On 2020/8/8 1:09, Ira Weiny wrote: > On Fri, Jul 31, 2020 at 06:59:32PM +0900, Yasunori Goto wrote: >> On 2020/07/30 8:21, 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-. >> >> For example, there are 2 containers executed in a system, which is named as >> container A and container B, and these host gives FS-DAX files to each >> containers. >> If the user of container A would like to change DAX-off for tuning, then he >> will stop his application >> and change DAX flag, but the flag may not be changed. >> >> Then he will "need" to ask host operator to execute drop_cache, and the >> operator did it. >> As a result, not only container A, but also container B get the impact of >> drop_cache. >> >> Especially, if this is multi tenant container system, then I think this is >> not acceptable. >> >> Probably, there are 2 problems I think. >> 1) drop_cache requires root permission. >> 2) drop_cache has too wide effect. >> >>>> 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. >> The current drop_cache feature tries to drop ALL of cache (page cache and/or >> slab cache). >> Then, I agree that normal user should not drop all of them. >> >> But my intention was that drop cache of ONE file which is changed dax flag, >> (and if possible, drop only the inode cache.) >> Do you mean it will be still cause of weakness against DoS attack? >> If so, I should give up to solve problem 1) at least. > FWIW changing the on disk flag automatically flags the inode to be dropped as > soon as all references are done. > > See: > > 2c567af418e3 fs: Introduce DCACHE_DONTCACHE > dae2f8ed7992 fs: Lift XFS_IDONTCACHE to the VFS layer Hi, I find that DCACHE_DONTCACHE doesn't work well. If DCACHE_REFERENCED is not set, dput() can drop the inode successfully as soon as all references are gone. By contrast, if DCACHE_REFERENCED is set, dput() only decreases the reference count of dentry and don't evict inode. Example 1: echo abcdefg > test.txt echo 3 > /proc/sys/vm/drop_caches xfs_io -c 'chattr +x' test.txt In this example, we can say the DAX policy takes effects immediately as we don't need to drop cache after chattr. In this circumstance, DCACHE_REFERENCED is not set, and DCACHE_DONTCACHE can drop the inode as expected. Example 2: echo abcdefg > test.txt xfs_io -c 'chattr +x' test.txt In this example, we must drop caches after chattr to make DAX policy take effects. This is because DCACHE_REFERENCED is set, and fast_dput() will return true, and then retain_dentry() have no chance to check DCACHE_DONTCACHE. If this is the desired behavior, I can't understand the necessity of DCACHE_DONTCACHE. Regards, Hao Li > > But from a users perspective you just don't know when that will happen. The > system just can't guarantee it. The best the user can do is stop taking > references to the file and close all references, and periodically check the > state. But this will take a reference so... Kind of a catch-22 here... :-( > > Ira > >> >> Thanks, >> >>> Cheers, >>> >>> Dave. >> -- >> Yasunori Goto >> >
