On Thu, Sep 15, 2016 at 05:16:42PM -0700, Dan Williams wrote: > On Thu, Sep 15, 2016 at 4:07 PM, Dave Chinner <david@xxxxxxxxxxxxx> wrote: > > On Thu, Sep 15, 2016 at 10:01:03AM -0700, Dan Williams wrote: > >> On Thu, Sep 15, 2016 at 1:26 AM, Christoph Hellwig <hch@xxxxxx> wrote: > >> > On Wed, Sep 14, 2016 at 11:54:38PM -0700, Dan Williams wrote: > >> >> The DAX property, page cache bypass, of a VMA is only detectable via the > >> >> vma_is_dax() helper to check the S_DAX inode flag. However, this is > >> >> only available internal to the kernel and is a property that userspace > >> >> applications would like to interrogate. > >> > > >> > They have absolutely no business knowing such an implementation detail. > >> > >> Hasn't that train already left the station with FS_XFLAG_DAX? > > > > No, that's an admin flag, not a runtime hint for applications. Just > > because that flag is set on an inode, it does not mean that DAX is > > actually in use - it will be ignored if the backing dev is not dax > > capable. > > What's the point of an admin flag if an admin can't do cat /proc/<pid > of interest>/smaps, or some other mechanism, to validate that the > setting the admin cares about is in effect? Sorry, I don't follow - why would you be looking at mapping file regions in /proc to determine if some file somewhere in a filesystem has a specific flag set on it or not? FS_XFLAG_DAX is an inode attribute flag, not something you can query or administrate through mmap: I.e. # xfs_io -c "lsattr" -c "chattr +x" -c lsattr -c "chattr -x" -c "lsattr" foo --------------- foo --------------x foo --------------- foo # What happens when that flag is set on an inode is determined by a whole bunch of other things that are completely separate to the management of the inode flag itself. Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>