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

Dave Chinner
2020-07-28T02:21:07Z

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 also wonder what will
> happen if we set S_DAX unconditionally. Thanks!

As per the documentation, that's exactly what the dax=always mount
option does.

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com