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Subject: Re: Can we change the S_DAX flag immediately on XFS without dropping
 caches?
To: Ira Weiny <ira.weiny@intel.com>
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From: Yasunori Goto <y-goto@fujitsu.com>
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Date: Fri, 31 Jul 2020 19:04:03 +0900
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CC: Dave Chinner <david@fromorbit.com>, "Li, Hao" <lihao2018.fnst@cn.fujitsu.com>, "linux-xfs@vger.kernel.org" <linux-xfs@vger.kernel.org>, "linux-nvdimm@lists.01.org" <linux-nvdimm@lists.01.org>
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On 2020/07/30 1:10, 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.  So there is no need
> to switch the file mode directly as a normal user.
>
> Would that work for your use case?
Though I wrote it on another mail, your information was very helpful for me.
Thank you for your response.

Bye,

>
> Ira
>
>> Thanks,
>>
>> -- 
>> Yasunori Goto
>>

-- 
Yasunori Goto
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