On Thu, Feb 13, 2020 at 04:16:17PM -0800, Dan Williams wrote: > On Thu, Feb 13, 2020 at 3:29 PM Ira Weiny <ira.weiny@xxxxxxxxx> wrote: > > > > On Thu, Feb 13, 2020 at 11:58:39AM -0800, Darrick J. Wong wrote: > > > On Thu, Feb 13, 2020 at 11:05:13AM -0800, Ira Weiny wrote: > > > > On Thu, Feb 13, 2020 at 11:01:57AM -0800, 'Ira Weiny' wrote: > > > > > On Wed, Feb 12, 2020 at 02:49:48PM -0500, Jeff Moyer wrote: > > > > > > Ira Weiny <ira.weiny@xxxxxxxxx> writes: > > > > > > > > > > > > > > [snip] > > > > > > > > > > Given that we document the dax mount > > > > > > option as "the way to get dax," it may be a good idea to allow for a > > > > > > user to selectively disable dax, even when -o dax is specified. Is that > > > > > > possible? > > > > > > > > > > Not with this patch set. And I'm not sure how that would work. The idea was > > > > > that -o dax was simply an override for users who were used to having their > > > > > entire FS be dax. We wanted to depreciate the use of "-o dax" in general. The > > > > > individual settings are saved so I don't think it makes sense to ignore the -o > > > > > dax in favor of those settings. Basically that would IMO make the -o dax > > > > > useless. > > > > > > > > Oh and I forgot to mention that setting 'dax' on the root of the FS basically > > > > provides '-o dax' functionality by default with the ability to "turn it off" > > > > for files. > > > > > > Please don't further confuse FS_XFLAG_DAX and S_DAX. > > > > Yes... the above text is wrong WRT statx. But setting the physical > > XFS_DIFLAG2_DAX flag on the root directory will by default cause all files and > > directories created there to be XFS_DIFLAG2_DAX and so forth on down the tree > > unless explicitly changed. This will be the same as mounting with '-o dax' but > > with the ability to turn off dax for individual files. Which I think is the > > functionality Jeff is wanting. > > To be clear you mean turn off XFS_DIFLAG2_DAX, not mask S_DAX when you > say "turn off dax", right? Yes. [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. [IF the underlying 'device' (superblock) does _not_ support DAX] ... S_DAX is _never_ set but the underlying XFS_DIFLAG2_DAX flags can be toggled and will be inherited as above. Because S_DAX is never set access to that file will be restricted to "not dax"...[1] I could go into that level of detail in the doc if needed? I feel like we need a more general name for XFS_DIFLAG2_DAX if I do.[2] > > The mount option simply forces "S_DAX" on all regular files as long as > the underlying device (or soon to be superblock for virtiofs) supports > it. There is no method to mask S_DAX when the filesystem was mounted > with -o dax. Otherwise we would seem to need yet another physical flag > to "always disable" dax. Exactly. I don't think we want to support that. From this thread alone it seems we have enough complexity and that would be another layer... ;-) Ira [1] I'm beginning to think that if I type dax one more time I'm going to go crazy... :-P [2] I have patches in the wings to introduce EXT4_DAX_FL as an ext4 on disk bit which would be equivalent to XFS_DIFLAG2_DAX. If anyone wants a better name let me know.
