On Mon, Oct 10, 2016 at 05:07:45PM +1100, Dave Chinner wrote:

quoted

quoted

*However*, the DAX IO path locking in XFS  has changed in 4.9-rc1 to
match the buffered IO single writer POSIX semantics - the test is a
bad test based on the fact it exercised a path that is under heavy
development and so can't be used as a regression test across
multiple kernels.

That being said - I wonder if we should allow the shared lock on DAX
files IFF the user is specifying O_DIRECT in the open mode..

It should do - if it doesn't then we screwed up the IO path
selection logic in XFS and we'll need to fix it.

Depends on your defintion of "we".  The DAX code has always abused the
direct I/O path, and that abuse is ingrained in the VFS path in a way that
we can't easily undo it in XFS, e.g. take a look at io_is_direct and
iocb_flags in include/linux/fs.h, which ensure that DAX I/O will always
appear as IOCB_DIRECT to the fs.  It will take some time to untagle
this, but it's on my todo list once the last file system (ext4)
untangles the DAX and direct I/O path.