On Mon, Aug 11, 2014 at 10:51:47AM +0200, Jan Kara wrote:

So I'm afraid we'll have to find some other way to synchronize
page faults and truncate / punch hole in DAX.

What if we don't?  If we hit the race (which is vanishingly unlikely with
real applications), the consequence is simply that after a truncate, a
file may be left with one or two blocks allocated somewhere after i_size.
As I understand it, that's not a real problem; they're temporarily
unavailable for allocation but will be freed on file removal or the next
truncation of that file.

I'm also still considering the possibility of having truncate-down block
until all mmaps that extend after the new i_size have been removed ...

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