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

From: Yasunori Goto <y-goto@fujitsu.com>
To: Dave Chinner <david@fromorbit.com>,
	"Li, Hao" <lihao2018.fnst@cn.fujitsu.com>
Cc: "linux-xfs@vger.kernel.org" <linux-xfs@vger.kernel.org>,
	"linux-nvdimm@lists.01.org" <linux-nvdimm@lists.01.org>,
	"ira.weiny@intel.com" <ira.weiny@intel.com>
Subject: Re: Can we change the S_DAX flag immediately on XFS without dropping caches?
Date: Wed, 29 Jul 2020 11:23:21 +0900	[thread overview]
Message-ID: <573feb69-bc38-8eb4-ee9b-7c49802eb737@fujitsu.com> (raw)
In-Reply-To: <20200728022059.GX2005@dread.disaster.area>

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