Re: [PATCH v5 2/5] fuse: make DAX mount option a tri-state

[JeffleXu <jefflexu@linux.alibaba.com>]

On 9/27/21 8:21 AM, Dave Chinner wrote:
> On Thu, Sep 23, 2021 at 09:02:26PM -0400, Vivek Goyal wrote:
>> On Fri, Sep 24, 2021 at 08:26:18AM +1000, Dave Chinner wrote:
>>> On Thu, Sep 23, 2021 at 03:02:41PM -0400, Vivek Goyal wrote:
>>>> On Thu, Sep 23, 2021 at 05:25:23PM +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'.
>>>>
>>>> So will "-o dax=inode" be used for per file DAX where dax mode comes
>>>> from server?
>>>>
>>>> I think we discussed this. It will be better to leave "-o dax=inode"
>>>> alone. It should be used when we are reading dax status from file
>>>> attr (like ext4 and xfs). 
>>>>
>>>> And probably create separate option say "-o dax=server" where server
>>>> specifies which inode should use dax.
>>>
>>> That seems like a poor idea to me.
>>>
>>> The server side already controls what the client side does by
>>> controlling the inode attributes that the client side sees.  That
>>> is, if the server is going to specify whether the client side data
>>> access is going to use dax, then the server presents the client with
>>> an inode that has the DAX attribute flag set on it.
>>
>> Hi Dave,
>>
>> Currently in fuse/virtiofs, DAX is compltely controlled by client. Server
>> has no say in it. If client is mounted with "-o dax", dax is enabled on
>> all files otherwise dax is disabled on all files. One could think of
>> implementing an option on server so that server could deny mmap()
>> requests that come from client, but so far nobody asked for such
>> an option on server side.
> 
> Can you please refer to the new options "always|never|inode" in
> disucssions, because the "-o dax" option is deprecated and we really
> need to center discussions around the new options, not the
> deprecated one.

Consistent with ext4/xfs, "-o dax" is equal to "-o dax=always", and DAX
is always enabled/disabled with "-o dax=always|never" regardless the per
inode dax flag.

With "-o dax=inode", the client gets the per inode dax flag from FUSE
protocol (during aops->lookup()) fed by server, deciding if DAX shall be
enabled or not for this specific file.

> 
>> When you say "inode that has DAX attribute flag set on it", are you
>> referring to "S_DAX (in inode->i_flags)" or persistent attr
>> "FS_XFLAG_DAX"?
> 
> Neither. The S_DAX flags is the VFS struct inode state that tells
> the kernel what to do with that inode. The FS_XFLAG_DAX is the
> userspace API status/control flag that represents the current state
> of the persistent per-inode DAX control attribute.
> 
> What I'm talking about the persistent attribute that is present
> on stable storage. e.g. in the XFS on-disk inode the flag is
> XFS_DIFLAG2_DAX which is never seen outside low level XFS code. In
> the case of FUSE, this is the persistent server-side DAX attribute
> state. The client gets this information from the server via an
> inode attribute defined in the FUSE protocol. The client translates
> that network protocol attribute to S_DAX status on FS inode
> instantiation, and to/from FS_XFLAG_DAX for user control under
> dax=inode.

Got it.

> 
> In the case that the user changes FS_XFLAG_DAX, the FUSE client
> needs to communicate that attribute change to the server, where the
> server then changes the persistent state of the on-disk inode so
> that the next time the client requests that inode, it gets the state
> it previously set. Unless, of course, there are server side policy
> overrides (never/always).

One thing I'm concerned with is that, is the following behavior
consistent with the semantics of per-file DAX in ext4/xfs?

Client changes FS_XFLAG_DAX, this change is communicated to server and
then server also returns **success**. Then client finds that this file
is not DAX enabled, since server doesn't honor the previously set state.

IOWs, shall server always honor the persistent per-inode attribute of
host file (if the change of FS_XFLAG_DAX inside guest is stored as
persistent per-inode attribute on host file)?

> 
> IOWs, dax=inode on the client side is essentially "follow server
> side policy" because the server maintains the persistent DAX flag
> state in the server side inode...
> 
>> As of now S_DAX on client side inode is set by fuse client whenever
>> client mounted filesystem with "-o dax". And I think you are assuming
> 
> Of course. Similarly, if the client uses dax=never, the S_DAX is
> never set on the VFS inode. But if dax=inode is set, where does the
> DAX attribute state that determines S_DAX comes from .....?
> 
>> that DAX attribute of inode is already coming from server. That's not
>> the case. In fact that seems to be the proposal. Provide capability
>> so that server can specify which inode should be using DAX and which
>> inode should not be.
> 
> Yup, that's exactly what I'm talking about - this is "dax=inode"
> behaviour, because the client follows what the server tells it
> in the inode attributes that arrive over the wire.
> 
>>> In that case, turning off dax on the guest side should be
>>> communicated to the fuse server so the server turns off the DAX flag
>>> on the server side iff server side policy allows it.
>>
>> Not sure what do you mean by server turns of DAX flag based on client
>> turning off DAX. Server does not have to do anything. If client decides
>> to not use DAX (in guest), it will not send FUSE_SETUPMAPPING requests
>> to server and that's it.
> 
> Where does the client get it's per-inode DAX policy from if
> dax=inode is, like other DAX filesystems, the default behaviour?
> 
> Where is the persistent storage of that per-inode attribute kept?

In the latest patch set, it is not supported yet to change FS_XFLAG_DAX
(and thus setting/clearing persistent per-inode attribute) inside guest,
since this scenario is not urgently needed as the real using case.
Currently the per-inode dax attribute is completely fed from server
thourgh FUSE protocol, e.g. server could set/clear the per-inode dax
attribute depending on the file size.

The previous path set indeed had ever supported changing FS_XFLAG_DAX
and accordingly setting/clearing persistent per-inode attribute inside
guest. For "passthrough" type virtiofsd, the persistent per-inode
attribute is stored as XFS_DIFLAG2_DAX/EXT4_DAX_FL on host file
directly, since this is what "passthrough" means.

> 
>>> When the guest
>>> then purges it's local inode and reloads it from the server then it
>>> will get an inode with the DAX flag set according to server side
>>> policy.
>>
>> So right now we don't have a mechanism for server to specify DAX flag.
>> And that's what this patch series is trying to do.
>>
>>>
>>> Hence if the server side is configured with "dax=always" or
>>> dax="never" semantics it means the client side inode flag state
>>> cannot control DAX mode. That means, regardless of the client side
>>> mount options, DAX is operating under always or never policy,
>>
>> Hmm..., When you say "server side is configured with "dax=always", 
>> do you mean shared directory on host is mounted with "-o dax=always",
>> or you mean some virtiofs server specific option which enables
>> dax on all inodes from server side.
> 
> I don't care. That's a server side implementation detail. You can
> keep it in a private xattr for all I care.
> 
>> In general, DAX on host and DAX inside guest are completely independent.
>> Host filesystem could very well be mounted with dax or without dax and
>> that has no affect on guests's capability to be able to enable DAX or
>> not. 
> 
> If you have both the host and guest accessing the same files with
> different modes, then you have a data coherency problem that is
> guaranteed to corrupt data.
> 
>>> seems to be exactly what 'dax=inode' behaviour means on the client
>>> side - it behaves according to how the server side propagates the
>>> DAX attribute to the client for each inode.
>>
>> Ok. So "-o dax=inode" in fuse will have a different meaning as opposed
>> to ext4/xfs. This will mean that server will pass DAX state of inode
>> when inode is instantiated and client should honor that. 
> 
> No, that's exactly what dax=inode means: DAX behaviour is
> per-inode state that users must probe via statx() to determine if
> dax is active or not.
> 
>> But what about FS_XFLAG_DAX flag then. Say host file system
>> does support this att and fuse/virtiofs allows querying and
>> setting this attribute (I don't think it is allowed now).
> 
> FS_XFLAG_DAX is the ioctl-based control API for the client side.
> It's not a persistent flag.
> 
>> So will we not create a future conflict where in case of
>> fuse/virtiofs "-o dax=inode" means something different and it does
>> look at FS_XFLAG_DAX file attr.
> 
> Like many, I suspect you might be mis-understanding the DAX API.
> 
> There are 4 parts to it:
> 
> - persistent filesystem inode state flag
> - ioctls to manipulate persistent inode state flag (FS_XFLAG_DAX)
> - mount options to override persistent inode state flag
> - VFS inode state indicating DAX is active (S_DAX, STATX_ATTR_DAX)
> 
> You seem to be conflating the user API FS_XFLAG_DAX flag with
> whatever the filesystem uses to physically storage that state. They
> are not the same thing. FS_XFLAG_DAX also has no correlation with
> the mount options - we can change the on-disk flag state regardless
> of the mount option in use. We can even change the on-disk state
> flag on *kernels that don't support DAX*.  Changing the on-disk
> attribute *does not change S_DAX*.
> 
> That's because the on-disk persistent state is a property of the
> filesystem's on-disk format, not a property of the kernel that is
> running on that machine. The persistent state flag is managed as a
> completely independent filesystem inode attribute, but it only
> affects behaviour when the dax=inode mode has been selected.
> 
> Control of the behaviour by this persistent inode flag is what
> dax=inode means. It does not define how that attribute flag is
> managed, it just defines the policy that an the inode's behaviour
> will be determined by it's dax attribute state.
> 
> OTOH, S_DAX is used by the kernel to enable DAX access. It is the
> _mechanism_, not the policy. We control S_DAX by mount option and/or
> the filesystems persistent inode state flag, and behaviour is
> determined by these policies at VFS inode instantiation time. Change
> the policy, turf the inode out of cache and re-instantiate it, and
> S_DAX for that inode is recalculated from the new policy.
> 
> For FUSE, the server provides the persistent storage and nothing
> else. The ioctls to manipulate dax state are client side ioctls, as
> are the mount options and the S_DAX vfs inode state. Hence for
> server side management of the per-inode S_DAX state, the FUSE
> protocol needs to be able to pass the per-inode persistent DAX
> attribute state to the client and the client needs to honor
> that attribute from the server.
> 
> How the FUSE server stores this persistent attribute is up to the
> server implementation. If the server wants FUSE access to be
> independent of host access, it can't use the persistent inode flags
> in the host filesystem - it will need to use it's own xattrs. If the
> server wants host access to be coherent with the guest, then it will
> need to implement that in a different way.
> 
> But the FUSE client doesn't care about any of this server side
> policy mumbo-jumbo - it just needs to do what the server sends
> to it in the DAX inode attribute. And that's exactly what dax=inode
> means...
> 
>> In summary, there seem to be two use cases.
>>
>> A. virtiofsd/fuse-server wants do be able to come up with its own
>> policy to decide which inodes should use guest.
>>
>> B. guest client decides which inode use DAX based on FS_XFLAG_DAX
>> attr on inode (and server does not play a role).
>>
>> To be able to different between these two cases, I was suggesting
>> using "-o dax=inode" for B and "-o dax=server" for A.
> 
> dax=inode covrees both these cases.  All the server side needs to do
> is set the inode attribute according to policy, and all the client
> needs to do is obey the server side per-inode attribute. IOWs,
> client side using "dax=inode" means the server side controls the DAX
> behaviour via the FUSE DAX attribute. 
> 
> If the server wants the client to always use DAX, then it always
> sets the FUSE inode attribute. If the server says "never use DAX",
> then it never sets the FUSE inode attribute.  If the server doesn't
> want the client to control policy, then it just rejects attempts to
> change the per-inode persistent DAX flag via client side
> ioctl(FS_XFLAG_DAX) calls. Hence we have use case A completely
> covered.
> 
> For case B, which is true dax=inode behaviour at the client, then
> the server side honours the client side requests to change the
> persistent FUSE DAX inode attribute via client side FS_XFLAG_DAX
> ioctls.
> 
> See? At the client side, there is absolutely no difference between
> server side policy control and true dax=inode support. The client is
> completely unaware of server side policies and that's how the client
> side should be implemented. The applications have to be prepared for
> policy to change dynamically with either dax=server or dax=inode, so
> there's no difference to applications running in the guest either.
> 
> Hence I just don't see the justification for a special DAX mode from
> an architectural POV. It's no more work to implement per-inode DAX
> properly form the start and we don't end up with a special, subtly
> different mode.
> 
> Cheers,
> 
> Dave.
> 

-- 
Thanks,
Jeffle