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Re: [PATCH v3 13/15] block, dax: make dax mappings opt-in by default

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On Tue, Nov 3, 2015 at 3:04 PM, Dan Williams <dan.j.williams@intel.com> wrote:
> On Tue, Nov 3, 2015 at 12:20 PM, Dave Chinner <david@fromorbit.com> wrote:
>> On Mon, Nov 02, 2015 at 11:35:04PM -0800, Dan Williams wrote:
>>> On Mon, Nov 2, 2015 at 4:32 PM, Dave Chinner <david@fromorbit.com> wrote:
> [..]
>>> Only in the mmap path:
>>
>> which means blkdev_direct_IO() is now always going to go down the
>> dax_do_io() path for any driver with a ->direct_access method rather
>> than the direct IO path, regardless of whether DAX is enabled on the
>> device or not.
>>
>> That really seems wrong to me - you've replace explicit "is DAX
>> enabled" checks with "is DAX possible" checks, and so DAX paths are
>> used regardless of whether DAX is enabled or not. And it's not
>> obvious why this is done, nor is it now obvious how DAX interacts
>> with the block device.
>>
>> This really seems like a step backwards to me.
>
> I think the reason it is not obvious is the original justification for
> the bypass as stated in commit bbab37ddc20b "block: Add support for
> DAX reads/writes to block devices" was:
>
>     "instead of allocating a DIO and a BIO"
>
> It turns out it's faster and as far as I can tell semantically
> equivalent to the __blockdev_direct_IO() path.  The DAX mmap path in
> comparison has plenty of sharp edges and semantic differences that
> would be avoided by turning off DAX.
>
> I'm not opposed to also turning off dax_do_io() when S_DAX is clear,
> but I don't currently see the point.  At the very least I need to add
> the above comments to the code, but do you still think opt-in DAX is a
> backwards step?

I thought of one way dax_do_io() breaks current semantics, it defeats
blktrace and i/o stat tracking.  I'll restore the existing behavior
that gates dax_do_io() on S_DAX.
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[PATCH v3 00/15] block, dax updates for 4.4 Dan Williams <dan.j.williams@intel.com> - 2015-11-02 05:40 +0100
  [PATCH v3 12/15] block: enable dax for raw block devices Dan Williams <dan.j.williams@intel.com> - 2015-11-02 05:40 +0100
  [PATCH v3 04/15] libnvdimm,  pmem: move request_queue allocation earlier in probe Dan Williams <dan.j.williams@intel.com> - 2015-11-02 05:40 +0100
    Re: [PATCH v3 04/15] libnvdimm, pmem: move request_queue allocation  earlier in probe Ross Zwisler <ross.zwisler@linux.intel.com> - 2015-11-03 20:20 +0100
  [PATCH v3 07/15] kvm: rename pfn_t to kvm_pfn_t Dan Williams <dan.j.williams@intel.com> - 2015-11-02 05:40 +0100
  [PATCH v3 13/15] block, dax: make dax mappings opt-in by default Dan Williams <dan.j.williams@intel.com> - 2015-11-02 05:40 +0100
    Re: [PATCH v3 13/15] block, dax: make dax mappings opt-in by default Dave Chinner <david@fromorbit.com> - 2015-11-03 01:40 +0100
      Re: [PATCH v3 13/15] block, dax: make dax mappings opt-in by default Dan Williams <dan.j.williams@intel.com> - 2015-11-03 08:40 +0100
        Re: [PATCH v3 13/15] block, dax: make dax mappings opt-in by default Dave Chinner <david@fromorbit.com> - 2015-11-03 21:30 +0100
          Re: [PATCH v3 13/15] block, dax: make dax mappings opt-in by default Dan Williams <dan.j.williams@intel.com> - 2015-11-04 00:10 +0100
            Re: [PATCH v3 13/15] block, dax: make dax mappings opt-in by default Dan Williams <dan.j.williams@intel.com> - 2015-11-04 20:30 +0100
  [PATCH v3 05/15] libnvdimm,  pmem: fix size trim in pmem_direct_access() Dan Williams <dan.j.williams@intel.com> - 2015-11-02 05:40 +0100
    Re: [PATCH v3 05/15] libnvdimm, pmem: fix size trim in  pmem_direct_access() Ross Zwisler <ross.zwisler@linux.intel.com> - 2015-11-03 20:40 +0100
      Re: [PATCH v3 05/15] libnvdimm, pmem: fix size trim in pmem_direct_access() Dan Williams <dan.j.williams@intel.com> - 2015-11-03 22:40 +0100
  [PATCH v3 14/15] dax: dirty extent notification Dan Williams <dan.j.williams@intel.com> - 2015-11-02 05:40 +0100
    Re: [PATCH v3 14/15] dax: dirty extent notification Dave Chinner <david@fromorbit.com> - 2015-11-03 02:20 +0100
      Re: [PATCH v3 14/15] dax: dirty extent notification Dan Williams <dan.j.williams@intel.com> - 2015-11-03 06:10 +0100
        Re: [PATCH v3 14/15] dax: dirty extent notification Dave Chinner <david@fromorbit.com> - 2015-11-03 06:50 +0100
          Re: [PATCH v3 14/15] dax: dirty extent notification Dan Williams <dan.j.williams@intel.com> - 2015-11-03 08:30 +0100
            Re: [PATCH v3 14/15] dax: dirty extent notification Dave Chinner <david@fromorbit.com> - 2015-11-03 22:00 +0100
              Re: [PATCH v3 14/15] dax: dirty extent notification Dan Williams <dan.j.williams@intel.com> - 2015-11-03 22:20 +0100
              Re: [PATCH v3 14/15] dax: dirty extent notification Ross Zwisler <ross.zwisler@linux.intel.com> - 2015-11-03 22:40 +0100
                Re: [PATCH v3 14/15] dax: dirty extent notification Dan Williams <dan.j.williams@intel.com> - 2015-11-03 22:50 +0100
        Re: [PATCH v3 14/15] dax: dirty extent notification Ross Zwisler <ross.zwisler@linux.intel.com> - 2015-11-03 22:20 +0100
          Re: [PATCH v3 14/15] dax: dirty extent notification Dan Williams <dan.j.williams@intel.com> - 2015-11-03 22:40 +0100
  [PATCH v3 10/15] dax,  pmem: introduce zone_device_revoke() and devm_memunmap_pages() Dan Williams <dan.j.williams@intel.com> - 2015-11-02 05:40 +0100
  [PATCH v3 06/15] um: kill pfn_t Dan Williams <dan.j.williams@intel.com> - 2015-11-02 05:40 +0100

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