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

Li, Hao
2020-08-05T08:11:29Z

Hello,

Ping.

Thanks,
Hao Li

On 2020/7/31 17:12, Li, Hao wrote:
> 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