, Dave Chinner 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 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@fromorbit.com -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. 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