On Wed, Jul 29, 2020 at 11:23:21AM +0900, Yasunori Goto wrote:
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
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.