Re: [PATCH v2 0/4] dax: require 'struct page' and other fixups

[Dan Williams <dan.j.williams@intel.com>]

On Sun, Oct 1, 2017 at 2:11 PM, Dave Chinner <david@fromorbit.com> 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@lst.de> 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.

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