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

[Christoph Hellwig <hch@lst.de>]

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.