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

Yasunori Goto
2020-07-29T02:34:39Z

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?

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.

Do you have any opinion making such feature?
(Agree/opposition, or any other comment?)

Thanks,

--
Yasunori Goto