On Fri, Jul 28, 2017 at 11:38:21AM +0200, Jan Kara wrote:

Well, you are right I can make the implementation work with struct file
flag as well - let's call it O_DAXDSYNC. However there are filesystem
operations where you may need to answer question: Is there any fd with
O_DAXDSYNC open against this inode (for operations that change file offset
-> block mapping)? And in that case inode flag is straightforward while
file flag is a bit awkward (you need to implement counter of fd's with that
flag in the inode).

We can still keep and inode flag as the internal implementation
detail.  As mentioned earlier the right flag to control behavior
of a mapping is an mmap flag.  And the initial naive implementation
would simply mark the inode as sync once the first MAP_SYNC open happens
on it.  We could then move to more precise tracking if/when needed.