Hello, Ping. Thanks, Hao Li On 2020/7/31 17:12, Li, Hao wrote: > 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
