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. > > They are two > separate flags with two separate behaviors: > > FS_XFLAG_DAX is a filesystem inode metadata flag. > > Setting FS_XFLAG_DAX on a directory causes all files and directories > created within that directory to inherit FS_XFLAG_DAX. > > Mounting with -o dax causes all files and directories created to have > FS_XFLAG_DAX set regardless of the parent's status. I don't believe this is true, either before _or_ after this patch set. '-o dax' only causes XFS_MOUNT_DAX to be set which then cause S_DAX to be set. It does not affect FS_XFLAG_DAX. This is important because we don't want '-o dax' to suddenly convert all files to DAX if '-o dax' is not used. > > The FS_XFLAG_DAX can be get and set via the fs[g]etxattr ioctl. Right statx was the wrong tool... fs[g|s]etattr via the xfs_io -c 'chatttr|lsattr' is the correct tool. > > ------- > > S_DAX is the flag that controls the IO path in the kernel for a given > inode. > > Loading a file inode into the kernel (via _iget) with FS_XFLAG_DAX set > or creating a file inode that inherits FS_XFLAG_DAX causes the incore > inode to have the S_DAX flag set if the storage device supports it. Yes after reworking "Clean up DAX support check" I believe I've got it correct now. Soon to be in V4. > > Files with S_DAX set use the dax IO paths through the kernel. > > The S_DAX flag can be queried via statx. Yes as a verification that the file is at that moment operating as dax. It will not return true for a directory ever. My bad for saying that. Sorry I got my tools flags mixed up... Ira
