On 2020/7/30 7:21, Dave Chinner 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? > Step back for a minute and explain why you want to be able to change > the DAX mode of a file -as a user-. chattr command can be executed by normal users as long as they want. I think if they do this, they may get confused because the dax mode doesn't take effects immediately. >> 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. drop_caches have to be limited for root user, but we may need to find a way for normal users to make dax changing take effect if they have run chattr. Regards, Hao Li > Cheers, > > Dave.
