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Re: [PATCH v2 0/5] fs, xfs: block map immutable files for dax, dma-to-storage, and swap

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On Sun, Aug 06, 2017 at 11:51:50AM -0700, Dan Williams wrote:
> Of course it's a useful API. An application already needs to worry
> about the block map, that's why we have fallocate, msync, fiemap
> and...

Fallocate and msync do not expose the block map in any way.  Proof:
they work just fine over say nfs.

fiemap does indeed expose the block map, which is the whole point.
But it's a debug tool that we don't event have a man page for.  And
it's not usable for anything else, if only for the fact that it doesn't
tell you what device your returned extents are relative to.

> > We've been through this a few times but let me repeat it:  The only
> > sensible API gurantee is one that is observable and usable.
> 
> I'm missing how block-map immutable files violate this observable and
> usable constraint?

What is the observable behavior of an extent map change?  How can you
describe your immutable extent map behavior so that when I violate
them by e.g. moving one extent to a different place on disk you can
observe that in userspace?

> This immutable approach should also go in, it solves the same problem
> without the the latency drawback,

How is your latency going to be any different from MAP_SYNC on
a fully allocated and pre-zeroed file?

> Beyond flush from userspace it also
> can be used to solve the swapfile problems you highlighted

Which swapfile problem?

> and it
> allows safe ongoing dma to a filesystem-dax mapping beyond what we can
> already do with direct-I/O.

Please explain how this interface allows for any sort of safe userspace
DMA.

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[PATCH v2 0/5] fs, xfs: block map immutable files for dax,  dma-to-storage, and swap Dan Williams <dan.j.williams@intel.com> - 2017-08-04 04:40 +0200
  [PATCH v2 2/5] fs, xfs: introduce FALLOC_FL_SEAL_BLOCK_MAP Dan Williams <dan.j.williams@intel.com> - 2017-08-04 04:40 +0200
    Re: [PATCH v2 2/5] fs, xfs: introduce FALLOC_FL_SEAL_BLOCK_MAP "Darrick J. Wong" <darrick.wong@oracle.com> - 2017-08-04 21:50 +0200
      Re: [PATCH v2 2/5] fs, xfs: introduce FALLOC_FL_SEAL_BLOCK_MAP Dan Williams <dan.j.williams@intel.com> - 2017-08-04 22:00 +0200
    Re: [PATCH v2 2/5] fs, xfs: introduce FALLOC_FL_SEAL_BLOCK_MAP Dave Chinner <david@fromorbit.com> - 2017-08-05 01:40 +0200
      Re: [PATCH v2 2/5] fs, xfs: introduce FALLOC_FL_SEAL_BLOCK_MAP Dan Williams <dan.j.williams@intel.com> - 2017-08-05 01:50 +0200
        Re: [PATCH v2 2/5] fs, xfs: introduce FALLOC_FL_SEAL_BLOCK_MAP Dave Chinner <david@fromorbit.com> - 2017-08-05 02:10 +0200
  [PATCH v2 1/5] fs, xfs: introduce S_IOMAP_IMMUTABLE Dan Williams <dan.j.williams@intel.com> - 2017-08-04 04:40 +0200
    Re: [PATCH v2 1/5] fs, xfs: introduce S_IOMAP_IMMUTABLE "Darrick J. Wong" <darrick.wong@oracle.com> - 2017-08-04 22:10 +0200
      Re: [PATCH v2 1/5] fs, xfs: introduce S_IOMAP_IMMUTABLE Dan Williams <dan.j.williams@intel.com> - 2017-08-04 22:40 +0200
    Re: [PATCH v2 1/5] fs, xfs: introduce S_IOMAP_IMMUTABLE Christoph Hellwig <hch@lst.de> - 2017-08-05 11:50 +0200
      Re: [PATCH v2 1/5] fs, xfs: introduce S_IOMAP_IMMUTABLE Dave Chinner <david@fromorbit.com> - 2017-08-07 02:30 +0200
        Re: [PATCH v2 1/5] fs, xfs: introduce S_IOMAP_IMMUTABLE Christoph Hellwig <hch@lst.de> - 2017-08-11 12:40 +0200
  Re: [PATCH v2 0/5] fs, xfs: block map immutable files for dax,  dma-to-storage, and swap Dan Williams <dan.j.williams@intel.com> - 2017-08-04 04:40 +0200
    Re: [PATCH v2 0/5] fs, xfs: block map immutable files for dax,         dma-to-storage, and swap Christoph Hellwig <hch@lst.de> - 2017-08-05 12:00 +0200
      Re: [PATCH v2 0/5] fs, xfs: block map immutable files for dax,  dma-to-storage, and swap Dan Williams <dan.j.williams@intel.com> - 2017-08-06 21:00 +0200
        Re: [PATCH v2 0/5] fs, xfs: block map immutable files for dax,         dma-to-storage, and swap Christoph Hellwig <hch@lst.de> - 2017-08-11 12:50 +0200
  [PATCH v2 5/5] xfs: toggle XFS_DIFLAG2_IOMAP_IMMUTABLE in response  to fallocate Dan Williams <dan.j.williams@intel.com> - 2017-08-04 04:40 +0200
    Re: [PATCH v2 5/5] xfs: toggle XFS_DIFLAG2_IOMAP_IMMUTABLE in  response to fallocate "Darrick J. Wong" <darrick.wong@oracle.com> - 2017-08-04 22:20 +0200
      Re: [PATCH v2 5/5] xfs: toggle XFS_DIFLAG2_IOMAP_IMMUTABLE in  response to fallocate Dan Williams <dan.j.williams@intel.com> - 2017-08-04 22:50 +0200
        Re: [PATCH v2 5/5] xfs: toggle XFS_DIFLAG2_IOMAP_IMMUTABLE in  response to fallocate Dan Williams <dan.j.williams@intel.com> - 2017-08-04 23:00 +0200
        Re: [PATCH v2 5/5] xfs: toggle XFS_DIFLAG2_IOMAP_IMMUTABLE in  response to fallocate "Darrick J. Wong" <darrick.wong@oracle.com> - 2017-08-04 23:00 +0200
  [PATCH v2 4/5] xfs: introduce XFS_DIFLAG2_IOMAP_IMMUTABLE Dan Williams <dan.j.williams@intel.com> - 2017-08-04 04:40 +0200
    Re: [PATCH v2 4/5] xfs: introduce XFS_DIFLAG2_IOMAP_IMMUTABLE "Darrick J. Wong" <darrick.wong@oracle.com> - 2017-08-04 22:40 +0200
      Re: [PATCH v2 4/5] xfs: introduce XFS_DIFLAG2_IOMAP_IMMUTABLE Dan Williams <dan.j.williams@intel.com> - 2017-08-04 22:50 +0200
      Re: [PATCH v2 4/5] xfs: introduce XFS_DIFLAG2_IOMAP_IMMUTABLE Dave Chinner <david@fromorbit.com> - 2017-08-05 01:50 +0200
        Re: [PATCH v2 4/5] xfs: introduce XFS_DIFLAG2_IOMAP_IMMUTABLE "Darrick J. Wong" <darrick.wong@oracle.com> - 2017-08-05 02:00 +0200

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