On Fri, Jun 24, 2016 at 09:24:46AM +1000, Dave Chinner wrote: > Except we did that *intentionally* - by definition there is no > cache to bypass with DAX and so all IO is "direct". That, combined > with the fact that all Linux filesystems except XFS break the POSIX > exclusive writer rule you are quoting to begin with, it seemed > pointless to enforce it for DAX.... No file system breaks the exclusive writer rule - most filesystem don't make writers atomic vs readers. More importantly every other filesystem (well there only are ext2 and ext4..) exludes DAX writers against other DAX writers. > So, before taking any patches to change that behaviour in XFS, a > wider discussion about the policy needs to be had. I don't think > we should care about POSIX here - if you have an application that > needs this serialisation, turn off DAX. That's why I made it a > per-inode inheritable flag and why the mount option will go away > over time. Sorry, but this is simply broken - allowing apps to opt-in behavior (e.g. like we're using O_DIRECT) is always fine. Requriring filesystem-specific tuning that has affect outside the app to get existing documented behavior is not how to design APIs. Maybe we'll need to opt-in to use DAX for mmap, but giving the same existing behavior for read and write and avoiding a copy to the pagecache is an obvious win. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
