I can see no sign of that in the ext4 git changelog. AFAIK barriers were
always enabled for ext4... but commit
5f3481e9a80c240f169b36ea886e2325b9aeb745 causes an fdatasync() in the
middle of an already-allocated file to always flush its blocks out (with a
barrier). PostgreSQL would be 'bitten' by this hard (really bitten by the
bug it fixes): almost all its writes are in the middle of
already-allocated files, and before this change the fdatasync() wouldn't
actually have synced anything but the inode, AFAICS.