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* Can we change the S_DAX flag immediately on XFS without dropping caches?
@ 2020-07-28  2:00 Li, Hao
  2020-07-28  2:20 ` Dave Chinner
  0 siblings, 1 reply; 10+ messages in thread
From: Li, Hao @ 2020-07-28  2:00 UTC (permalink / raw)
  To: linux-xfs, linux-nvdimm; +Cc: ira.weiny, Gotou, Yasunori

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.

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? I also wonder what will
happen if we set S_DAX unconditionally. Thanks!

Regards,
Hao Li



^ permalink raw reply	[flat|nested] 10+ messages in thread

* Re: Can we change the S_DAX flag immediately on XFS without dropping caches?
  2020-07-28  2:00 Can we change the S_DAX flag immediately on XFS without dropping caches? Li, Hao
@ 2020-07-28  2:20 ` Dave Chinner
  2020-07-29  2:23   ` Yasunori Goto
  0 siblings, 1 reply; 10+ messages in thread
From: Dave Chinner @ 2020-07-28  2:20 UTC (permalink / raw)
  To: Li, Hao; +Cc: linux-xfs, linux-nvdimm, ira.weiny, Gotou, Yasunori

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 also wonder what will
> happen if we set S_DAX unconditionally. Thanks!

As per the documentation, that's exactly what the dax=always mount
option does.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

^ permalink raw reply	[flat|nested] 10+ messages in thread

* Re: Can we change the S_DAX flag immediately on XFS without dropping caches?
  2020-07-28  2:20 ` Dave Chinner
@ 2020-07-29  2:23   ` Yasunori Goto
  2020-07-29 16:10     ` Ira Weiny
  2020-07-29 23:21     ` Dave Chinner
  0 siblings, 2 replies; 10+ messages in thread
From: Yasunori Goto @ 2020-07-29  2:23 UTC (permalink / raw)
  To: Dave Chinner, Li, Hao; +Cc: linux-xfs, linux-nvdimm, ira.weiny

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


^ permalink raw reply	[flat|nested] 10+ messages in thread

* Re: Can we change the S_DAX flag immediately on XFS without dropping caches?
  2020-07-29  2:23   ` Yasunori Goto
@ 2020-07-29 16:10     ` Ira Weiny
  2020-07-31  9:12       ` Li, Hao
  2020-07-31 10:04       ` Yasunori Goto
  2020-07-29 23:21     ` Dave Chinner
  1 sibling, 2 replies; 10+ messages in thread
From: Ira Weiny @ 2020-07-29 16:10 UTC (permalink / raw)
  To: Yasunori Goto; +Cc: Dave Chinner, Li, Hao, linux-xfs, linux-nvdimm

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?

Ira

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

^ permalink raw reply	[flat|nested] 10+ messages in thread

* Re: Can we change the S_DAX flag immediately on XFS without dropping caches?
  2020-07-29  2:23   ` Yasunori Goto
  2020-07-29 16:10     ` Ira Weiny
@ 2020-07-29 23:21     ` Dave Chinner
  2020-07-31  9:15       ` Li, Hao
  2020-07-31  9:59       ` Yasunori Goto
  1 sibling, 2 replies; 10+ messages in thread
From: Dave Chinner @ 2020-07-29 23:21 UTC (permalink / raw)
  To: Yasunori Goto; +Cc: Li, Hao, linux-xfs, linux-nvdimm, ira.weiny

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-.

> 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.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

^ permalink raw reply	[flat|nested] 10+ messages in thread

* Re: Can we change the S_DAX flag immediately on XFS without dropping caches?
  2020-07-29 16:10     ` Ira Weiny
@ 2020-07-31  9:12       ` Li, Hao
  2020-08-05  8:10         ` Li, Hao
  2020-07-31 10:04       ` Yasunori Goto
  1 sibling, 1 reply; 10+ messages in thread
From: Li, Hao @ 2020-07-31  9:12 UTC (permalink / raw)
  To: Ira Weiny
  Cc: Dave Chinner, linux-xfs, linux-nvdimm, yangx.jy, ruansy.fnst,
	gujx, Yasunori Goto

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.


Regards,

Hao Li

>
> Would that work for your use case?
>
> Ira



^ permalink raw reply	[flat|nested] 10+ messages in thread

* Re: Can we change the S_DAX flag immediately on XFS without dropping caches?
  2020-07-29 23:21     ` Dave Chinner
@ 2020-07-31  9:15       ` Li, Hao
  2020-07-31  9:59       ` Yasunori Goto
  1 sibling, 0 replies; 10+ messages in thread
From: Li, Hao @ 2020-07-31  9:15 UTC (permalink / raw)
  To: Dave Chinner
  Cc: linux-xfs, linux-nvdimm, ira.weiny, ruansy.fnst, yangx.jy, gujx,
	Yasunori Goto

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.



^ permalink raw reply	[flat|nested] 10+ messages in thread

* Re: Can we change the S_DAX flag immediately on XFS without dropping caches?
  2020-07-29 23:21     ` Dave Chinner
  2020-07-31  9:15       ` Li, Hao
@ 2020-07-31  9:59       ` Yasunori Goto
  1 sibling, 0 replies; 10+ messages in thread
From: Yasunori Goto @ 2020-07-31  9:59 UTC (permalink / raw)
  To: Dave Chinner; +Cc: Li, Hao, linux-xfs, linux-nvdimm, ira.weiny

On 2020/07/30 8:21, 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-.


For example, there are 2 containers executed in a system, which is named as
container A and container B, and these host gives FS-DAX files to each 
containers.
If the user of container A would like to change DAX-off for tuning, then 
he will stop his application
and change DAX flag, but the flag may not be changed.

Then he will "need" to ask host operator to execute drop_cache, and the 
operator did it.
As a result, not only container A, but also container B get the impact 
of drop_cache.

Especially, if this is multi tenant container system, then I think this 
is not acceptable.

Probably, there are 2 problems I think.
1) drop_cache requires root permission.
2) drop_cache has too wide effect.

>
>> 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.

The current drop_cache feature tries to drop ALL of cache (page cache 
and/or slab cache).
Then, I agree that normal user should not drop all of them.

But my intention was that drop cache of ONE file which is changed dax flag,
(and if possible, drop only the inode cache.)
Do you mean it will be still cause of weakness against DoS attack?
If so, I should give up to solve problem 1) at least.


Thanks,

>
> Cheers,
>
> Dave.

-- 
Yasunori Goto


^ permalink raw reply	[flat|nested] 10+ messages in thread

* Re: Can we change the S_DAX flag immediately on XFS without dropping caches?
  2020-07-29 16:10     ` Ira Weiny
  2020-07-31  9:12       ` Li, Hao
@ 2020-07-31 10:04       ` Yasunori Goto
  1 sibling, 0 replies; 10+ messages in thread
From: Yasunori Goto @ 2020-07-31 10:04 UTC (permalink / raw)
  To: Ira Weiny; +Cc: Dave Chinner, Li, Hao, linux-xfs, linux-nvdimm



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


^ permalink raw reply	[flat|nested] 10+ messages in thread

* Re: Can we change the S_DAX flag immediately on XFS without dropping caches?
  2020-07-31  9:12       ` Li, Hao
@ 2020-08-05  8:10         ` Li, Hao
  0 siblings, 0 replies; 10+ messages in thread
From: Li, Hao @ 2020-08-05  8:10 UTC (permalink / raw)
  To: Ira Weiny
  Cc: Dave Chinner, linux-xfs, linux-nvdimm, yangx.jy, ruansy.fnst,
	gujx, Yasunori Goto

Hello,

Ping.

Thanks,
Hao Li


On 2020/7/31 17:12, Li, Hao wrote:
> 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.
>
>
> Regards,
>
> Hao Li
>
>> Would that work for your use case?
>>
>> Ira



^ permalink raw reply	[flat|nested] 10+ messages in thread

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Thread overview: 10+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2020-07-28  2:00 Can we change the S_DAX flag immediately on XFS without dropping caches? 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
2020-08-05  8:10         ` Li, Hao
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

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