Re: Can we change the S_DAX flag immediately on XFS without dropping caches?

[Dave Chinner <david@fromorbit.com>]

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@fromorbit.com
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