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? Step back for a minute and explain why you want to be able to change the DAX mode of a file -as a user-. > 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. No, drop_caches is not going to be made available to users. That makes it s trivial system wide DoS vector. Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx
