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

Li, Hao
2020-07-31T09:15:54Z

On 2020/7/30 7:21, Dave Chinner 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?
> Step back for a minute and explain why you want to be able to change
> the DAX mode of a file -as a user-.
chattr command can be executed by normal users as long as they want.
I think if they do this, they may get confused because the dax mode
doesn't take effects immediately.
>> 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.
> No, drop_caches is not going to be made available to users. That
> makes it s trivial system wide DoS vector.
drop_caches have to be limited for root user, but we may need to find
a way for normal users to make dax changing take effect if they have
run chattr.

Regards,
Hao Li

> Cheers,
>
> Dave.