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

Li, Hao
2020-07-31T09:12:47Z

On 2020/7/30 0:10, Ira Weiny 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?
>>
>> 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?)
> I would not be opposed but there were many hurdles to that implementation.
>
> What is the use case you are thinking of here?
>
> The compromise of dropping caches was reached because we envisioned that many
> users would simply want to chose the file mode when a file was created and
> maintain that mode through the lifetime of the file. To that end one can
> simply create directories which have the desired dax mode and any files created
> in that directory will inherit the dax mode immediately.
Inheriting mechanism for DAX mode is reasonable but chattr&drop_caches
makes things complicated.
> So there is no need
> to switch the file mode directly as a normal user.

The question is, the normal users can indeed use chattr to change the DAX
mode for a regular file as long as they want. However, when they do this,
they have no way to make the change take effect. I think this behavior is
weird. We can say chattr executes successfully because XFS_DIFLAG2_DAX has
been set onto xfs_inode->i_d.di_flags2, but we can also say chattr doesn't
finish things completely because S_DAX is not set onto inode->i_flags.
The user may be confused about why chattr +/-x doesn't work at all. Maybe
we should find a way for the normal user to make chattr take effects
without calling the administrator, or we can make the chattr +/x command
request root permission now that if the user has root permission, he can
make DAX changing take effect through echo 2 > /proc/sys/vm/drop_caches.

Regards,

Hao Li

>
> Would that work for your use case?
>
> Ira