On 2020/7/30 0:10, Ira Weiny wrote: > On Wed, Jul 29, 2020 at 11:23:21AM +0900, Yasunori Goto wrote: >> Hi, >> >> On 2020/07/28 11:20, Dave Chinner wrote: >>> On Tue, Jul 28, 2020 at 02:00:08AM +0000, Li, Hao wrote: >>>> Hi, >>>> >>>> I have noticed that we have to drop caches to make the changing of S_DAX >>>> flag take effect after using chattr +x to turn on DAX for a existing >>>> regular file. The related function is xfs_diflags_to_iflags, whose >>>> second parameter determines whether we should set S_DAX immediately. >>> Yup, as documented in Documentation/filesystems/dax.txt. Specifically: >>> >>> 6. When changing the S_DAX policy via toggling the persistent FS_XFLAG_DAX flag, >>> the change in behaviour for existing regular files may not occur >>> immediately. If the change must take effect immediately, the administrator >>> needs to: >>> >>> a) stop the application so there are no active references to the data set >>> the policy change will affect >>> >>> b) evict the data set from kernel caches so it will be re-instantiated when >>> the application is restarted. This can be achieved by: >>> >>> i. drop-caches >>> ii. a filesystem unmount and mount cycle >>> iii. a system reboot >>> >>>> I can't figure out why we do this. Is this because the page caches in >>>> address_space->i_pages are hard to deal with? >>> Because of unfixable races in the page fault path that prevent >>> changing the caching behaviour of the inode while concurrent access >>> is possible. The only way to guarantee races can't happen is to >>> cycle the inode out of cache. >> I understand why the drop_cache operation is necessary. Thanks. >> >> BTW, even normal user becomes to able to change DAX flag for an inode, >> drop_cache operation still requires root permission, right? >> >> So, if kernel have a feature for normal user can operate drop cache for "a >> inode" with >> its permission, I think it improve the above limitation, and >> we would like to try to implement it recently. >> >> Do you have any opinion making such feature? >> (Agree/opposition, or any other comment?) > I would not be opposed but there were many hurdles to that implementation. > > What is the use case you are thinking of here? > > The compromise of dropping caches was reached because we envisioned that many > users would simply want to chose the file mode when a file was created and > maintain that mode through the lifetime of the file. To that end one can > simply create directories which have the desired dax mode and any files created > in that directory will inherit the dax mode immediately. Inheriting mechanism for DAX mode is reasonable but chattr&drop_caches makes things complicated. > So there is no need > to switch the file mode directly as a normal user. The question is, the normal users can indeed use chattr to change the DAX mode for a regular file as long as they want. However, when they do this, they have no way to make the change take effect. I think this behavior is weird. We can say chattr executes successfully because XFS_DIFLAG2_DAX has been set onto xfs_inode->i_d.di_flags2, but we can also say chattr doesn't finish things completely because S_DAX is not set onto inode->i_flags. The user may be confused about why chattr +/-x doesn't work at all. Maybe we should find a way for the normal user to make chattr take effects without calling the administrator, or we can make the chattr +/x command request root permission now that if the user has root permission, he can make DAX changing take effect through echo 2 > /proc/sys/vm/drop_caches. Regards, Hao Li > > Would that work for your use case? > > Ira
