On Fri, Feb 14, 2020 at 04:23:19PM -0500, Jeff Moyer wrote: > Ira Weiny <ira.weiny@xxxxxxxxx> writes: > > > [disclaimer: the following assumes the underlying 'device' (superblock) > > supports DAX] > > > > ... which results in S_DAX == false when the file is opened without the mount > > option. The key would be that all directories/files created under a root with > > XFS_DIFLAG2_DAX == true would inherit their flag and be XFS_DIFLAG2_DAX == true > > all the way down the tree. Any file not wanting DAX would need to set > > XFS_DIFLAG2_DAX == false. And setting false could be used on a directory to > > allow a user or group to not use dax on files in that sub-tree. > > > > Then without '-o dax' (XFS_MOUNT_DAX == false) all files when opened set S_DAX > > equal to XFS_DIFLAG2_DAX value. (Directories, as of V4, never get S_DAX set.) > > > > If '-o dax' (XFS_MOUNT_DAX == true) then S_DAX is set on all files. > > One more clarifying question. Let's say I set XFS_DIFLAG2_DAX on an > inode. I then open the file, and perform mmap/load/store/etc. I close > the file, and I unset XFS_DIFLAG2_DAX. Will the next open treat the > file as S_DAX or not? My guess is the inode won't be evicted, and so > S_DAX will remain set. The inode will not be evicted, or even it happens to be xfs_io will reload it to unset the XFS_DIFLAG2_DAX flag. And the S_DAX flag changes _with_ the XFS_DIFLAG2_DAX change when it can (when the underlying storage supports S_DAX). Trying to change XFS_DIFLAG2_DAX while the file is mmap'ed returns -EBUSY. Ira > > The reason I ask is I've had requests from application developers to do > just this. They want to be able to switch back and forth between dax > modes. > > Thanks, > Jeff > > > [1] I'm beginning to think that if I type dax one more time I'm going to go > > crazy... :-P > > dax dax dax! >
