[Virtio-fs] [PATCH v6 2/7] fuse: make DAX mount option a tri-state

[JeffleXu]

On 10/29/21 9:03 PM, Vivek Goyal wrote:
> On Fri, Oct 29, 2021 at 04:33:06PM +0800, JeffleXu wrote:
>>
>>
>> On 10/20/21 10:48 PM, Vivek Goyal wrote:
>>> On Wed, Oct 20, 2021 at 10:52:38AM +0800, JeffleXu wrote:
>>>>
>>>>
>>>> On 10/18/21 10:10 PM, Vivek Goyal wrote:
>>>>> On Mon, Oct 11, 2021 at 11:00:47AM +0800, Jeffle Xu wrote:
>>>>>> We add 'always', 'never', and 'inode' (default). '-o dax' continues to
>>>>>> operate the same which is equivalent to 'always'. To be consistemt with
>>>>>> ext4/xfs's tri-state mount option, when neither '-o dax' nor '-o dax='
>>>>>> option is specified, the default behaviour is equal to 'inode'.
>>>>>
>>>>> Hi Jeffle,
>>>>>
>>>>> I am not sure when  -o "dax=inode"  is used as a default? If user
>>>>> specifies, "-o dax" then it is equal to "-o dax=always", otherwise
>>>>> user will explicitly specify "-o dax=always/never/inode". So when
>>>>> is dax=inode is used as default?
>>>>
>>>> That means when neither '-o dax' nor '-o dax=always/never/inode' is
>>>> specified, it is actually equal to '-o dax=inode', which is also how
>>>> per-file DAX on ext4/xfs works.
>>>>
>>>> This default behaviour for local filesystem, e.g. ext4/xfs, may be
>>>> straightforward, since the disk inode will be read into memory during
>>>> the inode instantiation, and checking for persistent inode attribute
>>>> shall be realatively cheap, except that the default behaviour has
>>>> changed from 'dax=never' to 'dax=inode'.
>>>
>>> Interesting that ext4/xfs allowed for this behavior change.
>>>
>>>>
>>>> Come back to virtiofs, when neither '-o dax' nor '-o
>>>> dax=always/never/inode' is specified, and it actually behaves as '-o
>>>> dax=inode', as long as '-o dax=server/attr' option is not specified for
>>>> virtiofsd, virtiofsd will always clear FUSE_ATTR_DAX and thus guest will
>>>> always disable DAX. IOWs, the guest virtiofs atually behaves as '-o
>>>> dax=never' when neither '-o dax' nor '-o dax=always/never/inode' is
>>>> specified, and '-o dax=server/attr' option is not specified for virtiofsd.
>>>>
>>>> But I'm okay if we need to change the default behaviour for virtiofs.
>>>
>>> This is change of behavior from client's perspective. Even if client
>>> did not opt-in for DAX, DAX can be enabled based on server's setting.
>>> Not that there is anything wrong with it, but change of behavior part
>>> concerns me.
>>>
>>> In case of virtiofs, lot of features we are controlling from server.
>>> Client typically just calls "mount" and there are not many options
>>> users can specify for mount.  
>>>
>>> Given we already allowed to make client a choice about DAX behavior,
>>> I will feel more comfortable that we don't change it and let client
>>> request a specific DAX mode and if client does not specify anything,
>>> then DAX is not enabled.
>>>
>>
>> Hi Vivek,
>>
>> How about the following design about the default behavior to move this
>> patchset forward, considering the discussion on another thread [1]?
>>
>> - guest side: '-o dax=inode' acts as the default, keeping consistent
>> with xfs/ext4
> 
> This sounds good.
> 
>> - virtiofsd: the default behavior is like, neither file size based
>> policy ('dax=server') nor persistent inode flags based policy
>> ('dax=attr') is used, though virtiofsd indeed advertises that
>> it supports per inode DAX feature (so that it won't fail FUSE_INIT
>> negotiation phase when guest advertises dax=inode by default)... In
>> fact, it acts like 'dax=never' in this case.
> 
> Not sure why virtiofsd needs to advertise that it supports per inode
> DAX even if no per inode dax policy is in affect. Guest will know that
> server is not supporting per inode DAX. But it will not return an
> error to user space (because dax=inode seems to be advisory).
> 
> IOW, this is very similar to the case of using dax=inode on a block
> device which does not support DAX. No errors and no warnings.

OK. I will adopt this behavior. That is, if virtiofsd is not specified
with 'dax=server|attr' option, it won't advertise support for per inode
DAX in FUSE_INIT either. And then client will fallback to 'dax=never'
even if it is mounted with 'dax=inode'.

> 
>>
>> Then when guest opts-in and specifies '-o dax=inode' manually, then it
>> shall realize that proper configuration for virtiofsd is also needed (-o
>> dax=server|attr).
> 
> I gave some comments w.r.t dax=server naming in your posting. Not sure if
> you got a chance to look at it.
> 
> Thanks
> Vivek
> 
>>
>> [1] https://www.spinics.net/lists/linux-xfs/msg56642.html
>>
>> -- 
>> Thanks,
>> Jeffle
>>

-- 
Thanks,
Jeffle