On Mon, Feb 22, 2016 at 01:52:28PM -0500, Jeff Moyer wrote:

I see.  So, at write fault time, you're saying that new blocks may be
allocated, and that in order to make that persistent, we need a sync
operation.

Yes.

Presumably this MAP_SYNC option could sync out the necessary
metadata updates to the log before returning from the write fault
handler.  The arguments against making this work are that it isn't
generally useful, and that we don't want more dax special cases in the
code.  Did I get that right?

The argument is that it's non-trivial, and we haven't even sorted out
basic semantics for directly mapped storaged.  Let's finish up getting
this right, and then look into optimizing it further in the next step.

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