On 2020/7/30 7:21, Dave Chinner wrote:
On Wed, Jul 29, 2020 at 11:23:21AM +0900, Yasunori Goto wrote:
> On 2020/07/28 11:20, Dave Chinner wrote:
>> On Tue, Jul 28, 2020 at 02:00:08AM +0000, Li, Hao wrote:
>>> 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
>> 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