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From: "Li, Hao" <>
To: Ira Weiny <>
Cc: Dave Chinner <>,
	"" <>,
	"" <>,
	<>, <>,
	<>, Yasunori Goto <>
Subject: Re: Can we change the S_DAX flag immediately on XFS without dropping caches?
Date: Fri, 31 Jul 2020 17:12:38 +0800
Message-ID: <> (raw)
In-Reply-To: <>

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.


Hao Li

> Would that work for your use case?
> Ira

  reply index

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-28  2:00 Li, Hao
2020-07-28  2:20 ` Dave Chinner
2020-07-29  2:23   ` Yasunori Goto
2020-07-29 16:10     ` Ira Weiny
2020-07-31  9:12       ` Li, Hao [this message]
2020-08-05  8:10         ` Li, Hao
2020-08-05 15:44           ` Darrick J. Wong
2020-08-07 16:57             ` Ira Weiny
2020-07-31 10:04       ` Yasunori Goto
2020-07-29 23:21     ` Dave Chinner
2020-07-31  9:15       ` Li, Hao
2020-07-31  9:59       ` Yasunori Goto
2020-08-07 17:09         ` Ira Weiny

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